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Lactation Studies. II. Effect of Estrus¹

State College of Washington, Department of Dairy Husbandry, Pullman

ABSTRACT

Eighty estrous cycles of 19 Holstein cows were studied for variation in yield of milk, F.C.M., fat and per cent fat. All cows in estrus were kept separated from other cows in the herd. Observations were made 8 days before estrus and 11 days after. Cows milked two times daily (42 cycles) showed more variability in yield of milk but less variability in yield of F.CM., fat and per cent fat than cows milked three times daily (38 cycles). Yield generally was higher 2 days before heat and 2 to 3 days after heat than during any other period of the cycle. Decline the day of estrus occurred when these cows were in lactation over 100 days. This decline was more apparent as stage of lactation increased. Per cent fat was higher for the 5-day period centered around estrus than any other time of the cycle, regardless of stage of lactation.

In addition to the above study, milk yield during 636 estrous cycles (over 18 days in length) was analyzed for the Guernsey, Jersey and Holstein breeds. The differences in variability of milk yield between two- and three-time milkings observed on the more limited data were confirmed. No major breed differences were observed with respect to general trend in milk yield, although the Jerseys tended to vary proportionately more.

Decline in milk yield was greatest when estrus occurred during late lactation. This decline increased as time in lactation increased. Age, month of estrus and length of estrous cycles appeared to have little consistent effect on patterns of milk yield during the estrous cycle.

This study indicates that testing cows on the day of estrus actually may be advantageous to the cows. This particularly is true for cows in lactation less than 100 days.

¹ Scientific Paper no. 1056, Washington Agricultural Experiment Stations, State College of Washington, Pullman.

² Acknowledgment is made to W. T. Southwarth for assistance with the I.B.M. analysis of the data.