16 INDUCTION OF FOLLICULAR TURNOVER TO IMPROVE FERTILITY IN DAIRY COWS EXPOSED TO SUMMER HEAT STRESS

Abstract

The reduced conception rate of high-lactating cows during the hot season is a well-documented phenomenon. Previous studies have provided evidence of the ovarian follicle and the susceptibility of its enclosed oocyte to thermal stress, suggesting that heat-shock-induced disruption of follicular and oocyte function is involved in the mechanism underlying the reduced fertility. Evaporative cooling improves summer fertility to some extent. However, there is a compelling need to find additional ways to improve fertility further during the hot season. In the current study, we hypothesized that induction of consecutive follicular waves by administration of GnRH + PGF_2α would improve fertility, with the aim of removing damaged follicles and enhancing the emergence of healthier ones. The study was conducted from July to November (2006 to 2007) on healthy, cyclic cows yielding an average of 43.6 kg of milk day^–1 in 2 commercial herds exposed to an efficient cooling systems. Control cows (C; n = 187) were artificially inseminated (AI) after estrus around Days 80 to 90 postpartum. Treated cows (T; n = 195) were hormonally treated to induce consecutive follicular cycles, beginning with an injection of GnRH (1.5 mL, Gonabreed) to induce ovulation and followed by injection of PGF_2α (2.5 mL, Estromat) 7 days later to regress the corpora lutea. In this manner, 3 successive 9-day follicular waves were induced. Cows manifesting estrus (determined by pedometric monitoring, Afimilk, Kibbutz Afikim, Israel) following the third PGF_2α injection were inseminated, and pregnancy was determined 44 days post-AI. Statistical analysis (logistic regression, SAS) was performed, taking into account the following parameters: season, milk level, body condition score (BCS), somatic-cell count, and number of lactations. Overall, the GnRH-PGF_2α treatment resulted in a slight increase in conception rate (27 and 32% for C v. T, respectively; NS). Conception rate was higher in cows exhibiting estrus within 5 days after the third PGF_2α injection than in those exhibiting estrus later (41 v. 31%, respectively). The most prominent effect was associated with the number of lactations: treatment did not affect the conception of multiparous cows, but a significantly higher conception rate was noted for first-calf heifers (37 and 53% for C v. T, respectively; P < 0.05). A similar pattern was noted for cows yielding less than 40 kg of milk day^–1 (36 and 51% for C v. T, respectively; NS), but not in those yielding more than 40 kg of milk day^–1. Numerically, but not significantly, higher conception rates were noted for T cows with high BCS at parturition (+15%) or at peak lactation (+13%), and in cows with a low somatic cell count (+10%) relative to the C group. Interestingly, there was no treatment × season (summer v. fall) interaction. Taken together, the combination of hormonal treatment-induced follicular turnover and efficient cooling appears to improve summer and autumn fertility, mainly in first-calf heifers.