Precalving effects on metabolic responses and postpartum anestrus in grazing primiparous dairy cows.

ABSTRACT

The effect of increased access to pasture feeding during the last 6 wk of gestation on metabolic responses and postpartum anestrous interval was investigated. Heifers with a body condition score (BCS) of 5.0 (BC5+FF; on a 1-to-10 scale, US = 1.5 + 0.32 x New Zealand) were offered unrestricted pasture, and those with BCS 4.0 were fed either pasture unrestricted (BC4+FF) or restricted (BC4+RES) for the last 6 wk of gestation. After calving, all groups were offered unrestricted pasture. Mean BCS at calving for BC5+FF, BC4+FF, and BC4+RES were 4.7 +/- 0.1, 4.3 +/- 0.1, and 3.5 +/- 0.1, respectively. At 35 d postpartum, LH pulse frequency was lower in BC4+RES than in BC4+FF and BC5+FF, which were similar. At 77 d after calving, 8% of BC4+RES cows had ovulated compared with 75% of BC4+FF and 69% of BC5+FF cows. Metabolic hormonal differences between BC4+FF and BC4+RES were not reflected in the differences between BC4+FF and BC5+FF for LH pulse frequency or ovulation. Unrestricted access to pasture during the final 6 wk of gestation for BC4 heifers reduced the risk of prolonged postpartum anestrus. Systemic factors, tissue sensitivity, and critical developmental set points are probably involved in the integrated control of ovulation by body condition.