PS5-17. Effects of Protein Supplementation on Weight, Intake, and SUN in Rangeland Ewes.

Abstract Whether protein supplementation is necessary to meet the nutritional requirements of ewes grazing diverse rangelands remains uncertain. This study evaluated how protein supplementation level affects body weight, supplement intake, and serum urea nitrogen (SUN) in grazing ewes differing in age and breed. From June to October 2024, ewes (N = 201) grazed four pastures in the Cedar Mountains near Cedar City, Utah. Ewes were stratified by initial weight and breed; black face [BLF; Suffolk, n = 43) or white face [WF; Targhee, Rambouillet, and crosses, n = 158)] into one of three different protein levels: no supplement (N; n = 26 BLF, 108 WF); low protein (L, 10.5% CP DM; n = 9 BLF, 21 WF) or high protein (H, 23.2% CP DM; n = 8 BLF, 29 WF). Supplement was offered at 113.4 g/day via a SmartFeed™ trailer, which also recorded individual intake. Body weight, BCS, and blood samples were collected on d 0, 54, and 83. Forage samples were collected before and after each pasture rotation and analyzed for CP levels. Ewes were also categorized by age: adolescence (n = 117, age 1-2), adult (n = 61, age 3-5), or old (n = 23, age 6-9) to determine whether age impacted productivity. Data were analyzed using the MIXED procedure in SAS to test the main effects of treatment, age, breed, and day, and their interactions, on body weight, supplement intake, and SUN levels. Day 0 was used to compare weight and SUN for days 54 and 83. Crude protein (CP) levels in the forage were also analyzed using pasture, functional group (forb, grass, shrub, tree), and day as fixed effects. A treatment×day interaction (P = 0.014) was seen, showing the greatest change in body weight in treatment N from day 54 to 83 (P = 0.002). Also, a breed×treatment×day interaction (P = 0.0496) demonstrated that intake from treatment L was highest in both breeds, and increased over time, while treatment H only increased intake at day 83, with BLF ewes consuming the most. Breed (P < 0.0001) and age (P < 0.0001) influenced weight, with BLF ewes averaging higher, as well as adolescent ewes exhibiting lower body weights. Age influenced supplement intake (P = 0.0005), with adult ewes consuming the highest amounts overall. SUN was influenced by treatment (P = 0.005), with treatment N being the lowest overall. Also, day 83 had the lowest SUN levels out of the study (P < 0.0001), and WF ewes exhibited lower SUN levels (P = 0.043) than BLF. The CP was highest (P = 0.0007) in the first pasture of all available forages compared to the other three pastures. These data suggest that throughout the grazing season, supplementation type may not affect body weight but may impact how much supplementation ewes consume as well as SUN levels. Differences in intake may reflect changing forage CP available across the grazing season.