Elk and Summer Nutrition
ByDavid Hewitt, B&C Professional Member
The distribution of elk in North America is primarily in mountains, northern parklands, prairies, and high deserts where winter can be severe. Deep snows, poor quality forage, and cold temperatures can be a hardship for elk, especially when elk are concentrated on the winter range. In contrast, summer conditions are much better, with warmer temperatures, green forage, and large areas where elk can roam. These seasonal differences have traditionally caused wildlife biologists to look for problems on the winter range when elk herds underperform.
A two-decade research effort that began at the Starkey Experimental Forest and Range in northeastern Oregon has turned the traditional view of the importance of winter range upside down. Using a combination of carefully controlled studies with captive elk and assessment of reproduction and body condition of free-ranging elk in the Northwest and Rocky Mountains, researchers have shown that in many habitats of the western US, inadequate summer nutrition limits growth and reproduction, both of which could have impacts at the population level.
The story begins with the high nutritional cost of raising an elk calf. Although reproduction can be influenced by many nutrients, digestible energy has a large effect on many aspects of the reproductive cycle. Energy requirements of a female elk increase 75% at peak lactation relative to maintenance. To meet these requirements, a female elk must either eat a lot of high-quality forage or use her own fat reserves. If a female has to rely too much on fat reserves to raise her calf, she will enter autumn in poor condition, and body condition of female elk during autumn has pronounced effects on elk reproduction and recruitment the following year.
To understand effects of summer nutrition on reproduction, John and Rachel Cook and their colleagues conducted a 3-year experiment in which they manipulated the summer diet of female elk. One group was fed a high-quality diet throughout the spring and summer, a second group was placed on a dietary regime in which digestible energy declined 10% from late June through early November, and third group was placed on a similar dietary regime, but digestible energy in the diet declined about 20% during summer and early autumn.
Effects of these seemingly modest changes in digestible energy during summer were dramatic. Eighty percent of cows in the low-nutrition group did not become pregnant and cows in the medium-nutrition group conceived up to 2 weeks later than those in the high-nutrition group. At 6 months of age, calves from the high-nutrition group were 40% heavier than calves born into the medium-nutrition group and were 70% heavier than calves in the low-nutrition group. Survival of calves over winter was related to autumn body mass such that calves born to mothers on high-quality forage had higher survival rates during winter than calves raised by mothers on a lower-quality diet.
Summer nutrition not only affected body size of calves raised that year and pregnancy rates of cows that autumn. Cows on poor summer diets began winter with low fat reserves and survived the winter at lower rates than females who raised calves while eating high-quality forage. Female calves raised on a high-quality diet had high pregnancy rates as yearlings, whereas few female calves raised under poor conditions their first summer conceived as yearlings.
This study with captive elk provided benchmarks for female fat reserves and calf body sizes during autumn that could be used to assess summer and early autumn nutrition of free-ranging elk. The Cooks used those benchmarks and captures of 861 female elk in 21 herds in the Pacific Northwest and Rocky Mountains to evaluate adequacy of summer nutrition to elk.
This second study identified widespread inadequate summer nutrition. Only 3 of 21 herds had summer ranges that enabled female elk to raise a calf, become pregnant that autumn, and enter winter with relatively high levels of body fat (>11%). On the remaining ranges, females that raised calves had 5-9% body fat in autumn and up to 40% of the females that raised a calf during the summer failed to conceive that autumn. Six-month old calves were weighed in 3 herds (2 in WA and 1 in CO) and in all 3 herds >70% of the calves had poor or marginal body weights entering winter, potentially influencing probability of over-winter survival.
What do these findings mean for elk management?
- Managers need to provide good forage on summer ranges to ensure elk populations remain sufficiently productive to support hunter harvest and large mammalian predators.
- Early-succession habitat produces more and better-quality forage for elk than late- succession habitat. Managers should consider actions that result in more early succession habitat in places where it is scarce. Such actions may be a less aggressive approach to controlling wildfires, more prescribed fires, and timber harvest.
- High densities of elk and other herbivores relative to carrying capacity may reduce forage quality and quantity on summer ranges. The effect of high elk densities was demonstrated by Kelly Stewart and her colleagues at the Starkey Experimental Forest and Range in northeastern Oregon. They showed that increasing elk density resulted in lower fat reserves and reduced pregnancy in female elk as a result of limited nutrition obtained from summer range. State wildlife agencies should use hunter harvest to ensure elk do not negatively affect summer forage. Federal land management agencies and private landowners should manage livestock numbers to ensure good summer forage for all animals using the range.
- These results do not reduce the importance of adequate winter range for elk populations. Maintaining and managing winter range should continue to be a priority for managers in the western United States. These studies clearly illustrate interactions between summer and winter range in that high-quality summer range and increased body condition will carry over and influence elk survival on poor and moderate winter range.
To learn more about these studies, see the original publications:
Cook, J. G. et al. 2004. Effects of summer-autumn nutrition and parturition date on reproduction and survival of elk. Wildlife Monographs 155:1-61.
Cook, R. C. et al. 2013. Regional and seasonal patterns of nutritional condition and reproduction in elk. Wildlife Monographs 184:1-44.
Stewart, K.M. et al. 2005. Density-dependent effects on physical condition and reproduction in North American elk: an experimental test. Oecologia 143:85-93