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State College of Washington, Department of Dairy Husbandry, Pullman
ABSTRACT
A total of 82 production records (Holstein, Guernsey and Jersey breeds) were utilized to study the effect of gestation on yield. The cows were fed and managed for near-maximum yield and did not have access to pasture during the period studied.
Daily milk weights and butterfat tests were available for 19 Holstein cows, eight of which carried calves a minimum of 223 days. In the latter group, five were milked three times daily and three were milked twice daily. There was no difference in rate of decline in lactation between the two groups (milk, F.C.M. and butterfat) with advancing pregnancy. The rate of decline in yield was three to four times faster from 181–223 days of pregnancy as compared with 101–180 days of pregnancy. Butterfat test was more variable during preconception periods and progressively less variable as length of gestation increased.
Milk records for 53 cows milked two and three times daily and carrying calf for 210 days revealed that inhibition started at approximately the same time regardless of the number of times milked daily, age of the cow, breed, month of conception, production level of the cow or calving interval up to 18 mo.
A lactation curve for 10 cows not pregnant during the first 365 days in lactation revealed a tendency for rate of decline in milk yield to slow after 330 days. This is in contrast to an accelerating rate of decline in cows pregnant more than 180 days. Lactation decline appears to follow a cyclic pattern following conception not unlike that observed for estrual cycles.
1 Scientific paper no. 1064, Washington Agricultural Experiment Stations, State College of Washington, Pullman.
2 Acknowledgment is made to W. T. Southworth for assistance with I. B. M. analysis of the data.
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