Effects of stage of lactation and pregnancy and their interactions on milk yield and constituents.

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 less than .1 to 3.0% in yields and percentages for Holsteins, and less than .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.