Effect of feeding yeast culture on reproduction and lameness in dairy cows under heat stress.

Multiparous Holstein cows (n=717) from two dairy farms were blocked at calving by parity and previous lactation milk yield and, within each block, randomly assigned to one of two treatments: a diet containing no yeast culture (Control; n=359) or 30 g/d of a culture of Saccharomyces cerevisiae (YC; n=358) from 20 to 140 d postpartum. Only cows calving during months of heat stress, May-August were enrolled. Lameness score (1-5 scale) was evaluated at study enrollment and again at 100 d postpartum. The body condition score (BCS, 1-5 scale) was evaluated at calving, 28, 58 and 140 d postpartum. Cows received two injections of PGF(2alpha) at 37 and 51 d postpartum, and those observed in estrus were inseminated. Cows not in estrus were enrolled in a timed AI protocol at 65 d postpartum and inseminated at 75 d postpartum. Ovaries were examined by ultrasonography at 37 and 51 d postpartum to determine whether estrous cycling had been initiated by the presence of a corpus lutem (CL) in at least one of the two examinations. Pregnancy was diagnosed at 31, 38 and 66 d after the first AI and at 38 and 66 d after the second and third AI. Diet did not affect time of onset of estrous cycles postpartum, and 8.2% of the cows were anovular. Detection of estrus in the 7d after the second injection of PGF(2alpha) was similar for control and YC. For control and YC, conception rates 38 d after AI at first (30.8% and 31.4%), second (39.3% and 35.1%) and third (25.8% and 30.6%) inseminations, and pregnancy losses did not differ, which resulted in similar median days to pregnancy and proportion of pregnant cows at 140 d postpartum. Yeast culture did not affect incidence of lameness, but tended to reduce lameness score. Lame cows and anovular cows had lesser conception rates at first AI, and extended interval from calving to conception. A THI of 71 was identified as the critical point in which fertility was reduced in lactating dairy cows, although the sensitivity and specificity were minimal. Cows exposed to a THI>71 on the day of first AI had a 33% reduction in the rate of pregnancy resulting in extended interval to pregnancy. Feeding a yeast culture of S. cerevisiae had minor effects on lameness score, but no impact on reproduction of multiparous cows under heat stress.