VARIATION OF MILK, FAT, PROTEIN, AND SOMATIC-CELLS FOR DAIRY-CATTLE

Sample day observations of Guernseys, Holsteins, and Jerseys were evaluated to determine contributions to variation and relations among milk, fat, and protein yields; fat and protein percentages; and somatic cell score. Somatic cell scores and mean yields for sample days increased with parity, whereas fat and protein percentages decreased. Lactation curves were approximated, examined, and compared. Fat and protein yield decreased at different rates, and persistency of yields decreased with increasing parity. Among breeds, curves differed more for fat than protein yield. Estimates for fat percentage also differed more than for protein percentage among breeds and parities. Curves for protein percentage reached the nadir earlier in lactation than curves for fat percentage. Somatic cell scores were highest at the beginning of lactation for cows in first parity but were highest at the end of lactation for cows in later parities. Somatic cell scores increased more rapidly following the nadir for cows in later lactations. Effects of month and age of calving were estimated for all traits. © 1990, American Dairy Science Association. All rights reserved.