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Our purpose is to discuss contemporary and pertinent considerations related to mineral-supplementation strategies for sheep grazing marginal rangelands in extensively managed production systems.
Sources
Grey literature, peer-reviewed literature, and data (published and unpublished) from co-authors were used.
Synthesis
Precision trace-mineral nutrition of sheep that are grazing extensive production systems requires a comprehensive understanding of the sheep and the grazing environment. Generally, extensive sheep production systems are found in remote geographical regions composed of marginal rangelands not suitable for cultivated crop farming or improved forages. Sheep production is subject to the accessibility and availability of grazable forage, which may vary greatly within and across years. Sheep common to extensive systems include mostly wool-, meat-, and some hair-type sheep. Furthermore, contemporary sheep have changed with regard to mature BW, dietary intake, and prolificacy. Accordingly, it is important to consider both landscape and animal challenges when developing trace-mineral nutrition programs.
Conclusions and Applications
Dietary mineral heterogeneity of marginal rangelands has made precision mineral supplementation of sheep challenging. Knowledge of plant differences, plant phenology temporal changes, metabolic mineral antagonism, and soil geochemical mapping can facilitate prediction of site-specific mineral shortfalls. Furthermore, an appreciation of recent genetic improvement of sheep breeds common in extensive production systems can enable producers to accurately estimate specific mineral requirements respective of breed and production stage. Future research efforts that use contemporary sheep genotypes and emerging trace-mineral sources with site-specific environmental data are critical to further refine mineral nutrition management of sheep managed in extensive systems.
Globally, extensive sheep operations rely on broad expanses of marginal agricultural land not suitable for cultivated crop farming (e.g., grains, fruits, hay) or improved pasture lands with planted exotic forages for large domestic livestock (e.g., dairy, beef;
). These lands are commonly referred to as rangelands and are generally maintained in a “native” state with regard to the dominant plant species and inherent terrain features and typically have few to no agricultural improvements other than fencing and water development. Rangelands are characterized by heterogeneous plant communities, plant productivity challenges, complex topography, and harsh/extreme climatic conditions, which in combination, create complicated production scenarios (
Fuhlendorf, S. D., R. W. Fynn, D. A. McGranahan, and D. Twidwell. 2017. Heterogeneity as the basis for rangeland management. Pages 169–196 in Rangeland Systems. Springer, Cham, Switzerland.
). Accordingly, sheep production is limited to the accessibility, availability or quantity, and quality of vegetation, which may vary greatly both within and across years (
), macrominerals (Ca. P, Na, K, Mg, Na, Cl), and microminerals (Cu, Mn, Zn, Se, Co, I) collectively constrain sheep productivity. A persistent obstacle to effective trace-mineral management in grazing-based sheep production systems has been implementing supplemental solutions to overcome spatial variation in soil geochemistry, plant species, and plant phenological stages that influences the availability of plant-derived minerals. Additionally, challenges exist related to water availability and quality, inter-relationships of minerals, and difficulty of matching these micronutrients to the dynamic requirements of the ewe throughout the production year.
Precision trace-mineral management is further nuanced by mineral supplement intake variability on extensive landscapes and the associated challenges of flock age structure, breed differences, and phenotypic changes due to genetic improvement of sheep breeds (USDA-NASS, 1970–2019;
). Considering the low economic input of extensively managed sheep production systems, strategically developed mineral-supplementation strategies are necessary to effectively supply mineral to offset deficiencies in a dynamic production system. The following review will attempt to provide both theoretical and practical trace-mineral management considerations for practitioners and producers alike to apply in extensive and semi-extensive sheep management systems.
REVIEW AND DISCUSSION
Dynamics of Soil, Vegetation, and Dietary Mineral Content
A fundamental understanding of sheep mineral nutrition in a rangeland setting must begin at the soil level. Current spatial tools are available for mapping soil mineral concentrations with regional precision. In particular, the United States Geological Survey (USGS) has developed geochemical and mineralogical maps derived from consistent soil sampling methods (e.g., https://mrdata.usgs.gov/ds-801/). These maps are “heat” maps that visualize areas of lower (e.g., purple or cooler colors) and greater (e.g., red or hotter colors) mineral content. For example, the drastic variation of Se and Zn across the US West is readily apparent in Figures 1a and 1b. Areas with greater soil Se such as the northern Great Plains are in contrast with areas of lower soil Se such as the Rocky Mountains (Figure 1a). An opposite pattern for soil Zn is demonstrated in Figure 1b, where greater soil Zn in the Pacific Northwest is in contrast with lower soil Zn in the lower US Southwest. It is important to understand, however, that these mapping tools are regional estimates and not necessarily location specific nor always indicative of plant mineral content. For example, authors compared Zn concentrations in leaves of Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis), a dominant and characteristic shrub to the region, collected from 25 different sheep winter-grazing ranges across Wyoming and western Colorado with soil Zn concentrations (upper 5 cm) from the nearest USGS soil sampling sites, which ranged 5 to 37 km away. Based on the analysis, USGS soil Zn was a significant predictor of Wyoming big sagebrush leaf Zn, although the amount of variation explained was relatively low (P = 0.02; r2 = 0.25; Figure 2), thus highlighting the utility but additional complexities of predicting forage mineral concentrations from soil geochemistry.
Figure 1Spatial patterns of soil microminerals for (a) Se and (b) Zn in the western and central United States. Data are based on United States Geological Survey geochemical and mineralogical maps that are derived from consistent soil sampling methods and sample spacing with 1 field sample per 1,600 km2 (
USGS (United States Geological Survey). 2021. Geochemical and mineralogical data for soils of the conterminous United States. Accessed Nov. 13, 2020. https://mrdata.usgs.gov/ds-801/.
Figure 2Comparison of Zn concentrations of Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis; a dominant and characteristic shrub to the region) leaves collected from 25 different sheep winter-grazing ranges across Wyoming and western Colorado (
) with soil Zn concentrations (upper 5 cm) from the nearest United States Geological Survey (USGS) soil sampling sites, which ranged 5 to 37 km away. Gray lines indicate 95% CI.
The ability to predict forage mineral composition from soil geochemistry is limited due to the many factors influencing the availability of minerals from soils. Factors include soil fertility, geologic parent material, precipitation, temperature, elevation and topographic position, pH, OM, drainage, and root proliferation (
McDowell, L. R. 1985. Contribution of Tropical Forages and Soil Toward Meeting Mineral Requirements of Grazing Ruminants. Pages 165–166. Acad. Press, New York, NY.
Masters, D. G., and C. L. White. 1996. Detection and Treatment of Mineral Nutrition Problems in Grazing Sheep. Australian Cent. Int. Agric. Res., Canberra, Australia.
). The soil–forage mineral relationship is further nuanced by some mineral elements being more closely correlated than others. For example, soil Cu is a poor predictor of forage Cu (
). Still, most research efforts to date have only used grass species and the direct relationship with soil mineral elements; thus, future research using multivariate approaches that combine fine-scale soil types and conditions across multiple plant species may yield more conclusive results to help predict both forage and animal mineral status.
Mineral content of rangeland vegetation varies greatly. Species, climate, phenological stage, plant part, palatability, and accessibility all influence the amount of plant-derived mineral ultimately consumed by a grazing animal. Forbs (also referred to as flowering plants or weeds) are generally more palatable and have a greater mineral composition than grasses (
). Analysis of plant material samples of grass and shrub species from Wyoming collected during winter months across 25 sheep operations also illustrate this variation (Figure 3). Shrubs tended to have greater Ca and Cu concentrations than grasses (Figure 4a and 4b). Some shrubs had greater Na than other shrubs and grasses [particularly 2 halophytic shrubs colloquially known as “salt shrubs”; Gardner’s saltbush (ATGA, Atriplex gardneri) and shadscale saltbush (ATCO, Atriplex confertifolia); Figure 4c]. Selenium and Zn were present at very low concentrations in both shrubs and grasses with notable concentrations of Se in Gardner’s saltbush, which is not surprising as Atriplex shrubs are known Se accumulators (
). Regardless of the greater mineral concentrations in shrubs compared with grasses, some shrubs, such as sagebrush, are much less palatable, and thus may be limiting in the diet (
Genetic and phenotypic parameters for dietary selection of mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana [Rydb] Beetle) in Rambouillet sheep..
). Nevertheless, an appreciation of mineral differences in rangeland forages can provide a qualitative assessment for nutritionists to prioritize supplementation based on the forage species available in a particular pasture or area. For example, sheep grazing reclaimed or improved grass-dominated pastures would likely ingest less minerals than winter pastures with a greater shrub component (Figure 4). Thus, in the absence of available nutritional analysis, a working knowledge of plant diversity can enable producers and nutritionists to prioritize the logistical and economic inputs associated with mineral-supplementation strategies.
Figure 3Annual mineral variation (±SE) for native perennial C3 grass western wheatgrass (Pascopyrum smithii; ●) and the exotic perennial C3 grass crested wheatgrass (Agropyron cristatum; ○) based on monthly samples collected near Laramie, Wyoming.
measured mineral concentrations of grasses in an arid climate in the US Pacific Northwest and observed significant changes in mineral concentrations from April to December (e.g., spring → summer → winter). With the exception of Na and Fe, mineral concentrations generally declined significantly as plants went through phenological stages of maturity (Table 1). Likewise,
reported mineral concentrations continue to decline from late-summer (senescence) to late-winter (dormancy; Table 1). Furthermore, bimodal patterns of mineral composition have also been observed when moisture is not limited especially in semi-arid environments (
). Taken together, the studies demonstrate the need to consider seasonal and climatic impacts on forage trace-mineral content within and across plant species.
Table 1Forage mineral concentration changes (±%) across season in the US Pacific Northwest (
). Sheep can specifically select individual plant species, as well as specific plant parts, both of which may have less cell wall components (greater in digestibility) and be greater in mineral content (
). Consequently, plant species availability and palatability across seasons influence the proportional amount of grasses, shrubs, and forbs in diets of grazing sheep (
). Pastures with greater plant-species richness and diversity will result in greater availability of plant-derived minerals compared with homogeneous monoculture pastures, especially during the fall and winter months (
). For example, in shrubland ecosystems, browse (e.g., shrubs and forbs) may comprise 50 to 90% of the diet depending on the season or climatic conditions (
Cook, W. C., and L. E. Harris. 1950. Bulletin No. 342—The nutritive content of the grazing sheep’s diet on summer and winter ranges of Utah. UAES Bulletins. Accessed Nov. 2, 2020. https://digitalcommons.usu.edu/uaes_bulletins/303.
Genetic and phenotypic parameters for dietary selection of mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana [Rydb] Beetle) in Rambouillet sheep..
). This ability to selectively consume browse is especially important during the fall and winter periods when digestibility and mineral content of other forage species is decreasing and animal mineral requirements are increasing in preparation for breeding and gestation. In grazing areas with minimal or no plant diversity, mineral supplementation should be considered because lack of plant species diversity may be the primary limiting factor. The general decline in mineral content of rangeland vegetation in the fall and winter months (discussed above) should be considered as well. This is especially important as these periods of lower mineral nutrition coincide with the increased mineral requirements of breeding and gestation. Efforts to quantify site- and animal-specific temporal changes in mineral concentrations and requirements, respectively, complement the development of effective mineral-supplementation regimens that prevent nutrient shortfalls.
Mineral Interactions
The spatial and temporal heterogeneity of grazing sheep diets (including both forage and water) creates many complex mineral interactions and potential bioavailability concerns. Prediction of mineral interactions is most effective when producers and nutritionists have a working knowledge of spatial and temporal dynamics of soil, plant, and even water quality in a grazing system. Micronutrient absorption and bioavailability are influenced by organic and inorganic dietary interactions, principal among these being the antagonistic effects of Mo, S, and Fe with Cu.
Greater attention has been focused on Cu toxicity in sheep due to low dietary Cu tolerance (15 mg of Cu/kg of DM;
), and this concern is warranted due to contamination of sheep diets with high-Cu feedstuffs and fertilizers or sheep consuming mineral formulated for cattle (
). Still, the potential for clinical and subclinical Cu deficiency should not be rejected outright as provision of a Cu-containing mineral supplement for sheep is rare and sheep Cu deficiencies have been reported in extensively managed sheep populations (
). Broadly, circumstances of Cu deficiency in sheep in extensive grazing systems may be attributable to: 1) high dietary Mo, 2) low dietary Cu:Mo ratios of <2:1, 3) low Cu forage concentrations (<5 mg of Cu/kg of DM), 4) dietary S-Fe-Mo antagonism, and 5) Cu concentration differences across forage species and phenological stages and uneven Cu distribution in plant tissues (
). Furthermore, adequate dietary Cu from young vegetation early in the growing season tends to decline to inadequate levels by the end of the growing season. Ewes grazing dormant, low-quality forages would require diets with more than 5 mg of Cu/kg of DM to meet the increased Cu requirements during gestation and early lactation, concentrations which are unlikely to be met (
measured water quality of various sources (e.g., flowing surface water, groundwater, reservoirs, and springs) over a 5-yr period and observed that 66, 42, 37, and 36% of livestock water sources exceeded recommended quality standards for Fe, Na, sulfates, and pH, respectively. Similar reports from
across 20 Montana sheep operations reported that 40, 35, 20, and 10% of livestock water source samples exceeded recommended quality standards for Na, sulfates, pH, and Fe, respectively. Sulfate concentrations in water are often associated with deep ground water wells (
The additive effect of elevated S from water sources and the presence of elevated dietary Mo may limit dietary availability of Cu for sheep. Dietary S, via high water sulfate or S consumption from soil ingestion, has the potential to reduce Cu availability (
). Gestating cows consuming high sulfate water (500 mg of S/L) had 54% lower hepatic Cu concentrations than cows consuming de-sulfated water (42 mg of S/L) over the course of 1 yr when cows consumed 10 mg of Cu/kg of DM (
of 366 mg/L (e.g., 120 mg of S/L) combined with forage dietary S of ≈1.67 to 2.45 g/kg would total ≈1.79 to 2.57 g/kg daily S intake, which is within the bounds of lower to mid-range antagonistic thresholds for Cu (1 to 4 g of S/kg;
). Consequently, thresholds where Cu antagonism might occur from high sulfate water might approximate the 4,650 to 6,990 mg of S/L range.
Sulfur antagonism of Se in sheep exists but is unlikely to exert clinical signs of Se deficiency under most field conditions, especially when Se status of the animal and diet are adequate (0.3 mg of Se/kg of DM;
). However, the combined effects of Mo and S on reducing absorptivity of Cu is of more practical concern. Low dietary Cu and elevated S and Mo results in formation of thiomolybdates in the rumen digesta, which bind Cu and reduce its absorption (
suggest that dietary S concentrations of 1 g/kg combined with 0.5 to 4.5 mg of Mo/kg of DM did not reduce Cu availability, but >4 g/kg dietary S with 4.5 mg of Mo/kg of DM greatly reduced Cu availability in grazing ewes. More recent estimates from
Knowles, S. O., N. D. Grace, J. R. Rounce, A. Litherland, D. M. West, and J. Lee. 2002. Dietary Mo as an antagonist to Cu absorption. Pages 717–721 in Trace Elements in Man and Animals. Springer, New York, NY.
suggest Mo concentrations as low as 1 mg of Mo/kg of DM can reduce Cu absorption in sheep. Increased probability of elevated forage Mo has been tied to soil parent material, environmental pollution, or mining wastes high in Mo. Accordingly, soil geochemical maps are helpful in identifying regions with greater concentrations of antagonistic minerals. Still, potential for interactions of antagonistic minerals with Cu on extensive landscapes will vary by forage species and, notably, phenological stage. For example,
reports that although the absorptivity of Cu from mature forage is less compared with vegetative stages, so is the antagonistic influence of S, Mo, and S + Mo on Cu. Thus, it is likely that this potential antagonistic effect on Cu might be greatest in the early periods of the grazing period versus when sheep are grazing dormant pastures later in the season.
Iron concentration in water and its contribution to total dietary Fe is minor as concentrations have been documented to range from 0 to 1,192 mg of Fe/L in livestock watering sources (
). Dietary Fe thresholds, where hepatic Cu concentrations may be inhibited, range from 300 to 1,200 mg of Fe/kg of DM. Reports of reductions in hepatic Cu concentrations have ranged from 22 to 40% over 84-d periods (
Effect of Co, Cu, Fe, Mn, Mo, Se, and Zn supplementation on the elemental content of soft tissues and bone in sheep grazing ryegrass/white clover pasture..
). In major sheep-producing regions of the US Intermountain West, surveys of plant Fe concentrations ranged from 250 to 1,000 mg of Fe/kg of DM. The greatest concentrations were in the winter and fall, indicating potential antagonism with dietary Cu during this period (
). Still, the solubility of Fe from forages, quantity of ingested soil, and their related antagonism with Cu in extensive grazing scenarios warrants additional research (
Spears, J. W. 1994. Minerals in forages. Pages 281–317 in Forage Quality, Evaluation, and Utilization. G. C. Fahey, ed. Am. Soc. Agron., Crop Sci. Soc. Am., Soil Sci. Soc. Am., Madison, WI. http://doi.wiley.com/10.2134/1994.foragequality.c7.
Mineral-Supplementation Challenges in Extensive Environments
Two important goals of any free-choice mineral-supplementation program should include 1) providing compensatory amounts and types of minerals to effectively offset minerals lacking in the plant community available (and preferred) for grazing and 2) achieving targeted consumption of supplement in all ewes. Although the literature is populated with information about how to formulate supplements to meet nutritional requirements, very few studies were focused on minimizing animal-to-animal variation in voluntary intake and achieving a consistent targeted daily intake of supplemental minerals.
Precise information about voluntary mineral intake by sheep are lacking in the literature, especially in US West sheep-production systems.
provided the greatest insight as they compared mineral intake of ewes in both confined and grazing environments over a 2-yr period. They observed that 3 to 10% of confined ewes failed to consume mineral, whereas all grazing ewes consumed mineral. In fact, ewes in the grazing treatment consumed over 1.5-fold more mineral than ewes in confinement, while variability in individual intake among confined ewes was much greater than grazing ewes (CV = 61 vs. 40%, respectively). Importantly, overconsumption of targeted intake of free-choice mineral blocks containing 4% sodium chloride posed more of a challenge than underconsumption, which was a reported concern in
The effects of offering mineral blocks to ewes pre-mating and in late pregnancy on block intake, pregnant ewe performance and immunoglobulin status of the progeny..
. Regardless of environment, approximately 83% of ewes exceeded manufacturer’s recommended intake of mineral block at 7 to 14 g/d, with 60% of ewes consuming 29 to 84 g/d and 19% consuming ≥85 g/d. Overconsumption of mineral and added costs associated with mineral supplementation may contribute to producers’ reluctance to implement a mineral-supplementation program. This reluctance is reflected in a recent field survey conducted across the largest US sheep-producing region, the Upper Mountain West, where it was reported that 33 to 50% of producers did not consistently supplement grazing sheep with a fortified mineral (
monitored mineral-feed block (17 to 24% CP) consumption in gestating ewes across 15 flocks on 9 farms (4,284 ewes) in an upland pasture grazing environment and observed significant variation of intake in the mineral block within and across flocks. Ewes not consuming feed blocks ranged from 0 to 67%, and averaged 19% across flocks. Although percentage sodium chloride composition of the feed blocks and desired target intake was not specified, as grazing area per ewe increased from 0.5 to 1.0 to 1.5 ha, the proportion of ewes consuming mineral decreased from 85 to 74 to 63%, respectively. Considering this evidence from
). Evident in both studies is that stocking density should be considered when developing a precision trace-mineral supplement regimen. Readers are encouraged to read the reviews of
summarized specific factors affecting variability of intake in grazing sheep consuming mineral blocks (range = 70 to 440 g/d) and the extent of the mean intake CV across different mineral block formulations (CV = 60 to 96%). Similarly,
highlighted the need to account for variation in supplement and intake when evaluating the efficacy of a supplementation program in addition to the effects of supplement type and behavioral factors that influence consumption of free-choice supplements in grazing sheep.
Protein and energy supplements are often fortified with mineral to help ensure uniform consumption.
observed greater uniformity of feed-block consumption when ewes were housed versus grazing but also greater variation of mean intake with molasses-based blocks compared with a mineral-fortified grain supplement (56 vs. 39%). Consumption of loose salt has resulted in less variation of mean intake when compared with a salt block (58 vs. 115%;
reported lower variation of mean intake with limit-fed fortified pellets (32%) than cooked molasses-based blocks (99.5%) provided to mature ewes grazing winter range. In the US West, the commercial feed industry has the ability to fortify pelleted supplements with a complete mineral package at an added cost of $10 to $25/t (2020) depending on the location and mineral sources used, which enables producers to integrate mineral supplementation with their protein and energy supplementation program. This strategy compared with utilization of a free-choice loose mineral supplement that can range from $1,100 to $1,800/t (2020) may allow producers to optimize the input costs related to supplementation, while also achieving more uniform target intake.
In addition to more uniform consumption with a fortified pellet,
also hypothesized that the gregarious behavior of sheep combined with the limited availability of physical space with blocks may concomitantly result in the greater variation of supplement intake. Similar hypotheses were mentioned by
where the proportion of sheep not consuming mineral decreased (31, 19, 3.8, 0.5, and 0.0%) as trough space increased (4, 8, 12, 16, and 24 cm, respectively). In grazing environments, fortified supplements are often dispensed into troughs or directly onto the ground across large management cohorts (>1,000 head of sheep), which may result in excessive competition for supplement. Ensuring adequate feeding space of ≥16 cm/animal has been proposed (
observed that Merino wethers were less competitive for trough space when compared with Corriedale, Dorset Horn, and Border Leicester crossbred wethers. Breeds exhibiting gregarious flocking behavior versus individualistic tendencies (e.g., breeds of Merino origin;
Lynch, J. J., G. N. Hinch, and D. B. Adams. 1992. The behaviour of sheep: Biological principles and implications for production. CSIRO Publ., Melbourne, Australia.
) may require greater trough space or number of feeding apparatuses (tubs and blocks), especially when managed as a large flock in extensive grazing environments with mineral supplement in a fixed location. Typical features of arid grazing environments include the use of gregarious fine-wool sheep breeds that are allotted to a large grazing area and may only have access to stationary free-choice mineral supplements at overnight bedding grounds; however, this may be limited to herded flocks. In nonherded dispersed grazing locations, provision of a mineral supplement in close proximity to water sources or highly congregated areas may also be a strategy to minimize supplement intake variation.
Neophobia has been observed in sheep and results in nervous, restricted feeding behavior due to a novel feed or feeding apparatus where reported nonconsumption of supplement can range from 6 to 50% of a contemporary group (
indicated the importance of exposing lambs to molasses-based blocks before weaning, which resulted in increased block intake and a reduction in nonconsumers over time. Early exposure to novel feeds in lambs 60 d of age has been shown to increase subsequent consumption of novel feed ingredients by 20 d after introduction (
). Extensive and semi-extensive sheep operations often will wean lambs from ewes in a pastoral environment and move to novel dry-lot environments, which can result in ≈20% of the lambs being reluctant to consume feed and water (
). Thus, strategies to familiarize lambs to a trace-mineral supplement and novel feeding apparatuses weeks before moving to the weaning environment will help limit the amount of nonconsumers and minimize reluctant feeding behavior (
Meeting the mineral requirements for specific stages of production is often not given adequate attention in both research and producer education efforts, especially in extensive management systems. Although, optimal mineral management is constrained by many factors outside of the manager’s control, prioritizing efforts for time points when physiological demands are greatest (e.g., breeding, gestation, lactation) will have a greater return on investment than an arbitrary year-round approach. As mentioned earlier, periods of breeding, early gestation, and late gestation coincide with reliance on senesced plant communities of the lowest nutritional value for many extensively managed sheep operations. According to
, an 80-kg ewe expected to gestate 2 lambs would experience an increased requirement for minerals from breeding to late gestation (e.g., Ca 74%, P 89%, Na 20%, Cl 88%, K 39%, Mg 40%, S 52%, Co 42%, Cu 128%, I 42%, Fe 200%, Mn 108%, Se 25%, Zn 41%). Attention to these increased requirements is important because dietary inadequacy of Cu, I, Fe, Mn, Se, and Zn can reduce embryonic and fetal survival as well as all aspects of reproductive efficiency (
). Retention and sequestration of specific trace minerals and the supplemental chemical sources used are important considerations for precision trace-mineral management in extensively managed sheep flocks. Placental transfer of trace minerals (Cu, I, Fe, Mn, Se, and Zn), although varied in transport mechanisms and tissue accumulation, does occur throughout gestation, and thus, maternal supplementation is a sound strategy of ensuring optimal nutritional status for the neonatal lamb in utero. Dietary trace-mineral management of the gestating ewe for optimal neonatal viability was reviewed by
The potential for improving physiological, behavioural and immunological responses in the neonatal lamb by trace element and vitamin supplementation of the ewe..
, and the authors recommend this as a supplementary resource on this topic.
Increased concentrations of minerals in colostrum and milk contribute to the increased requirements from late gestation to early lactation. With the exception of Ca, Mn, and Fe, which have the greatest requirement during gestation, all other minerals have a greater relative requirement in early lactation (
). Prenatal supplementation and the subsequent trace-mineral fortification of colostrum and milk are effective strategies to provide increased concentrations to the neonate, but the effects on lamb performance are nuanced depending on the mineral concentration and chemical form supplemented (
Effects of zinc source and dietary concentration on serum zinc concentrations, growth performance, wool and reproductive characteristics in developing rams..
Differential responses in the literature regarding neonatal lamb performance for Se are likely due to the chemical form and relative bioavailability of the mineral (
). For example, flocks grazing Se-deficient regions may benefit from Se containing by-products and Se yeast supplements (e.g., selenomethionine and selenocysteine) that show longer duration of retention in tissues (
concentrations of Se yeast weaned heavier lambs, with even more pronounced effects in ewes rearing multiples, compared with lambs from ewes provided NASEM-recommended Se concentrations (
). Interestingly, in the same study, this positive effect was not observed in ewes fed the same concentrations of Se using sodium selenate.
Zinc transfer is more tightly regulated and nuanced in its accumulation in the fetal lamb. The accretion of Zn is minimal during the first 80 d of gestation but increases steadily in the fetal liver and bone until 144 d of gestation (
). Continual supply of dietary Zn to the gestating ewe and the fetal lamb is critical as short-term Zn inadequacy can result in relatively rapid depletion of physiological reserves (
), especially in situations of ewes grazing dormant forages with low concentrations (13 to 20 mg of Zn/kg of DM). Strategies to load fetal tissues and improve the neonatal Zn status of the lamb differ from that observed with Se. For example, feeding 1, 4, and 7 times
recommended concentrations to gestating ewes did not improve neonatal lamb Zn status or growth performance and survival when a zinc sulfate source was used (
Effects of increasing dietary zinc sulfate fed to primiparous ewes: I. Effects on serum metabolites, mineral transfer efficiency, and animal performance..
Effects of zinc source and dietary concentration on serum zinc concentrations, growth performance, wool and reproductive characteristics in developing rams..
). Considering the important physiological role of Zn, additional research regarding optimal concentrations for the preruminant lamb are warranted.
Proper fetal nervous system and skeletal matrix development requires that ewes consume more than 5 to 6 mg of Cu/kg of DM during pregnancy. Prevention of enzootic ataxia (sway back) in Cu-deficient lambs has been observed after prenatal Cu supplementation (
), but prenatal Cu supplementation of the ewe to increase milk Cu available for the neonatal lamb has not been thoroughly investigated. Copper-methionine was more effective at increasing plasma and liver Cu concentrations compared with Cu-sulfate (
), yet more research is also needed regarding effectiveness of chelated Cu as a means of increasing neonatal Cu status and increasing Cu concentrations in milk.
Cobalt supplementation has indirect benefit to neonatal lamb survival due to its essential role in vitamin B12 synthesis and essential role in rumen fermentation pathways. Because inappetence is a major consequence of inadequate dietary Co/vitamin B12, improved lamb birth weight, growth performance, survival to weaning, and ewe milk vitamin B12 concentrations have been reported when ewes were fed supplemental Co (
The effect of maternal supplementation of zinc, selenium, and cobalt as slow-release ruminal bolus in late pregnancy on some blood metabolites and performance of ewes and their lambs..
). Provision of Co to ewes beyond the required 0.10 to 0.20 mg of Co/kg requirement may enhance rumen digestion of forages via microbial vitamin B12 production but fetal loading strategies may otherwise be ineffective (
The strategy of providing mineral supplements to gestating or lactating ewes beyond recommended dietary concentrations should be employed judiciously as the neonatal lamb is not always benefited. Iodine supplementation of periparturient ewes beyond recommended daily amounts of 0.1 to 0.8 mg of I/kg of DM increased plasma, colostrum, and milk I and increased plasma thyroxine (T4), but immunoglobulin absorption in the neonatal lamb was impaired (
The effect of varying levels of mineral and iodine supplementation to ewes during late pregnancy on serum immunoglobulin G concentrations in their progeny..
Effect of the level of iodine in the diet of pregnant ewes on the concentration of immunoglobulin G in the plasma of neonatal lambs following the consumption of colostrum..
elucidated this adverse effect of excess iodine in the maternal diet as evidenced by altered ileal gene expression in the neonatal lamb, ultimately impairing IgG absorption in the neonatal lamb.
Effects of chemical source of trace minerals on retention and bioavailability to the animal have been an important area of research, with the general consensus being that hydroxy trace minerals and chelated trace minerals resist antagonistic ruminal interactions and achieve greater relative bioavailability than oxides and sulfates (
Effects of methionine hydroxy copper supplementation on lactation performance, nutrient digestibility, and blood biochemical parameters in lactating cows..
). Similarly, a growing body of evidence in cattle suggests that replacing sulfate forms of Cu, Zn, and Mn with hydroxychloride forms of these minerals improved apparent total-tract NDF digestibility 1 to 5% (
observed greater NDF digestibility (5%) with lambs fed Zn hydroxychloride compared with Zn methionine. These considerations with digestibility may be even more pronounced in the low quality–heterogeneous diets from forage-based diets versus the higher quality homogeneous diets fed in more intensively managed systems.
Variation in Requirements Due to Age, Breed, and Selection
Just as the production stage and overall body mass across breeds account for differences in mineral requirements, it is expected that shifts in animal performance over time will also. Average ewe prolificacy across the United States has increased by 0.12 lambs from the 1970s to 2010s (USDA-NASS, 1970–2020; Figure 5). Ewe prolificacy increased at a near linear rate in Wyoming but appeared to peak in the 2000s in Idaho and Montana before decreasing in subsequent years. Nevertheless, combined mean ewe prolificacy for Colorado, Idaho, Montana, Utah, and Wyoming from 2010 to 2019 was 0.06 to 0.26 more lambs than observed from 1970 to 1979. Surveys of wool production cover a shorter time period (1999 to 2019), but average greasy fleece weight (GFW) appeared to be relatively constant across years (Figure 6; note the lower values for Colorado, which are attributed to the concentration of sheep feeding operations and shearing of feedlot lambs).
Figure 5Average number of lambs per ewe (e.g., prolificacy) at or near parturition across the United States (red triangles) and trends within extensive sheep-producing states [colored lines; 5 western states (CO = Colorado, ID = Idaho, MT = Montana, UT = Utah, and WY = Wyoming)] from 1970 to 2019 (USDA-NASS, 1970–2020).
Figure 6Average greasy fleece weight across the United States (red triangles) and trends within extensive sheep-producing states [colored lines; 5 western states (CO = Colorado, ID = Idaho, MT = Montana, UT = Utah, and WY = Wyoming)] from 1999 to 2019 (USDA-NASS, 1970–2020).
National surveys are limited in phenotypic information but can be augmented with more detailed data collected from research flocks throughout the country. In central Texas, ram off-test BW increased from 96.1 to 117.5 kg and clean fleece weight increased from 3.59 to 4.89 kg from 1942 to 2018, respectively (
reported off-test BW increased from 88.3 to 106.5 kg in rams with an accompanying 25% increase in clean fleece weight.
Summary statistics for ewe performance across age, breed, and time were estimated from recent (2010–2020) and historical (1980–1990) records collected at the USDA ARS US Sheep Experiment Station (USSES; Dubois, ID) and US Meat Animal Research Center (USMARC; Clay Center, NE). Ewe traits were analyzed for Suffolk, Polypay, and Rambouillet at USSES and Katahdin at USMARC (Table 2). Katahdin and Suffolk ewes differed greatly in BW and litter birth weight (LBW) but had similar levels of prolificacy. Additionally, the Polypay and Rambouillet were of similar BW but differed in prolificacy and GFW. Furthermore, selection in USSES Polypay and Rambouillet has favored maternal productivity (e.g., weight of lamb weaned) and correlated additive genetic and environmental effects have led to increased mature ewe prolificacy, BW, and LBW and decreased GFW since the 1980s. Taken together, US data from national, regional, and flock-specific sources clearly indicate that sheep have changed considerably in terms of BW and overall production output. It goes without saying that micronutrient requirements change concurrently to meet greater metabolic demands of increased performance.
Table 2Summary statistics (±SE) for ewe BW near mating, greasy fleece weight (GFW), frequencies of litter size classes, and total litter birth weight (LBW) of each litter size class within ewe age for breeds reared at USDA ARS sheep research facilities from 2010 to 2020 and from 1980 to 1990
account for stage of production and the relative demands of each. Zinc requirements in the last one-third of gestation are greatest due to fetal growth, followed by BW gain, maintenance, and fiber production. To emphasize the effects of breed and selection on mineral supplementation, a 3-yr-old ewe’s daily Zn requirements during the last one-third of gestation were estimated from average values in Table 2. Clean fleece weight for wool-producing breeds was estimated from GFW and assumed yields (Suffolk and Polypay = 55%, Rambouillet = 50%). Assumed daily BW gains for twin-bearing Katahdin, Suffolk, Polypay, and Rambouillet ewes in late gestation were 60, 80, 70, and 70 g/d, respectively. Daily estimated Zn requirements reflecting these breed and longitudinal differences in BW, LBW, and clean fleece weight (except for Katahdin) are displayed in Figure 7. Additionally, late-gestation Zn requirements across age for Rambouillet ewes were estimated assuming constant BW gain (70 g/d) and yield (50%) and are displayed in Figure 8.
daily Zn requirements of 3-yr-old Katahdin (K), Suffolk (S), Polypay (P), and Rambouillet (R) ewes for maintenance, wool growth, growth rate, and pregnancy. Values were estimated from ewe average performance for BW, litter birth weight, and greasy fleece weight reported in Table 2 as well as assumed wool yield (S and P = 55%, R = 50%) and daily BW gain (K = 60 g/d, S = 80 g/d, P = 70 g/d, and R = 70 g/d).
daily Zn requirements of 1- to 4-yr-old Rambouillet ewes for maintenance, wool growth, growth rate, and pregnancy. Values were estimated from ewe average performance for BW, litter birth weight, and greasy fleece weight reported in Table 2 as well as assumed wool yield (50%) and daily BW gain (70 g/d).
Historical increases in prolificacy and LBW in the Polypay and Rambouillet breeds have contributed to ≈4 mg of Zn/d increase in Zn requirements from the 1980s to 2010s. The stark contrast in the requirements of the larger terminal-sire Suffolk breed (48 mg of Zn/d) compared with the Katahdin hair breed (34 mg of Zn/d) also highlights the dramatic breed differences. Recent research has highlighted the international and breed-specific nuance in regard to mineral requirements, especially in hair sheep breeds (
Effects of increasing dietary zinc sulfate fed to primiparous ewes: I. Effects on serum metabolites, mineral transfer efficiency, and animal performance..
). Rambouillet ewe daily Zn requirement was also affected by performance increases due to age (Figure 8). If the proportion of 1-, 2-, 3-, and 4-yr-old ewes within a flock is assumed to be 35, 30, 20, and 15%, respectively, the average daily Zn requirement of the flock would be approximately 40 mg/d. However, whereas 2-yr-old ewes would have their requirements (40 mg of Zn/d) met in this scenario, 1-yr-old ewes (33 mg of Zn/d) would be oversupplemented and 3-yr-old (45 mg of Zn/d) and 4-yr-old ewes (48 mg of Zn/d) would be undersupplemented. These results highlight the importance of considering flock breed and age structure when implementing a mineral-supplementation program.
APPLICATIONS
When developing mineral-supplementation strategies for sheep grazing extensive rangelands, producers and nutritionists should consider plant, soil, and water mineral chemistry and if or how these may change within and across years for given pastures. Such information is useful for identifying limiting minerals and antagonistic factors that limit mineral availability. Also, producers must recognize the differences in mineral needs as affected by sheep breed, age, and production stage. Finally, producers should consider acclimation periods, space, and interanimal intake variation when providing supplement.
Future research efforts should account for the interaction of various mineral sources with low-quality forage diets to determine the mineral sources best suited to extensive production environments. Moreover, environmental interactions that could influence mineral consumption and bioavailability in grazing-based experiments should be quantified, rather than dismissed due to experimental constraints. Trace-mineral experiments that simulate extensive sheep production realities (e.g., forages high in cell wall dietary components, presence of antagonistic minerals, grazing behavior, variable intake of free-choice mineral, climatic extremes, periods of nutritional restriction, and diet composition complexity) will refine trace-mineral recommendations for sheep managed in extensive landscapes. Factorial arrangements using multiple sheep breeds can help refine requirements to optimize production for specific endpoints and products (lamb, wool, milk). Experimental design constraints related to administering and quantifying mineral supplement intake in grazing-based studies have been limited in part due to the spatial resources necessary to replicate grazed pastures. Emerging technologies (e.g., Super SmartFeeder; C-Lock Inc., Rapid City, SD) that provide the ability to dispense and quantify determined amounts of supplement in grazing environments will help close knowledge gaps related to managing intake variability and the production responses due to supplementation.
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