Managing Reproduction in Dairy Cattle: I. Progestogens for Control of Estrus in Dairy Cows

Abstract

Two experiments studied the effectiveness of medroxyprogesterone acetate (MAP) and melengestrol acetate (MGA) for control of estrus in normal and anestrous dairy cows. A third experiment investigated the use of MGA for controlling estrus during 17 successive 3-week breeding periods. In Experiment 1, 73 anestrous cows received MAP (180 mg/day) or a placebo for 10 days. Conception rate for first service, total conception rate, and services per conception were 59, 88,’ 1.7 and 54, 74, 1.8 for MAP and control groups. In Experiment 2, 232 cows in four herds were classified normal or anestrous according to estrous detection during the first 7 weeks postpartum. Beginning on Day 48 postpartum, some cows received MGA (1 mg/day) for 10 days while others served as controls. The mean interval from calving to first service, average days open for cows conceiving, and services per conception were 76, 107, 2.4; 76, 123, 2.5; 74, 98, 1.9 and 75, 107, 2.0 for MGA-normal, MGA-anestrous, control-normal and control-anestrous groups. Conception rates for first service were between 50 and 53% for normal cows and between 35 and 39% for anestrous cows irrespective of hormone treatment.In Experiment 3, 268 cows which calved in 17 successive 3-week periods in three herds were assigned to MGA or control groups. Some MGA cows received a single 14-day treatment (MGA-lX) while others received a double sequence (MGA-2X). Eighty-six per cent of the MGA cows and 60% of the controls were in estrus within 30 days following MGA withdrawal. Ninety-two per cent of the MGA cows were synchronized in two periods of 7 and 5 days. Cows which received the double sequence of MGA were synchronized to a greater degree than MGA-1X animals. The interval from calving to first service, average days open for cows conceiving, and services per conception were 80, 102, 1.8; 72, 102, 2.1 and 72, 97 and 1.9 for control, MGA-1 X and MGA-2X groups. The MGA reduced the variation among herds in interval from calving to first service and average days open. Progestogen administration did not affect milk yield. The system for controlling estrus in groups of cows could be beneficial as part of a total system to manage for a high reproductive rate in large herds.