Effect of Pre and Postpartum Herbage Allowances of Grasslands on Reproductive Parameters of Primiparous Beef Cows

The aim of this study was to evaluate the effect of herbage allowance of grasslands during the pre (PRE) and postpartum (POST) on beef cow productive and reproductive performance. Fifty pregnant Hereford heifers (5.6±0.4 of BCS) were used in a randomized block design with a factorial arrangement of PRE (fall; high vs. low, H vs. L) and POST (spring-summer, high vs. low, H vs. L) herbage allowance (4 vs. 2.5 kgDM/kgBW of annual mean for high vs. low), determining four treatments (HH, HL, LH, LL). Progesterone (P4) was measured weekly from 80 to 160 days postpartum (DPP; mating period) to confirm commencement of luteal phase (CLA; plasma P4≥1 ng/mL) and early (first month of mating period) and final pregnancy were recorded. Calf weight was recorded at birth and weaning. Data were analyzed as repeated measures with a mixed model that included PRE and POST within PRE as fixed effects and block as a random effect. Means were considered to differ when P<0.05. During the prepartum, all cows lost BW and BCS, but calving BCS was not affected by PRE and averaged 3.9±0.1 units. However, at the initiation of mating period, BCS was greater for LH cows than for the other three groups (4.1, 4.0, 4.5 and 4.0 ± 0.1 for HH, HL, LH, and LL, respectively). The CLA was not different between treatments and averaged 133±5 days but it was affected by BCS at mating as it decreased 25 days for every unit of BCS. However, plasma P4 during mating was greater for LH than HL cows, being intermediate for HH and LL cows (4.6, 2.4, 5.6 and 3.1±1.0 ng/mL for HH, HL, LH, and LL, respectively). Early pregnancy was not affected by treatment and averaged 56% but it also increased with BCS at mating. Final pregnancy was greater (P=0.051) for LH than HL and intermediate for the other two groups (88.68, 80.0, 100.0 and 90.1% for HH, HL, LH, and LL, respectively). Calf birth weight did not differ due to PRE but weaning weight tended to be greater (P=0.08) in HH and LH than HL and LL cows as calf average daily gain was greater (P<0.01) in the former than latter cow groups (1.05, 0.88, 1.08 and 0.95±0.05 kg/d for HH, HL, LH, and LL, respectively). High POST improved calf performance independently of PRE herbage allowance, however, it improved cow reproductive performance only when cows grazed low PRE.