Effects of Selection for Fertility on Milk Production Traits

A previous study investigated the impact of selection for fertility upon milk yield in the first lactation. The current study extends this analysis to include the yield and content of fat and protein. Daughter test-day records were used to estimate Wilmink curve parameters of each trait for 2,405 sires. The sires also had breeding values for the production traits and their fertility index. Correlations and linear regression between curve parameters and breeding values were carried out with and without correction for environmental effects. Selection for fertility was found to negatively affect milk, fat and protein yield. Improved fertility was found to result in an increased initial fat and protein content, but also increased the rate of decline during early lactation causing a reduced nadir. The persistency of protein content reduced with increased fertility; whilst, fat content rebounded to a greater extent in fertile cows than those with lower fertility. Fat-to-protein ratio reached its maximum about 5 weeks before peak milk production and was higher for less fertile cows, coinciding with time of strongest energy imbalance. Correction for environmental effects resulted in overall lower production curves for yield traits and fat content, but higher protein content. After correction, cows with higher fertility produced more milk compared to lower fertile cows purely on their genetic merit. Similar patterns were found for fat and protein yield. Fat-to-protein ratio was lower for higher fertile cows throughout the entire lactation.