Glucogenic precursor source impacts reproductive performance in young range cows

The objective of this study was to determine the impact of increasing supplemental glucogenic potential (GP) of the diet on reproductive performance, milk production, and calf performance in young March-calving cows grazing native range. In a 3-yr study, 253 postpartum range cows (2-and 3-yr-old) were randomly assigned to one of four supplementation treatments: 1) 910 g/d of dried distillers grains with solubles with an estimated GP of 67 g (DDGS), 2) 908 g/d of DDGS plus 40 g/d of propionate salt (NutroCal 100, Kemin Animal Nutrition and Health) with an estimated GP of 97 g (PS), 3) 800 g/d of DDGS plus 136 g/d of porcine blood meal with an estimated 97 g of GP (BM1), and 4) 691 g/d of DDGS plus 265 g/d of porcine blood meal with an estimated 126 g of GP (BM2). Treatments were designed to provide increasing levels of GP from either increasing levels of rumen undegradable protein or a direct ruminal propionate source. Supplements were individually fed for an average of 98 d. Postpartum supplementation strategy did not influence ( P ≥ 0.66) cow body weight (BW) during the entire study. In addition, body condition score was not influenced ( P ≥ 0.31) by postpartum supplementation strategy. Calf BW at birth and pre-breeding were not influenced ( P ≥ 0.66) by their dams’ postpartum supplementation. Calf BW at weaning exhibited a treatment by cow age interaction ( P < 0.01). Similarly, milk production exhibited a treatment by cow age interaction ( P < 0.01). Milk components (protein, fat, lactose, solids non-fat) were not influenced ( P ≥ 0.21) by postpartum supplementation strategy. However, milk urea nitrogen had a tendency ( P = 0.09) to be influenced by glucogenic potential of the diet. Pregnancy rate was the greatest ( P = 0.05) in PS fed cows with no differences ( P ≥ 0.75) among the other postpartum supplements. The results of this study indicate that the source of post ruminal glucose potential (glucogenic AA vs propionate) has a larger impact on reproduction and cow-calf performance than overall glucogenic potential coming from increasing rumen undegradable protein.