Effect of supplementation on performance and reproduction of lactating beef cows grazing lush spring pasture

Our objectives were to use 230 Angus × Simmental beef cows in a 2-yr experiment to determine the effects of supplementation on cow BW and BCS, first-service AI conception rate, and blood metabolites (nonesterified fatty acids, BHBA, and BUN). Cows (110 in yr 1 and 120 in yr 2) were randomly assigned to 1 of 2 treatments: control or supplement (SUPP). There were 3 replicates per treatment each year, and both treatments grazed clover-tall fescue pasture in early spring. The supplement contained 45% ground corncobs, 45% soybean hulls, and 10% dry molasses (DM basis), and 1.81 kg/cow per d was offered to cows fed SUPP starting 10 d before breeding. Forage samples were collected as groups were rotated to new pastures. Throughout the grazing season, forage CP decreased (P < 0.01), whereas ADF and NDF increased (P < 0.01). Day-18 BUN concentration in yr 2 tended to be decreased (P = 0.10) in cows fed SUPP compared with control cows. Concentration of BHBA for cows fed SUPP tended to be greater (P = 0.07) compared with control cows; however, nonesterified fatty acids did not differ (P = 0.80). There was no difference (P ≥ 0.44) in final BW and BCS nor was there any difference (P ≥ 0.35) in AI conception and overall pregnancy rate. In conclusion, a high-fiber, low-protein supplement did not affect BW or BCS or significantly improve first-service AI conception rate and overall pregnancy rate in cows grazing lush, early-spring pasture.