Lactation Studies. II. Effect of Estrus

Paper I of this series (2) reports variations in lactation that are associated with pregnancy. Even after conception, there was a pronounced cyclic fluctuation in the yield of milk and butterfat not unlike those associated with the estrous cycle as reported by Morrison et al. (7). Hooper (3, 4) reported much earlier that some cows showed considerable variation in yield at the time of heat, while others showed little or no variation, tIooper and Brown (5) studied records from 29 cows and found an average decrease of 0.1 lb. of fat and 1.5 lb. in milk production on the day of most evident heat. McCandlish (6) compared milk production 3 days preceding and following the day of breeding on 868 cases. Milk production was 4 per cent higher 2 days before breeding than on the day of breeding. Milk production the day after breeding was higher than the day of breeding but lower than the other 4 days studied. Copeland (1) made 211 comparisons from Jersey Register of Merit records based on 1-day supervisors' reports. Milk yields were compared for 2 days preceding and following supervision. An average decrease of 0.63 lb. of milk on day of heat was observed. The variation was less than 2 lb. per day in 75 per cent of the cases. Butterfat percentage increased an average of 0.1 per cent. Milk yield 2 days preceding heat was higher in 112 cases and lower in 99 cases than yields 2 days following heat.