17 THE EFFECT OF EXOGENOUS PROGESTERONE ON CONCEPTION RATE OF COOLED COWS DURING THE SUMMER AND AUTUMN

Abstract

Dairy cows exhibit low concentrations of plasma progesterone during the summer. The effect of exogenous progesterone supplementation postinsemination (AI) in the summer on conception is equivocal. Moreover, the variability among studies in thermal stress severity contributes to the variable effect of exogenous progesterone on summer fertility. This study examined the effect of supplementing exogenous progesterone post-AI during the summer and autumn on conception rate of lactating cows that were efficiently cooled during the hot season. The study was conducted in Israel on 2 commercial farms of high-milk-producing cows, yielding, on average, 41.3 kg of milk day^–1 around AI (124 days postpartum). These farms were equipped with efficient cooling systems along the feeding line and in the holding area. Mean body temperatures during the afternoon and evening hours in summer (air temperature and relative humidity of 30 to 32°C and 41 to 57%) were 39.1 ± 0.1°C, with a peak value of 39.5°C. Cows at first, second, or third AI that were inseminated after estrus (Day 0; detected by visual observation and computerized pedometric system; Afimilk, Israel) from July to August (summer) or September to October (fall) were included in the study (n = 377 cows). Treated cows (n = 181 cows) were fitted with an intravaginal device containing progesterone (CIDR) on Day 5 ± 1 after AI for 12 days; the device was removed on Day 17. Administration of the CIDR increased progesterone concentration by 2.3 ng mL^–1 (P < 0.05). Conception rates were analyzed by a logistic regression procedure. Overall, the CIDR treatment increased conception rate by 6% compared with controls (39 v. 33%; NS). Day of CIDR insertion tended to affect conception rate (43, 39, and 34% for Days 4, 5, and 6, respectively; NS). The CIDR treatment significantly increased, compared with the control (+23%, P < 0.05), the conception rate of cows with a low body condition score at peak lactation (49.3 v. 26.6%). Similarly, the CIDR significantly increased (+22%, P < 0.05) the conception rate of cows that exhibited a uterine disorder at parturition (46.3 v. 24.6% in untreated control cows). The CIDR treatment also increased conception rate (NS) in the fall (+13%) in mature cows (+8%) and in the first AI (+12%). Similarly, cows treated with a CIDR before Day 120 postpartum, or cows yielding less than 40 kg of milk day^–1 exhibited a numerically higher (+8%; NS) conception rate. Results indicated that exogenous progesterone administered post-AI increases conception during the summer and autumn in efficiently cooled cows during the hot months. The most beneficial effect was documented in cows that had been diagnosed as having a uterine disease postpartum, and in those that exhibited a low body condition score.