Changes in Late Gestation Beef Cow Circulating Metabolite and Metabolic Hormone Concentrations Due to Feeding Stockpiled Tall Fescue Versus Tall Fescue Hay

Abstract Text: The objective was to investigate effects of grazing stockpiled tall fescue (STF) versus feeding summer-baled hay during late gestation on nutrient availability for fetal development. Multiparous, spring-calving crossbred beef cows (yr 1: n = 48; yr 2: n = 56) either strip-grazed STF (n = 4 pastures; 12.3% CP, 63.9% NDF; DM basis), or received ad libitum tall fescue hay (6.9% CP, 66.6% NDF; DM basis) fed in uncovered drylots (n = 4) until calving. Jugular blood samples were obtained from cows on d 0 (baseline; d 188 of gestation), 20, 35, 56, 77, and 99 (remaining prepartum cows). Effects of forage system, sampling day, year, and all interactions were included in the model. Sampling day was considered a repeated effect. Calf date of birth was included in the model when P < 0.25; pasture or drylot was the experimental unit. Correlations were determined between calf birth weight and the last prepartum maternal metabolite or hormone concentration. The forage system x day x year interaction affected (P < 0.001) NEFA and tended to affect thyroxine (P = 0.06). In year 1, NEFA was greater (P < 0.001) on d 56 in cows grazing STF than cows consuming hay. In year 2, NEFA tended to be greater (P = 0.09) on d 35 and was greater (P < 0.001) on d 77 and 99 in cows grazing STF. Cows grazing STF tended to have greater (P = 0.06) thyroxine on d 77 in year 1, but there were no differences (P ≥ 0.18) in year 2. There was a forage system x day interaction (P < 0.01) for BUN, cortisol, and triiodothyronine. After d 0, BUN was greater (P < 0.001) in cows consuming STF on all days measured. Cortisol was greater (P = 0.003) on d 35 and tended to be greater (P = 0.10) on d 99 in cows grazing STF. Triiodothyronine was less (P = 0.03) on d 0, but greater (P = 0.004) on d 99, in cows grazing STF. Cows grazing STF tended to have greater (P = 0.08) glucose than cows consuming hay. Calf birth weight was positively correlated with prepartum maternal BUN (r = 0.31, P = 0.002) and NEFA (r = 0.12, P = 0.005), but not glucose, cortisol, triiodothyronine, or thyroxine (P≥ 0.15). Results indicate cows grazing STF had altered metabolic status, which may impact fetal development and subsequent calf performance.

Keywords: developmental programming, forage systems, pregnancy