Effects of Feeding Melengestrol Acetate (MGA®) on Occurrence of Estrus, Fertility and Milk Yield in Dairy Cows

Beginning on the 43rd day of lactation, one of each of 32 pairs of Holstein cows was fed 0.5 mg melengestrol acetate daily for 18 days. Treated cows were bred at the first estrus following withdrawal of melengestrol acetate and control cows were bred at the first estrus following 60 days postpartum. Two-thirds of the treated cows showed estrus within 7 days after withdrawal of melengestrol acetate. Eighty-four percent of the treated cows and 72% of the control cows had been inseminated once by 81 days postpartum. First inseminations were given at an average of 73 days postpartum for treated cows and 74 days for control cows. Reproductive efficiency was poor in both groups—only nine treated and eight control cows conceived on first service. Six cows were culled from each group. Services per conception for the 26 remaining cows in each group averaged 2.85 for treated and 3.00 for control cows and average number of days open was 125 and 146 for treated and control cows, respectively. Although reproductive performance was better for the treated cows, there were no statistically significant differences between treated and control cows in any of the measures of breeding efficiency. Daily milk yield averaged 31.3 kg and 29.0 kg for each group of cows during the 18-day treatment period and the 30-day post-treatment period, respectively. Analyses of milk yield data revealed no significant influence of treatment on milk production.