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PRODUCTION AND MANAGEMENT: Original Research| Volume 36, ISSUE 1, P108-117, February 2020

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Supplemental monensin affects growth, physiology, and coccidiosis infestation of early-weaned beef calves consuming warm-season perennial or cool-season annual grasses

      ABSTRACT

      Objective

      Two experiments evaluated the effects of supplemental monensin on growth and physiology of early-weaned beef calves grazing ryegrass (Exp. 1) or bahiagrass (Exp. 2).

      Materials and Methods

      Brangus calves were weaned at 3 mo of age, stratified by sex and BW, and randomly assigned into 1 of 8 pastures (2 steers and 2 heifers per pasture per year) of ryegrass (Exp. 1; n = 2 yr) or bahiagrass (Exp. 2; n = 1 yr) from d 0 to 84. Treatments were assigned to pastures (4 pastures per treatment per year) and consisted of concentrate supplementation at 1 or 2% of BW (DM basis) in Exp. 1 and 2, respectively, with or without 20 mg of monensin/kg of DMI.

      Results and Discussion

      Herbage nutritive value and allowance did not differ (P ≥ 0.23) between treatments in Exp. 1 and 2, but herbage mass tended (P = 0.10) to increase by 5% for monensin versus control calves in Exp. 1. Calf overall ADG increased (P ≤ 0.005), whereas fecal coccidia egg count on d 84 decreased (P ≤ 0.0004), for monensin versus control calves in Exp. 1 and 2. Monensin supplementation tended (P ≤ 0.08) to increase plasma insulin concentrations in Exp. 1 and 2 and increased (P ≤ 0.03) plasma IGF-1 concentrations on d 56 in Exp. 1 and plasma urea nitrogen concentrations on d 84 in Exp. 2.

      Implications and Applications

      Supplemental monensin led to subtle changes to physiological parameters associated with energy metabolism, reduced coccidiosis infestation, and promoted the growth performance of early-weaned calves grazing ryegrass and bahiagrass pastures.

      Key words

      INTRODUCTION

      Early weaning (EW) beef calves at 2 to 3 mo of age is an effective management practice to enhance reproductive performance of primiparous cows (
      • Arthington J.D.
      • Kalmbacher R.S.
      Effect of early weaning on the performance of three-year-old, first-calf beef heifers and calves reared in the subtropics..
      ). Favorable climatic conditions during the winter season in the southern United States provide an opportunity to raise EW calves on annual cool-season (
      • Vendramini J.M.B.
      • Sollenberger L.E.
      • Dubeux Jr., J.C.B.
      • Interrante S.M.
      • Stewart Jr., R.L.
      • Arthington J.D.
      Concentrate supplementation effects on forage characteristics and performance of early weaned calves grazing rye–ryegrass pastures..
      ) and perennial warm-season grasses (
      • Vendramini J.M.B.
      • Sanchez J.M.D.
      • Cooke R.F.
      • Aguiar A.D.
      • Moriel P.
      • da Silva W.L.
      • Cunha O.F.R.
      • Ferreira P.D.S.
      • Pereira A.C.
      Stocking rate and monensin supplemental level effects on growth performance of beef cattle consuming warm-season grasses..
      ). However, regardless of forage type, EW calves have a relatively small rumen capacity and consequently require concentrate supplementation (i.e., 1 or 2% of BW; DM basis) to achieve growth performance similar to or greater than calves normally weaned at 8 mo of age (
      • Vendramini J.M.B.
      • Sollenberger L.E.
      • Dubeux Jr., J.C.B.
      • Interrante S.M.
      • Stewart Jr., R.L.
      • Arthington J.D.
      Concentrate supplementation effects on the performance of early weaned calves grazing Tifton 85 bermudagrass..
      ;
      • Moriel P.
      • Johnson S.E.
      • Vendramini J.M.B.
      • McCann M.A.
      • Gerrard D.E.
      • Mercadante V.R.G.
      • Hersom M.J.
      • Arthington J.D.
      Effects of calf weaning age and subsequent management systems on growth performance and carcass characteristics of beef steers..
      ,
      • Moriel P.
      • Johnson S.E.
      • Vendramini J.M.B.
      • Mercadante V.R.G.
      • Hersom M.J.
      • Arthington J.D.
      Effects of calf weaning age and subsequent management system on growth and reproductive performance of beef heifers..
      ). For example, concentrate DM supplementation at 1 and 2% of BW linearly increased the ADG of EW steers grazing Tifton 85 bermudagrass (Cynodon spp.;
      • Vendramini J.M.B.
      • Sollenberger L.E.
      • Dubeux Jr., J.C.B.
      • Interrante S.M.
      • Stewart Jr., R.L.
      • Arthington J.D.
      Concentrate supplementation effects on the performance of early weaned calves grazing Tifton 85 bermudagrass..
      ) and annual ryegrass (Lolium multiflorum;
      • Vendramini J.M.B.
      • Sollenberger L.E.
      • Dubeux Jr., J.C.B.
      • Interrante S.M.
      • Stewart Jr., R.L.
      • Arthington J.D.
      Concentrate supplementation effects on forage characteristics and performance of early weaned calves grazing rye–ryegrass pastures..
      ) compared with no concentrate supplementation.
      Ionophores should be provided to EW calves as a strategy to further increase their growth performance by changing physiological parameters while controlling coccidiosis, as these calves are highly susceptible to this disease until 8 mo of age (
      • Vendramini J.M.B.
      • Moriel P.
      Forage management and concentrate supplementation effects on performance of beef calve..
      ). Coccidiosis is detrimental to young cattle and is caused by a protozoan present in forage and water sources (
      • Keeton S.T.N.
      • Navarre C.B.
      Coccidiosis in large and small ruminants..
      ). Although forage DMI is limited in EW calves receiving large amounts of concentrate DM supplementation (
      • Vendramini J.M.B.
      • Sollenberger L.E.
      • Dubeux Jr., J.C.B.
      • Interrante S.M.
      • Stewart Jr., R.L.
      • Arthington J.D.
      Concentrate supplementation effects on forage characteristics and performance of early weaned calves grazing rye–ryegrass pastures..
      ,
      • Vendramini J.M.B.
      • Sollenberger L.E.
      • Dubeux Jr., J.C.B.
      • Interrante S.M.
      • Stewart Jr., R.L.
      • Arthington J.D.
      Concentrate supplementation effects on the performance of early weaned calves grazing Tifton 85 bermudagrass..
      ,
      • Vendramini J.M.B.
      • Moriel P.
      • Cooke R.F.
      • Arthington J.D.
      • da Silva H.M.
      • Piccolo M.B.
      • Sanchez J.M.D.
      • Gomes V.
      • Mamede P.A.
      Effects of monensin inclusion into increasing amount of concentrate on growth and physiological parameters of early-weaned beef calves consuming warm-season grasses..
      ), EW calves may get infected in areas of high animal and feces agglomeration (
      • Keeton S.T.N.
      • Navarre C.B.
      Coccidiosis in large and small ruminants..
      ), limiting their growth performance. Limited information about the potential benefits of monensin to the performance of EW beef calves grazing cool- and warm-season grass pastures and offered high concentrate DM supplementation is available in the literature (
      • Vendramini J.M.B.
      • Sollenberger L.E.
      • Dubeux Jr., J.C.B.
      • Interrante S.M.
      • Stewart Jr., R.L.
      • Arthington J.D.
      Concentrate supplementation effects on forage characteristics and performance of early weaned calves grazing rye–ryegrass pastures..
      ;
      • Vendramini J.M.B.
      • Moriel P.
      Forage management and concentrate supplementation effects on performance of beef calve..
      ). The hypothesis of the study was that monensin supplementation would reduce coccidia infestation and increase growth performance of EW beef calves grazing annual cool-season or perennial warm-season forages. The objective of this study was to test the effects of monensin supplementation on fecal coccidia egg counts, BW gain, and physiological indicators of energy and protein metabolism of EW calves grazing ryegrass (Exp. 1) and bahiagrass (Paspalum notatum; Exp. 2) pastures.

      MATERIALS AND METHODS

      Animals were cared for following procedures approved by the University of Florida Institute of Food and Agriculture Sciences Animal Research Committee (protocol #201709703). Experiments 1 and 2 were conducted at the University of Florida Institute of Food and Agricultural Sciences Range Cattle Research and Education Center, Ona, Florida (27°26′N and 82°55′W). Experiment 1 was conducted from January to April 2015 and repeated from January to April 2016 (grazing phase only), whereas Exp. 2 was conducted from January to March 2018 (grazing phase) and April to May 2018 (drylot phase).

      Animals and Diets

      Exp. 1—Grazing Phase (d 0 to 84)

      Approximately 10 d before the start of the experiment, 64 Angus × Brahman crossbred calves (n = 16 steers and 16 heifers per year; n = 2 yr) were early weaned on average at 107 ± 18 d of age and 84.6 ± 14 kg of BW. Calves were held in a drylot with access to long-stem stargrass (Cynodon nlemfuensis) hay ad libitum and 1 kg/d of preconditioning concentrate (guaranteed analysis, as fed: 14% CP, 1.0% fat, 18% fiber, 0.75% Ca, 0.40% P, and 0.40% NaCl, Land O’Lakes Purina Feed LLC, Gray Summit, MO) for 10 d until the start of the experiment (d 0). On d 0 of each year, EW calves were stratified by sex, initial BW, and age and randomly allocated into 1 of 8 ryegrass pastures (2 steers and 2 heifers per pasture; 0.30 ha/pasture). All calves remained in their respective pasture assignment for 84 d. Treatments were randomly assigned to pastures (4 pastures per treatment per year) and consisted of concentrate DM supplementation at 1% of BW with or without 20 mg of monensin (Rumensin 90; Elanco Animal Health, Greenfield, IN) per kilogram of an estimated total DMI of 2.5% of BW (
      • NASEM (National Academies of Sciences, Engineering, and Medicine)
      Nutrient Requirements of Beef Cattle.
      ). The supplement DM amount was selected based on previous results from
      • Vendramini J.M.B.
      • Sollenberger L.E.
      • Dubeux Jr., J.C.B.
      • Interrante S.M.
      • Stewart Jr., R.L.
      • Arthington J.D.
      Concentrate supplementation effects on forage characteristics and performance of early weaned calves grazing rye–ryegrass pastures..
      demonstrating that optimal growth and economic feasibility occurred when EW calves grazing annual rye–ryegrass mixtures were supplemented with concentrate DM at 1 versus 0 and 2% of BW. The proposed monensin amount was added daily to the concentrate immediately before feeding (0800 h). Concentrate consisted of (DM basis) 21.0% soybean hulls, 15.7% cottonseed meal, 15.0% cottonseed hulls, 8.8% wheat middlings, 8.0% dried distillers grains, 8.0% citrus pulp pellets, 7.8% cracked corn, 7.8% corn meal, 5.4% soybean meal, 2.0% sugarcane molasses, 0.50% Ca carbonate, 0.05% trace mineral premix, and 0.02% vitamin E (94% DM, 78% TDN, 16% CP). All calves received daily free-choice access to a commercial vitamin–mineral mix (Lakeland Animal Nutrition, Lakeland, FL; 14, 0.3, 24, and 9.0% of Ca, Mg, NaCl, and P, respectively, and 50, 1,500, 20, 40, and 3,000 mg/kg of Co, Cu, I, Se, and Zn, respectively).

      Exp. 2—Grazing Phase (d 0 to 84)

      Forty-eight Angus × Brahman crossbred calves (24 steers and 24 heifers) were weaned on January 9, 2018 (initial age = 92 ± 14 d; initial BW = 97 ± 12 kg). Calves were held in a drylot with access to long-stem stargrass hay ad libitum and 1 kg/d of preconditioning concentrate (same concentrate as described in Exp. 1) for 10 d. On d 0, calves were stratified by sex, initial BW, and age and randomly allocated into 1 of 8 bahiagrass pastures (3 steers and 3 heifers per pasture; 0.4 ha/pasture). All calves remained in their respective pasture assignment for 84 d. Treatments were randomly assigned to pastures (4 pastures per treatment) and consisted of concentrate DM supplementation at 2% of BW with or without 20 mg of monensin (Rumensin 90; Elanco Animal Health) per kilogram of an estimated total DMI of 2.5% of BW (
      • NASEM (National Academies of Sciences, Engineering, and Medicine)
      Nutrient Requirements of Beef Cattle.
      ). The supplement DM amount was selected based on previous results from
      • Vendramini J.M.B.
      • Sollenberger L.E.
      • Dubeux Jr., J.C.B.
      • Interrante S.M.
      • Stewart Jr., R.L.
      • Arthington J.D.
      Concentrate supplementation effects on the performance of early weaned calves grazing Tifton 85 bermudagrass..
      demonstrating that optimal growth and economic feasibility occurred when EW calves grazing perennial bermudagrass were supplemented with concentrate DM at 2 versus 0 and 1% of BW. The proposed monensin amount was added daily to the concentrate immediately before feeding (0800 h). Concentrate consisted of (DM basis) 21.0% soybean hulls, 15.7% cottonseed meal, 15.0% cottonseed hulls, 8.8% wheat middlings, 8.0% dried distillers grains, 8.0% citrus pulp pellets, 7.8% cracked corn, 7.8% corn meal, 5.4% soybean meal, 2.0% sugarcane molasses, 0.50% Ca carbonate, 0.05% trace mineral premix, 0.04% Bovatec 90 (Alpharma Inc., Fort Lee, NJ), and 0.02% vitamin E (94.1% DM, 75% TDN, 28.4% CP, 23.2% ADF, 1.78 Mcal/kg NEm, 1.17 Mcal/kg NEg, 1.57% Ca, 0.58% P, 0.44% S, 0.28% Mg, 1.32% K, 0.067% Na, 185 mg/kg Fe, 53 mg/kg Zn, 9 mg/kg Cu, 30 mg/kg Mn, and 1.9 mg/kg Mo). All calves received daily free-choice access to a commercial vitamin–mineral mix (University of Florida Cattle Research Winter Mineral; Vigortone, Brookville, OH; 16.8, 1.0, 20.7, and 4.0% of Ca, Mg, NaCl, and P, respectively, and 60, 1,750, 350, 60, and 5,000 mg/kg of Co, Cu, I, Se, and Zn, respectively).

      Exp. 2—Drylot Phase (d 85 to 101)

      On d 85, 12 calves per treatment (6 steers and 6 heifers) were randomly assigned to 1 of 24 individual concrete floor pens (18 m2/pen) in a fully covered drylot facility for a 17-d evaluation period (adaptation from d 85 to 94, daily forage intake data collection from d 95 to 101, and daily fecal sample collection from d 99 to 101) to evaluate forage and total DMI, and apparent DM digestibility. From d 85 to 101, calves remained on their respective treatment previously assigned on d 0 (same supplement type and amount as offered during the grazing phase) and were provided daily free-choice access to ground stargrass hay (11.2% CP, 43.9% ADF, 75.8% NDF, 53% TDN; DM basis).

      Sample and Data Collection

      Grazing Phase

      In Exp. 1 and 2, each pasture was sampled for herbage mass (HM) and nutritive value [CP and in vitro digestible OM (IVDOM)] every 14 d but reported at 28-d intervals from d 0 to 84. The double sampling technique was used to determine HM according to
      • Gonzalez M.A.
      • Hussey M.A.
      • Conrad B.E.
      Plant height, disk and capacitance meters used to estimate bermudagrass herbage mass..
      . Briefly, the indirect measure was the settling height of a 0.25-m2 aluminum disk, and the direct measure involved hand clipping all herbage to a 2-cm stubble from the same area using an electric clipper. Every 28 d, 1 or 2 double samples were taken from each experimental unit for a total of 20 samples per pasture that represented the HM range present on pastures. At each site, the disk settling height was measured and the forage clipped. Clipped forage was dried for 72 h at 60°C and weighed. The herbage mass from the clipped sample and the corresponding disk height were used to develop a regression equation, which was later used to estimate HM. Herbage allowance (HA) was calculated as the average HM divided by the average total BW of calves in each pasture (
      • Sollenberger L.E.
      • Moore J.E.
      • Allen V.G.
      • Pedreira C.G.S.
      Reporting forage allowance in grazing experiments..
      ). Hand-plucked forage samples were collected from each pasture every 14 d, dried at 55°C for 72 h in a forced-air oven, ground to pass a 1-mm screen (Model 4, Thomas-Wiley Laboratory Mill, Thomas Scientific, Swedesboro, NJ), and analyzed for IVDOM as described by
      • Moore J.E.
      • Mott G.O.
      Recovery of residual organic matter from “in vitro” digestion of forages..
      . Nitrogen concentration was determined using a micro-Kjeldahl method, a modification of the aluminum block digestion technique described by
      • Gallaher R.N.
      • Weldon C.O.
      • Futral J.G.
      An aluminum block digester for plant and soil analysis..
      . Crude protein was determined by multiplying N concentration by 6.25. Individual samples of the concentrate were collected every 28 d, pooled across months, and then sent in duplicate to a commercial laboratory (Dairy One Forage Laboratory, Ithaca, NY) for wet chemistry analysis of all nutrients.
      Calf BW were assessed immediately before concentrate feeding at 0800 h on d 0, 28, 56, and 84 (Exp. 1) and on d 0, 56, and 84 (Exp. 2). Blood samples (10 mL) were collected via jugular venipuncture into tubes containing sodium heparin (Vacutainer, Becton Dickinson, Franklin Lakes, NJ) for plasma harvest on d 0, 28, 56, and 84 (Exp. 1) and d 0, 56, and 84 (Exp. 2). Blood was centrifuged at 2,000 × g at 4°C for 30 min, and plasma was harvested and kept frozen at −80°C until further analysis to determine the plasma concentrations of insulin, glucose, plasma urea nitrogen (PUN), and IGF-1. Plasma concentrations of insulin were determined using Coat-A-Count solid-phase 125I RIA kits (Siemens Healthcare Diagnostics, Los Angeles, CA) previously validated for bovine samples (
      • Moriel P.
      • Scatena T.S.
      • Sá Filho O.G.
      • Cooke R.F.
      • Vasconcelos J.L.
      Concentrations of progesterone and insulin in serum of nonlactating dairy cows in response to carbohydrate source and processing..
      ). Plasma concentrations of glucose and PUN were determined using quantitative colorimetric kits (#G7521 and B7551, respectively; Pointe Scientific Inc., Canton, MI). Concentrations of IGF-1 were determined using a human-specific commercial ELISA kit (SG100; R&D Systems Inc., Minneapolis, MN) with 100% cross-reactivity with bovine IGF-1 and previously validated for bovine samples (
      • Moriel P.
      • Cooke R.F.
      • Bohnert D.W.
      • Vendramini J.M.B.
      • Arthington J.D.
      Effects of energy supplementation frequency and forage quality on performance, reproductive, and physiological responses of replacement beef heifers..
      ). The intra- and inter-assay CV were 2.9 and 4.8% for insulin, 3.2 and 4.5% for glucose, 3.9 and 4.9% for PUN, and 6.2 and 5.0% for IGF-1. The minimum detectable concentration was 0.01 μIU/mL for insulin and 0.056 ng/mL for IGF-1.
      Rectal fecal samples were collected from all calves on d 0 and 84 (Exp. 1 and 2). All fecal samples were stored in plastic bags, placed in an insulated container with ice, and then sent to a commercial laboratory (Myers Parasitology Services, Magnolia, KY) for analysis of fecal coccidia egg count using the modified Wisconsin sugar flotation technique (
      • Cox D.D.
      • Todd A.C.
      Survey of gastrointestinal parasitism in Wisconsin dairy cattle..
      ). Total fecal coccidia egg counts (observed egg count + 1) of each calf were log-transformed before statistical analyses and reported as log10 (
      • Martins P.G.M.A.
      • Moriel P.
      • Caputti G.P.
      • Vendramini J.M.B.
      • Arthington J.D.
      Effects of multiple oral administrations of fenbendazole on growth and fecal nematodes infection of early-weaned beef calves grazing perennial, warm-season or annual, cool-season grasses..
      ). In Exp. 1, all calves were negative for the fecal coccidia eggs on d 0.

      Drylot Phase

      In Exp. 2, BW of all calves were also assessed immediately before concentrate feeding at 0800 h on d 84 and 101. Hay and concentrate samples were collected daily, dried at 55°C for 72 h in a forced-air oven, and ground to pass a 1-mm screen to determine the forage and concentrate DM concentration and, consequently, calculate forage and total daily DMI from d 95 to 101. Rectal fecal samples from all calves in drylot were collected twice daily (0800 and 1500 h) from d 99 to 101 to determine in vivo apparent DM digestibility using the indigestible NDF procedure. Fecal samples were pooled across all fecal collection days for each calf. Concentrations of indigestible NDF in the forage, concentrate, and feces were determined as described by
      • Cole N.A.
      • McCuistion K.
      • Greene L.W.
      • McCollum F.T.
      Effects of concentration and source of wet distillers grains on digestibility of steam-flaked corn-based diets fed to finishing steers..
      with modifications proposed by
      • Krizsan S.J.
      • Huhtanen P.
      Effect of diet composition and incubation time on feed indigestible neutral detergent fiber concentration in dairy cows..
      . Four grams of forage was placed in 20 × 10-cm N-free polyester bags with pore sizes ranging from 50 to 60 μm. The bags were heat sealed and incubated for 288 h in one ruminally fistulated Braford steer. The steers were housed in a pen with ad libitum access to stargrass hay. All the bags representing all experimental units were incubated and removed from the steers simultaneously. The bags removed from the rumen were rinsed repeatedly until the rinsing water was colorless. Finally, bags were dried at 60°C for 48 h and weighed. Dried samples were analyzed for NDF concentration using the method of
      • Van Soest P.J.
      • Robertson J.B.
      • Lewis B.A.
      Methods for dietary fiber, neutral detergent fiber, and nonstarch polysaccharides in relation to animal nutrition..
      adapted for an Ankom 200 Fiber Analyzer (Ankom Technology Corp., Macedon, NY). Heat-stable α-amylase and sodium sulfite were used in the NDF assay, and the results are presented inclusive of residual ash. Total feces output and apparent DM digestibility were calculated as described by
      • Vendramini J.M.B.
      • Moriel P.
      • Cooke R.F.
      • Arthington J.D.
      • da Silva H.M.
      • Piccolo M.B.
      • Sanchez J.M.D.
      • Gomes V.
      • Mamede P.A.
      Effects of monensin inclusion into increasing amount of concentrate on growth and physiological parameters of early-weaned beef calves consuming warm-season grasses..
      .

      Statistical Analyses

      All data analyses included Satterthwaite approximation to determine the denominator df for the test of fixed effects using MIXED procedure (SAS Institute Inc., Cary, NC, version 9.4). Pasture and calf were considered the experimental units for the grazing (Exp. 1 and 2) and drylot phases (Exp. 2 only), respectively. In Exp. 1, pasture(treatment × year), calf sex, and calf(pasture) were included as random effects in all analyses of animal responses. In Exp. 2, pasture(treatment), calf sex, and calf(pasture) were included as random effects in all analyses of grazing phase, whereas calf(treatment) and calf sex were included as random effects in all analyses during the drylot phase. Pasture evaluation, growth performance, plasma parameters, and fecal egg counts (Exp. 2. only) during the grazing phase were analyzed as repeated measures and tested for fixed effects of treatment, day, year, and all resulting interactions (Exp. 1), and treatment, day, and treatment × day (Exp. 2). Pasture(treatment) was considered the subject for analyses of forage evaluation, whereas calf(treatment) was the subject in the analyses of growth and plasma measurements. In Exp. 1, all calves were negative for coccidia fecal egg count on d 0, and hence, fecal egg count on d 84 was tested for the fixed effects of treatment, year, and treatment × year using pasture(treatment) and calf(pasture) as random effects. Compound symmetry covariance structure was used for analyses of repeated measures. Plasma measurements and calf BW obtained on d 0 were used as covariates if P ≤ 0.05. In Exp. 2, calf growth and forage and total DMI during drylot phase were tested for fixed effects of treatment. All results were reported as least squares means. Significance was set at P ≤ 0.05, and tendencies were declared if 0.05 < P ≤0.10.

      RESULTS AND DISCUSSION

      Growth Performance

      In Exp. 1 and 2, calf age did not differ between treatments (P ≥ 0.67) but was included as a covariate (P ≤ 0.05) in the analyses of calf BW and ADG. Effects of treatment × day × year and treatment × year were not detected (P ≥ 0.42) for calf BW in Exp. 1. In agreement with our hypothesis, effects of treatment × day were detected (P ≤ 0.0007) for calf BW in Exp. 1 and 2 (Table 1). In Exp. 1, calf BW on d 0 and 28 did not differ (P ≥ 0.28) between treatments but increased (P ≤ 0.001) for monensin versus control calves on d 56 and 84. In Exp. 2, calf BW did not differ (P = 0.61) between treatments on d 0 but increased (P ≤ 0.0006) for monensin versus control calves on d 56 and 84. Calf ADG from d 0 to 28, 28 to 56, and 56 to 84 and overall ADG from d 0 to 84 in Exp. 1 were always greater (P ≤ 0.04) for monensin versus control calves (Table 1), whereas in Exp. 2, calf ADG from d 0 to 56 and 0 to 84 were greater (P ≤ 0.008) for monensin versus control calves and tended (P = 0.07) to increase for monensin versus control calves from d 56 to 84 (Table 1). Overall, monensin supplementation increased the ADG of EW calves by 0.17 and 0.14 kg/d in ryegrass (Exp. 1) and bahiagrass pastures (Exp. 2), respectively, which is in agreement with previous studies (
      • Rouquette Jr., F.M.
      • Griffin J.L.
      • Randel R.D.
      • Carroll L.H.
      Effect of monensin on gain and forage utilization by calves grazing bermudagrass..
      ;
      • Vendramini J.M.B.
      • Moriel P.
      • Cooke R.F.
      • Arthington J.D.
      • da Silva H.M.
      • Piccolo M.B.
      • Sanchez J.M.D.
      • Gomes V.
      • Mamede P.A.
      Effects of monensin inclusion into increasing amount of concentrate on growth and physiological parameters of early-weaned beef calves consuming warm-season grasses..
      ). Beef calves grazing bermudagrass (Cynodon dactylon) and receiving 0.90 kg/d concentrate added with 200 mg/d monensin had greater ADG (0.54 vs. 0.40 kg/d) compared with beef calves receiving concentrate without monensin (
      • Rouquette Jr., F.M.
      • Griffin J.L.
      • Randel R.D.
      • Carroll L.H.
      Effect of monensin on gain and forage utilization by calves grazing bermudagrass..
      ). Average daily gain of EW calves offered stargrass hay increased from 0.36 to 0.44 kg/d when monensin (20 mg/kg of total DMI) was added to the supplement (
      • Vendramini J.M.B.
      • Moriel P.
      • Cooke R.F.
      • Arthington J.D.
      • da Silva H.M.
      • Piccolo M.B.
      • Sanchez J.M.D.
      • Gomes V.
      • Mamede P.A.
      Effects of monensin inclusion into increasing amount of concentrate on growth and physiological parameters of early-weaned beef calves consuming warm-season grasses..
      ). In contrast, supplementation of 200 mg/d monensin did not affect the overall ADG of beef heifers grazing bahiagrass pastures and offered a soybean hulls–based supplement at 0.40 kg/d (
      • Vendramini J.M.B.
      • Sanchez J.M.D.
      • Cooke R.F.
      • Aguiar A.D.
      • Moriel P.
      • da Silva W.L.
      • Cunha O.F.R.
      • Ferreira P.D.S.
      • Pereira A.C.
      Stocking rate and monensin supplemental level effects on growth performance of beef cattle consuming warm-season grasses..
      ) or 2 kg/d sugarcane molasses + 0.50 kg/d cottonseed meal (
      • Moriel P.
      • Vendramini J.M.B.
      • Carnelos C.
      • Piccolo M.B.
      • da Silva H.M.
      Effects of monensin on growth performance of beef heifers consuming warm-season perennial grass and supplemented with sugarcane molasses..
      ). The monensin-induced mechanisms leading to greater growth performance will be discussed below, but plausible explanations for the discrepancy on growth performance of calves supplemented or not with monensin among the studies described above may include the supplement amount, composition, forage type, and level of coccidia infestation.
      Table 1Body weight and ADG of early-weaned calves grazing ryegrass pastures and offered concentrate DM supplementation at 1% of BW (Exp. 1) or bahiagrass pastures and offered concentrate DM supplementation at 2% of BW (Exp. 2) with or without monensin (20 mg of monensin/kg of an estimated total DMI of 2.5% of BW) from d 0 to 84
      ItemTreatmentSEMP-value
      P-value for the comparison of treatment within day.
      P-value
      ControlMonensinTreatment

      × day
      TreatmentDay
      Exp. 1
       BW,
      Covariate adjusted for calf age (P ≤ 0.05).
      kg
      d 085851.80.90<0.00010.009<0.0001
      d 281071101.80.29
      d 561151241.80.001
      d 841301451.8<0.0001
       ADG,
      Covariate adjusted for calf age (P ≤ 0.05).
      kg
      d 0 to 280.780.880.0340.04
      d 28 to 560.300.520.0570.02
      d 56 to 840.540.730.0420.002
      d 0 to 840.540.710.0360.005
      Exp. 2
       BW,
      Covariate adjusted for calf age (P ≤ 0.05).
      kg
      d 096971.40.61<0.00010.004<0.0001
      d 561431511.40.0006
      d 841641751.4<0.0001
       ADG,
      Covariate adjusted for calf age (P ≤ 0.05).
      kg
      d 0 to 560.830.990.0270.008
      d 56 to 840.720.840.0390.07
      d 0 to 840.800.940.0210.003
      1 P-value for the comparison of treatment within day.
      2 Covariate adjusted for calf age (P ≤ 0.05).

      Forage Responses

      Growth performance of cattle is regulated by intake (50–70%), digestibility (24–40%), and metabolism (5–15%,
      • Mertens D.
      Challenges in measuring forage quality.
      ). Effects of treatment × day and treatment × year × day were not detected (P ≥ 0.23) for HM, HA, IVDOM, and CP in Exp. 1 and 2. Herbage CP and IVDOM concentrations did not differ between treatments in Exp. 1 and 2 (P ≥ 0.23) but changed monthly similarly to our previous study (
      • Vendramini J.M.B.
      • Sollenberger L.E.
      • Dubeux Jr., J.C.B.
      • Interrante S.M.
      • Stewart Jr., R.L.
      • Arthington J.D.
      Concentrate supplementation effects on forage characteristics and performance of early weaned calves grazing rye–ryegrass pastures..
      ). In Exp. 1, the growth period was longer for herbage on d 0 (approximately 80 d from planting) relative to subsequent forage regrowth periods (28 d), and this may have reduced CP concentration on d 0 versus 28 and 56. Also, herbage CP concentration was less on d 84 compared with d 28 and 56 due to the greater presence of reproductive tillers on d 84. Similarly, bahiagrass CP and IVDOM in Exp. 2 decreased with maturity from d 0 to 84. The reduced leaf:stem ratio caused by the onset of reproductive-stem elongation is usually the main factor decreasing the nutritive value of warm-season grasses (
      • Sollenberger L.E.
      • Ocumpaugh W.R.
      • Euclides V.P.B.
      • Moore J.E.
      • Quesenberry K.H.
      • Jones Jr., C.S.
      Animal performance on continuously stocked ‘Pensacola’ bahiagrass and ‘Floralta’ limpograss pastures..
      ). Despite the monthly changes to forage CP and IVDOM, forage nutritive value in both experiments remained at adequate levels to meet the energy and protein requirements of EW calves during the entire study (
      • NASEM (National Academies of Sciences, Engineering, and Medicine)
      Nutrient Requirements of Beef Cattle.
      ).
      Herbage mass and HA in Exp. 1 and 2 gradually decreased (P < 0.0001) from d 0 to 84 (Table 2) due to the consumption of calves being greater than the forage accumulated during winter. The levels of ryegrass HA observed in Exp. 1 were greater during the first 56 d but almost half of the minimum threshold that limits forage intake of EW calves (0.5 kg of DM/kg of BW;
      • Vendramini J.M.B.
      • Arthington J.D.
      Effects of supplementation strategies on performance of early-weaned calves raised on pastures..
      ). Although ryegrass HM tended (P = 0.10) to increase for calves supplemented with monensin versus control (958 vs. 1,006 ± 15.0 kg of DM/ha, respectively), calves supplemented with monensin were heavier compared with control calves, and, consequently, overall ryegrass HA did not differ (P = 0.91) between treatments (1.09 vs. 1.08 ± 0.262 kg of DM/kg of BW, respectively). In Exp. 2, despite the gradual decrease in HA over time, the levels of bahiagrass HA were significantly greater during the entire grazing phase compared with the minimum threshold of 0.5 kg of DM/kg of BW described by
      • Vendramini J.M.B.
      • Arthington J.D.
      Effects of supplementation strategies on performance of early-weaned calves raised on pastures..
      . Therefore, growth performance of EW calves in both experiments was not limited by herbage mass during most (Exp. 1) and all (Exp. 2) of the grazing phase, and the greater growth performance of calves supplemented with monensin reported in both experiments was associated with factors beyond herbage mass, allowance, and nutritive value.
      Table 2Herbage mass (HM), herbage allowance (HA), in vitro digestible OM (IVDOM), and CP of ryegrass (Exp. 1) and bahiagrass (Exp. 2) pastures grazed by early-weaned calves offered concentrate supplementation with or without monensin (20 mg of monensin/kg of an estimated total DMI of 2.5% of BW) from d 0 to 84
      Item
      Pastures were sampled for HM and nutritive value (CP and IVDOM) every 14 d but reported at 28-d intervals from d 0 to 84. The double sampling technique was used to determine HM according to Gonzalez et al. (1990). Herbage allowance was calculated as the average HM divided by the average total BW of calves in each pasture (Sollenberger et al., 2005).
      Day of the studySEMP-value,

      day
      0285684
      Exp. 1
       HM, kg of DM/ha1,393a1,039b955c540d22.5<0.0001
       HA, kg of DM/kg of BW2.43a0.95b0.68c0.28d0.04<0.0001
       IVDOM, g/kg of DM796a798a725b708c4.57<0.0001
       CP, g/kg of DM195a301c237b205a5.5<0.0001
      Exp. 2
       HM, kg of DM/ha6,071a5,192b3,302c2,570d152.7<0.0001
       HA, kg of DM/kg of BW10.5a6.9b3.3c2.4d0.20<0.0001
       IVDOM, g/kg of DM477a430bc411c444b13.0<0.01
       CP, g/kg of DM177c127b105d138a5.0<0.01
      a–dWithin a row, means without a common superscript differ (P ≤ 0.05).
      1 Pastures were sampled for HM and nutritive value (CP and IVDOM) every 14 d but reported at 28-d intervals from d 0 to 84. The double sampling technique was used to determine HM according to
      • Gonzalez M.A.
      • Hussey M.A.
      • Conrad B.E.
      Plant height, disk and capacitance meters used to estimate bermudagrass herbage mass..
      . Herbage allowance was calculated as the average HM divided by the average total BW of calves in each pasture (
      • Sollenberger L.E.
      • Moore J.E.
      • Allen V.G.
      • Pedreira C.G.S.
      Reporting forage allowance in grazing experiments..
      ).

      Physiological Parameters

      Additional factors that may explain the greater growth performance of monensin-fed calves compared with those not supplemented with monensin are differences in total diet digestibility and metabolism. Although forage intake and digestibility were not estimated during the grazing phase, forage and total DM consumption and apparent DM digestibility did not differ (P ≥ 0.17) between calves supplemented or not with monensin during the drylot phase of Exp. 2 (Table 3) and in previous studies (
      • Vendramini J.M.B.
      • Moriel P.
      • Cooke R.F.
      • Arthington J.D.
      • da Silva H.M.
      • Piccolo M.B.
      • Sanchez J.M.D.
      • Gomes V.
      • Mamede P.A.
      Effects of monensin inclusion into increasing amount of concentrate on growth and physiological parameters of early-weaned beef calves consuming warm-season grasses..
      ;
      • Moriel P.
      • Vendramini J.M.B.
      • Carnelos C.
      • Piccolo M.B.
      • da Silva H.M.
      Effects of monensin on growth performance of beef heifers consuming warm-season perennial grass and supplemented with sugarcane molasses..
      ). Therefore, we evaluated the plasma concentrations of hormones and metabolites associated with energy and protein metabolism that could potentially explain the differences in calf growth performance observed in the current study.
      Table 3Drylot growth performance, DMI, and apparent DM digestibility of calves offered daily free-choice access to ground stargrass and offered concentrate DM supplementation at 2% of BW with or without monensin (20 mg of monensin/kg of an estimated total DMI of 2.5% of BW) from d 85 to 101 (Exp. 2)
      ItemTreatmentSEMP-value,

      treatment
      ControlMonensin
      BW,
      Full BW obtained on d 84 and 101 of calves randomly selected and assigned to the drylot phase from d 85 to 101.
      kg
       d 841761854.70.21
       d 1011972075.20.17
      Forage DMI,
      Forage and total DMI calculated as percentage of the average calf BW obtained on d 84 and 101.
      % of BW
      0.730.720.050.86
      Total DMI,
      Forage and total DMI calculated as percentage of the average calf BW obtained on d 84 and 101.
      % of BW
      2.832.810.050.73
      Apparent DM digestibility, g/kg of DM78177316.70.63
      1 Full BW obtained on d 84 and 101 of calves randomly selected and assigned to the drylot phase from d 85 to 101.
      2 Forage and total DMI calculated as percentage of the average calf BW obtained on d 84 and 101.
      Effects of treatment × day and treatment were not detected (P ≥ 0.27) for plasma concentrations of glucose in Exp. 1 and 2 (Table 4). Effects of treatment × day were not detected (P ≥ 0.30) for plasma concentrations of IGF-1 in Exp. 2 but tended to be detected (P = 0.009) in Exp. 1. Effects of treatment × day were not detected (P ≥ 0.43), but overall plasma concentrations of insulin tended to be greater for monensin versus control calves in Exp. 1 (P = 0.08) and 2 (P = 0.07; Table 4). Effects of treatment × day were not detected for plasma concentrations of PUN in Exp. 1 (P ≥ 0.43) but tended to be detected (P = 0.07) in Exp. 2 (Table 4).
      Table 4Plasma concentrations of glucose, IGF-1, insulin, and urea nitrogen (PUN) of early-weaned calves grazing ryegrass pastures and offered concentrate DM supplementation at 1% of BW (Exp. 1) or bahiagrass pastures and offered concentrate DM supplementation at 2% of BW (Exp. 2) with or without monensin (20 mg of monensin/kg of an estimated total DMI of 2.5% of BW) from d 0 to 84
      Plasma variableTreatmentSEMP-value
      P-value for the comparison of treatment within day.
      P-value
      ControlMonensinTreatment

      × day
      Treatment
      Exp. 1
      Concentrations of glucose, PUN, and insulin on d 0 of Exp. 1 were not included as a covariate (P ≥ 0.19). Plasma concentrations of IGF-1 on d 0 of Exp. 1 did not differ (P ≥ 0.59) between treatments but were included as a covariate (P = 0.03).
       Glucose, mg/dL78.381.41.870.420.28
       IGF-1, ng/mL
      d 0105.4109.718.320.870.090.51
      d 2877.1112.118.320.19
      d 5626.889.518.320.03
      d 8445.758.418.320.63
       Insulin, μIU/mL1.922.750.300.920.08
       PUN, mg/dL27.125.71.080.830.43
      Exp. 2
      Plasma concentrations of glucose, insulin, and IGF-1 on d 0 of Exp. 2 did not differ (P ≥ 0.59) between treatments but were included as a covariate (P < 0.0001). Concentrations of PUN on d 0 of Exp. 2 were not included as a covariate (P = 0.65).
       Glucose, mg/dL95.497.01.660.850.49
       IGF-1, ng/mL59.862.51.880.400.30
       Insulin, μIU/mL11.212.60.520.170.07
       PUN, mg/dL
      d 05.737.812.630.580.070.12
      d 5612.09.912.630.58
      d 8420.831.02.630.007
      1 P-value for the comparison of treatment within day.
      2 Concentrations of glucose, PUN, and insulin on d 0 of Exp. 1 were not included as a covariate (P ≥ 0.19). Plasma concentrations of IGF-1 on d 0 of Exp. 1 did not differ (P ≥ 0.59) between treatments but were included as a covariate (P = 0.03).
      3 Plasma concentrations of glucose, insulin, and IGF-1 on d 0 of Exp. 2 did not differ (P ≥ 0.59) between treatments but were included as a covariate (P < 0.0001). Concentrations of PUN on d 0 of Exp. 2 were not included as a covariate (P = 0.65).
      Monensin supplementation to cattle can enhance plasma concentrations of IGF-1 indirectly by stimulating the synthesis of glucose, leading to the release of insulin and IGF-1. In both experiments, effects of treatment × day and treatment were not detected (P ≥ 0.27) for plasma concentrations of glucose, whereas plasma concentrations of insulin tended (P = 0.08 and 0.07 in Exp. 1 and 2, respectively) to increase following monensin supplementation (Table 4; treatment × day effects for plasma insulin concentrations were not detected; P ≥ 0.43). Effects of treatment × day were not detected (P = 0.30) for plasma concentrations of IGF-1 in Exp. 2 but tended to be detected (P = 0.009) in Exp. 1, when plasma concentrations of IGF-1 were greater on d 56 (and numerically greater on d 28 and 84) after monensin was added to the concentrate supplement. The increases in plasma concentrations of glucose, insulin, and IGF-1 were expected (
      • Cappellozza B.I.
      • Cooke R.F.
      • Reis M.M.
      • Moriel P.
      • Keisler D.H.
      • Bohnert D.W.
      Supplementation based on protein or energy ingredients to beef cattle consuming low-quality cool-season forages: II. Performance, reproductive, and metabolic responses of replacement heifers..
      ,
      • Cappellozza B.I.
      • Cooke R.F.
      • Reis M.M.
      • Moriel P.
      • Keisler D.H.
      • Bohnert D.W.
      Supplementation based on protein or energy ingredients to beef cattle consuming low-quality cool-season forages: II. Performance, reproductive, and metabolic responses of replacement heifers..
      ).
      • Vendramini J.M.B.
      • Moriel P.
      • Cooke R.F.
      • Arthington J.D.
      • da Silva H.M.
      • Piccolo M.B.
      • Sanchez J.M.D.
      • Gomes V.
      • Mamede P.A.
      Effects of monensin inclusion into increasing amount of concentrate on growth and physiological parameters of early-weaned beef calves consuming warm-season grasses..
      reported no differences in plasma concentrations of glucose and insulin but increased plasma concentrations of IGF-1 when monensin was offered to EW calves grazing bahiagrass and supplemented with concentrate DM at 1 and 2% of BW.
      • Moriel P.
      • Vendramini J.M.B.
      • Carnelos C.
      • Piccolo M.B.
      • da Silva H.M.
      Effects of monensin on growth performance of beef heifers consuming warm-season perennial grass and supplemented with sugarcane molasses..
      reported no differences in plasma concentrations of glucose, insulin, and IGF-1 between heifers supplemented with molasses + cottonseed meal with or without monensin. Although supplement composition may have played a role, the discrepancy among these studies on plasma concentrations of glucose, insulin, and IGF-1 may be attributed to the timing of blood collection relative to time of day when supplements were offered. Plasma concentrations of glucose and insulin usually peak after 1 to 2 h after concentrate feeding (
      • Moriel P.
      • Scatena T.S.
      • Sá Filho O.G.
      • Cooke R.F.
      • Vasconcelos J.L.
      Concentrations of progesterone and insulin in serum of nonlactating dairy cows in response to carbohydrate source and processing..
      ), whereas blood samples were collected immediately before feeding in both experiments. Hence, peak concentrations of plasma glucose, insulin, and IGF-1 were likely missed. Despite the mismatch between timing of blood collection and peak of plasma concentrations of these hormones and metabolites, the observed differences in plasma concentrations of insulin in Exp. 1 and 2 and IGF-1 in Exp. 1 support the rationale that the energy metabolism of EW calves was positively affected by monensin supplementation.
      Concentrations of PUN are positively associated with intake of RDP, ruminal ammonia concentration, and ruminal protein:energy ratio (
      • Hammond A.C.
      Update on BUN and MUN as a guide for protein supplementation in cattle.
      ). Protein metabolism can also be affected by monensin, but variable responses have been reported for concentrations of PUN following monensin fortification of supplements. Monensin supplementation did not increase the PUN concentrations in some studies (
      • Vendramini J.M.B.
      • Moriel P.
      • Cooke R.F.
      • Arthington J.D.
      • da Silva H.M.
      • Piccolo M.B.
      • Sanchez J.M.D.
      • Gomes V.
      • Mamede P.A.
      Effects of monensin inclusion into increasing amount of concentrate on growth and physiological parameters of early-weaned beef calves consuming warm-season grasses..
      ;
      • Moriel P.
      • Vendramini J.M.B.
      • Carnelos C.
      • Piccolo M.B.
      • da Silva H.M.
      Effects of monensin on growth performance of beef heifers consuming warm-season perennial grass and supplemented with sugarcane molasses..
      ). However, it significantly increased the PUN concentrations in other studies (
      • Poos M.I.
      • Hanson T.L.
      • Klopfenstein T.J.
      Monensin effects on diet digestibility, ruminal protein bypass and microbial protein synthesis..
      ;
      • Muntifering R.B.
      • Theurer B.
      • Swingle R.S.
      • Hale W.H.
      Effect of monensin on nitrogen utilization and digestibility of concentrate diets by steers..
      ;
      • Vendramini J.M.B.
      • Sanchez J.M.D.
      • Cooke R.F.
      • Aguiar A.D.
      • Moriel P.
      • da Silva W.L.
      • Cunha O.F.R.
      • Ferreira P.D.S.
      • Pereira A.C.
      Stocking rate and monensin supplemental level effects on growth performance of beef cattle consuming warm-season grasses..
      ) likely due to an improved utilization of N associated with decreased proteolysis of dietary protein and altered site of protein digestion (
      • Poos M.I.
      • Hanson T.L.
      • Klopfenstein T.J.
      Monensin effects on diet digestibility, ruminal protein bypass and microbial protein synthesis..
      ). Effects of treatment × day were not detected for concentrations of PUN in Exp. 1 (P ≥ 0.43) but tended to be detected (P = 0.07) in Exp. 2, which did not differ between treatments on d 0 and 56 (P = 0.58; Table 4) and were below the optimal concentrations for growing animals (11 to 15 mg/dL;
      • Byers F.M.
      • Moxon A.L.
      Protein and selenium levels for growing and finishing beef cattle..
      ). However, concentrations of PUN in Exp. 2 increased on d 84 (P = 0.007; Table 4) for monensin versus control calves and were above the optimal concentration range. Despite the lack of treatment effects, concentrations of PUN in Exp. 1 were also above the 11 to 15 mg/dL range (
      • Byers F.M.
      • Moxon A.L.
      Protein and selenium levels for growing and finishing beef cattle..
      ). These results on PUN concentrations reflect the fluctuations in forage CP concentrations observed in both experiments. They also indicate that RDP and CP were consumed in excess from d 0 to 84 in Exp. 1 but in limited amounts until d 56 and in excess from d 56 to 84 in Exp. 1.

      Coccidiosis Infestation

      In addition to the subtle changes to protein and energy metabolism, monensin fortification of supplements may also improve calf performance by controlling coccidiosis (
      • Stromberg B.E.
      • Schlotthauer J.C.
      • Hamann K.J.
      • Oz H.S.
      • Bemrick W.J.
      Experimental bovine coccidiosis: Control with monensin..
      ;
      • Hurst J.J.
      • Wallace J.S.
      • Aga D.S.
      Method development for the analysis of ionophore antimicrobials in dairy manure to assess removal within a membrane-based treatment system..
      ). Coccidiosis is a parasitic disease caused by the protozoan parasite of the genus Eimeria that can lead to either clinical disease or subclinical losses to growth performance of young cattle (
      • Keeton S.T.N.
      • Navarre C.B.
      Coccidiosis in large and small ruminants..
      ). High concentrations of this protozoa can accumulate in areas where animals often congregate and feces accumulate, such as a drylot, heavily stocked pastures, and watering and feeding areas (
      • Keeton S.T.N.
      • Navarre C.B.
      Coccidiosis in large and small ruminants..
      ). Cattle become immune to coccidia at around 1 yr of age but afterward may continue to serve as a reservoir to younger animals (
      • Keeton S.T.N.
      • Navarre C.B.
      Coccidiosis in large and small ruminants..
      ). Hence, coccidiosis is an important clinical and subclinical disease with negative effects on performance of EW calves. Intact ionophore residues are excreted by treated animals, and the manure produced by these animals may be applied to croplands as fertilizers, leach into the groundwater, or enter surface water through runoff (
      • Hurst J.J.
      • Wallace J.S.
      • Aga D.S.
      Method development for the analysis of ionophore antimicrobials in dairy manure to assess removal within a membrane-based treatment system..
      ). Fecal coccidia eggs on d 0 were not detected in Exp. 1, whereas in Exp. 2, fecal coccidia egg counts on d 0 were minimal and did not differ between treatments (P = 0.51; Table 5). However, on d 84 of both experiments, fecal coccidia egg counts were 2.8- and 3.5-fold greater (P < 0.001) for control versus monensin calves in Exp. 1 and 2, respectively. These results are in agreement with other studies providing monensin to EW calves (
      • Vendramini J.M.B.
      • Moriel P.
      • Cooke R.F.
      • Arthington J.D.
      • da Silva H.M.
      • Piccolo M.B.
      • Sanchez J.M.D.
      • Gomes V.
      • Mamede P.A.
      Effects of monensin inclusion into increasing amount of concentrate on growth and physiological parameters of early-weaned beef calves consuming warm-season grasses..
      ) and are likely a result of ionophore controlling levels of coccidia in the body as well as decreasing ground level contamination. Therefore, regardless of forage type, the lower coccidia contamination of monensin-fed calves was likely the main factor contributing to their greater growth performance compared with calves not offered monensin supplementation in both experiments.
      Table 5Rectal fecal coccidia egg counts of early-weaned calves grazing ryegrass pastures and offered concentrate DM supplementation at 1% of BW (Exp. 1) or bahiagrass pastures and offered concentrate DM supplementation at 2% of BW (Exp. 2) with or without monensin (20 mg of monensin/kg of an estimated total DMI of 2.5% of BW) from d 0 to 84
      Coccidia egg count,
      Coccidia egg counts (observed egg count + 1) of each calf were log-transformed before statistical analyses and reported as log10 (Martins et al., 2017). Fecal coccidia egg count on d 0 was not detected in Exp. 1, did not differ (P = 0.51) between treatments in Exp. 2, and was not included as a covariate (P = 0.12).


      log10 of eggs/g of feces
      TreatmentSEMP-value
      P-value for the comparison of treatment within day.
      P-value
      ControlMonensinTreatment

      × day
      Treatment
      Exp. 1
       d 000<0.0001
       d 841.150.400.086<0.0001
      Exp. 2
       d 00.160.060.1110.510.0070.004
       d 841.140.330.1110.0004
      1 Coccidia egg counts (observed egg count + 1) of each calf were log-transformed before statistical analyses and reported as log10 (
      • Martins P.G.M.A.
      • Moriel P.
      • Caputti G.P.
      • Vendramini J.M.B.
      • Arthington J.D.
      Effects of multiple oral administrations of fenbendazole on growth and fecal nematodes infection of early-weaned beef calves grazing perennial, warm-season or annual, cool-season grasses..
      ). Fecal coccidia egg count on d 0 was not detected in Exp. 1, did not differ (P = 0.51) between treatments in Exp. 2, and was not included as a covariate (P = 0.12).
      2 P-value for the comparison of treatment within day.

      APPLICATIONS

      Supplement fortification with monensin in annual cool-season and perennial warm-season forage systems led to similar positive responses on performance of early-weaned beef calves. Supplemental monensin positively affected the plasma concentrations of energy metabolism–related hormones and metabolites, reduced the coccidia infestation, and increased the growth performance of early-weaned calves grazing annual ryegrass and perennial bahiagrass pastures.

      ACKNOWLEDGMENTS

      The authors are grateful for the contributions and efforts of Austin Bateman and Julien Warren (both from the Range Cattle Research and Education Center, University of Florida, Ona), who assisted in data collection and cared for the cattle.

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