While Wyoming is the least-populated state, it is still one of the key players in the U.S. sheep industry, according to University of Wyoming extension sheep specialist Whit Stewart and animal science doctoral student Chad Page.
In a post on the University of Wyoming agnews website, Stewart and Page said the importance of trace nutrients, specifically zinc, for sheep tends to be underestimated.
"Larger-scale operations and larger-scale ambitions for the sheep industry as a whole require more and more acutely refined precision management, even down to trace nutrients," they said.
According to Stewart and Page, zinc is an essential micro-nutrient in sheep production, and in the rugged landscapes of Wyoming where the state's sheep are produced, the physiologically challenging timepoints of breeding, pregnancy and lactation for sheep (fall/winter) occur when rangeland forages are at their lowest nutritional quality.
They also noted that while sheep are being selected for greater productivity, specific nutrient recommendations haven’t kept pace.
Zinc and modern production
Stewart and Page noted that scientists in the 1930s discovered that zinc was essential for sheep production. The element is the second-most abundant trace mineral in the body and has important functions in gene expression, immune function, reproduction, appetite regulation and wool production.
In most cases, the zinc content of grazing diets is adequate in early summer, but in later months, zinc levels decline to inadequate concentrations, the researchers said, noting that this results in suboptimal performance, especially during the most physiologically demanding times of breeding and pregnancy.
Stewart and Page pointed to a national forage survey conducted by the U.S. Department of Agriculture's National Animal Health Monitoring System in 1993 that found that 63% of native grasses failed to meet dietary zinc requirements for sheep.
Supplying additional zinc is a best practice to alleviate these seasonal shortfalls and demanding production periods; however, ensuring zinc consumption on extensive landscapes can be sporadic, and as a result, some producers opt not to provide the mineral during certain times of the year, they added.
Complicating the present-day picture is that previously recommended zinc supplementation levels may not be adequate for modern-day sheep to reach optimal performance, they said.
Over the past 50 years, fine-wool sheep in Wyoming have achieved selection milestones, including 50% greater growth rates, 30% greater mature body size and, because of that, a 25% increase in clean wool production, according to Stewart and Page. USDA data suggest that twin-bearing capacity has increased in the U.S. sheep flock since 1930.
They recommended increasing estimated zinc requirements from 34 mg/kg to 57 mg/kg, or 67% for the average ewe, because production goals and sheep have changed.
Current research is evaluating whether feed supplements contain adequate zinc levels for the best health and well-being of sheep, in addition to comparing different forms of zinc in sheep diets, Stewart and Page noted. They pointed out that recently completed research in 2017-18 found that rams grew 14% longer wool when fed double the recommended levels of zinc, resulting in more pounds of wool. Considering the record-high wool prices in 2018, this increase in production can increase margins, they said.
When these same rams were fed increasing levels of zinc, they also had greater feed conversion efficiency; they grew more with less feed when compared to rams eating the current recommended levels, benefitting the sustainability of the producers and the production environments, Stewart and Page reported.
Zinc's benefits extend beyond the ram, the researchers noted, adding that a separate study conducted at the University of Wyoming fed pregnant ewes increasing levels of zinc, which resulted in a 40% increase in lamb survival from birth to weaning.
On a related note, they said collaborative work among Montana State University, the University of Wyoming and the USDA Sheep Experiment Station in Dubois, Ida., concluded that ewes with lower zinc levels were associated with subclinical mastitis and produced 33 lb. lighter lambs than those with healthy udders.
Current research is examining whether greater zinc levels fed throughout pregnancy can reduce the incidence of bacterial infections of the udder, thus preventing lost production potential, Stewart and Page said, adding that their research has found that 20% of ewes in a flock suffer from subclinical bacterial infections of the udder.
They said there is still work to do in understanding the optimum levels of zinc in sheep diets, such as understanding breed differences that may require additional zinc levels is an important delineation. They noted that lambs from fine-wool breeds have greater zinc concentrations at birth than lambs from meat breeds.
The chemical forms of zinc — zinc sulfate versus zinc amino acid complex — have been shown to have differential effects on the rumen bacteria and subsequent performance of the animal, Stewart and Page said, noting that future research will look at what chemical forms should be utilized and at what ratios.
Another research area, they noted, is understanding the trace mineral composition of the plants sheep consume at different times of the year: What are the differences in grass, forb and shrub species, and how much zinc is actually available at different stages of plant maturity? They said efforts are underway to sample sheep ranches throughout Wyoming to identify trace mineral deficiencies and help producers assess where their forage resources might be falling short for a particular trace mineral.
The next step in supporting modern-day sheep breeds is communicating research results to feed companies that market products throughout Wyoming and surrounding regions, Stewart and Page said. By leveraging the extension arm of the land-grant university pipeline, Stewart and Page plan to continue to communicate translational research to industry partners and invite them to revisit their sheep mineral products.
Providing timely information to private industry also involves synthesizing late-breaking research into translational information that enhances decision-making, they added.