Impacts of Supplemental Glucogenic Precursors and Cow Age on Postpartum Range Cow Performance

Altering nutrient partitioning after calving from milk production to positive energy balance may improve reproductive performance. A 2004 study conducted at the Corona Range and Livestock Research Center evaluated responses of 2(n = 17), 3(n = 23), and 4-yr-old (n = 31) postpartum cows grazing native range (11.3% CP and 80% NDF, OM basis) to 3 protein supplements with increasing glucogenic potential (GP). Supplements were fed at 1,135 g·cow·d twice weekly for 65 d postpartum and provided: 1) 341 g CP, 142 g ruminally undegradable protein (RUP), 57 g GP (RUP0), 2) 341 g CP, 151 g RUP + 80 g propionate salt (NutroCALTM, Kemin Industries, Inc.), 121 g GP (RUP80), or 3) 341 g CP, 159 g RUP + 160 g propionate salt, 185 g GP (RUP160). A supplement × age interaction occurred for days to first estrus (P = 0.10). Days to first estrus was longest for 2-yr-old cows fed RUP0 and then decreased with cow age (P ≤ 0.04), while for RUP80 and RUP160, return to estrus was similar for 2and 3-yr-old cows (P ≥ 0.16) and shorter for 4-yr-old cows (P ≤ 0.10). Milk production exhibited a quadratic (P = 0.03) response to increasing supplemental GP, with cows fed RUP80 producing the least amount of milk at 55 d postpartum (9,982, 8,439, and 9,620 ± 473 g/d for RUP0, RUP80, and RUP160, respectively). Milk production differences did not impact 205-d calf weight (P = 0.96; 251 ± 5 kg). Days from BW nadir to estrus decreased linearly with cow age (P < 0.01; 33, 22, and 1 ± 4 d for 2-, 3-, and 4-yr-old cows, respectively). Milk production increased linearly with cow age (P < 0.01; 7,856, 9,407, and 10,777 ± 509 g/d for 2-, 3-, and 4-yr-old cows respectively. Calf 205-d weight reflected cow age differences in milk production (linear P < 0.01; 229, 249, and 274 ± 6 kg for 2-, 3-, and 4-yr-old cows, respectively). Moderate amounts of supplemental GP shifted nutrients away from milk production. Older cows returned to estrus at the same time they reached BW nadir, while younger cows needed to regain weight to return to estrus. Glucogenic precursor addition to protein supplements decreased days to first estrus in postpartum 2year-old range cows.