Genetic and environmental variation in reproductive performance of pasture-based dairy cows

Pasture-based dairy cows rely heavily on the availability of good quality grass and favourable environmental conditions to realize their maximum genetic potential for lactation and reproductive performance. Over one million records of gestation length, calving interval, number of services per conception, interval to first breeding and days open from four hundred and twenty eight pasture-based dairy farms in Tasmania, Australia, were evaluated. The hypothesis tested was that reproductive performance will be driven by multi-faceted genetic and environmental factors. It was evident that Holstein-Friesian and Jersey x Holstein-Friesian cows had similar reproductive performance, but calving intervals were longer in spring (390 days) than winter (362 days) and days open longer in autumn (136 days) than spring (77 days). There was a general decline in cow fertility that was mainly parity-driven because of the increases in interval to first breeding from 469 to 527 days, number of days open from 95 to 111 days and calving interval from 369 to 379 days as cow parity increased from 1 to 3. Our findings suggest that this decline in fertility could be attributable to decades of dairy cow breeding strategy that emphasized selection for high milk yield at the detriment of reproductive performance. In conclusion, there is the need for including reproductive traits in the selection index of pasture-based dairy cows and adjusting for seasonal variation