Body weight changes in lactating purebred and crossbred dairy cattle

Body weights of 396 first and 205 second lactation cows in the first, second, third, and fourth generations of the dairy cattle crossbreeding project at the Illinois Agricultural Experiment Station were studied. Body weights were taken regularly on each animal during the first 9 mo of each lactation with the first weight being taken 3 to 4 days after parturition and subsequent weights at intervals of 30 days thereafter. Among least-squares means there was a decrease in body weight during the first 1 or 2 mo of lactation and a steady increase thereafter in all breed groups for both first and second lactations. There were differences between cows as well as between months for all breed groups in each generation for both lactations. Sums of squares for months were partitioned into linear, quadratic, and cubic components. A third degree polynomial was fitted for each breed group within each generation and separately for each lactation showing the changes in body weights.