Lactation performance of purebred and crossbred dairy cows on pastures and impact on fertility

Robust knowledge of the critical determinants oflactation performance and correlations with fertility traits is essential for elucidating the underpinning relationships between genetic merit for high milk yield and reproduction. Holstein-Friesian, Jersey and Holstein-Friesian x Jersey breeds constitute the vast majority of milking cows in Tasmania's mainly pasture-based dairy farms in Australia where genetics and seasonal fluctuations in pasture quantity and quality influence lactation performance. We tested the hypothesis that many decades of selection for high milk yield will lead to a gradual but progressive decline in reproductive performance under pasture-based production systems. Therefore, the main objective of our research was to conduct a multi-trait analysis of lactation performance in purebred and crossbred dairy cows and quantify the relationships with fertility traits under grazing conditions. Test-day lactation and reproductive performance data from 2006-2010 in 428 dairy farms in Tasmania with an average herd size of 300 cows were subjected to General Linear Models and correlation analyses. Lactation performance was found to be a function of breed, physiological status and age of the cow, regional location, season and year of calving. Older, multi-parous, Holstein-Friesian cows gave significantly higher milk yields than younger, primi-parous calves v.ith a linear increase in milk yield, protein, fat and calf birth weight as cow parity increased from 1 to 3. Lactation and fertility traits had an antagonistic relationship as high milk yield was associated with increases in calving interval, interval to first breeding and number of days open. It was concluded that for unbiased comparisons of cow mating plans, milking routines between and within pasture-based and seasonally bred dairy herds, adjustments for breed, parity, regional location, season and year of calving will be necessary. Furthermore, a balance must be struck between selecting for high milk yield and fertility-related traits to minimize the impact of the antagonistic relationship between lactation and reproductive traits on cow performance