Influence of seasonal forage quality on glucose kinetics of young beef cows

¹ USDA - ARS, Fort Keogh Livestock and Range Research Laboratory, Miles City, MT 59301

^* To whom correspondence should be addressed. E-mail: richard{at}larrl.ars.usda.gov.

	Abstract

Extensive western range livestock production systems rely heavily on rangeland forages to meet nutritional needs of grazing livestock throughout the year. Interannual variation in quantity and quality of rangeland forage in the Northern Great Plains, as well as throughout much of the western United States, may play a pivotal role in how well grazing ruminants sequester nutrients in their tissues. This variation in forage quality may influence a beef cow's ability to utilize dietary nutrients via changes in tissue responsiveness to insulin. Identifying specific periods and production states where this phenomenon manifests will provide insight into the development and implementation of strategic and targeted supplementation practices that improve nutrient utilization during times of nutritional imbalances and may improve lifetime productivity of grazing range beef cows. A 2-yr study was conducted to monitor serum metabolites, glucose kinetics during glucose tolerance tests, and forage chemical composition every 90 d on young range cows (2 to 4 yr of age; n = 28). Cows were managed on 4 pastures in yr 1 and 2, varying in size from 36 to 76 ha in yr 1 and 49 to 78 ha in yr 2. Regardless of year, cow age, or cow physiological status, the main factor influencing glucose half-life was season of the year (P = 0.02). Effects of season on glucose half-life closely followed assessments describing forage quality with glucose half-lives of 46, 39, 43, and 51 ± 3.9 min for May, August, December, and March, respectively. Elevated glucose half-life during seasons when forage quality is of lower nutritive value indicates that tissue responsiveness to the actions of insulin follow seasonal changes in forage quality. Glucose half-life tended (P = 0.09) to decrease between May and August, increased (P = 0.04) between December and March, and showed a tendency (P = 0.10) for faster glucose half-life in seasons of greater nutrient density (May and August) when compared to seasons of lower nutrient density (December and March). Seasonal changes in serum metabolites were also observed and corresponded with changes in forage quality. Results support our hypothesis that as season progresses and forage quality declines, maternal tissues become less responsive to insulin, indicating that targeted supplementation with glucogenic precursors during these seasons of nutritional stress may improve cow performance.

Key Words: beef cows, forage quality, gestation, glucose kinetics, serum metabolites