Effects of Stage of Lactation and Pregnancy and Their Interactions on Milk Yield and Constituents

¹ Dairy Science Department, Institute of Food and Agricultural Sciences, University of Florida, Gainesville 32611

Effects of stage of lactation and pregnancy and their interactions on 17 milk yield and constituent traits of Holsteins and Jerseys were studied. Data were 11,120 Jersey and 11,092 Holstein daily milk samples. After adjustment for all other known and systematic genetic and environmental effects, stage of lactation influenced all traits and accounted for 11.8 to 26.3% of variation in yields and .2 to 9.9% in percentages. In general, yields declined and percentages increased after about 3 mo of lactation. Chloride percentage showed an initial decline and then a rise after 2 mo. Stage of pregnancy accounted for small but significant variation in most traits; variation was .2 to .4% and <.1 to 3.0% in yields and percentages for Holsteins, and <.1 to .2% and .1 to 1.1% for Jerseys. Interactions between stage of lactation and pregnancy were investigated by response surface methodology and found to be very small.

Key Words: lactation • pregnancy • composition

Submitted on March 10, 1989
Accepted on December 1, 1989