Quantitative Genetic Analysis of Extended Lactation and Lactation Persistency In Australian Dairy Cattle on Pasture Based Systems

Abstract

In the Australian dairy industry, and in particular Victoria, there has been a shift away from seasonally concentrated calving patterns. Reasons for such a shift stem largely from improved feeding of cows with year round feed supplies, and the introduction of new germplasm from North American Holstein Friesian animals into some Australian dairy herds. Such impacts have resulted in an increase in the phenotypic and genetic potential of cows to produce more milk and be milked for longer than the standard 305 days under seasonal calving systems. The increase in high output cows has come at an associated cost with a national decrease in reproductive performance and many highly productive cows being prematurely culled for failing to get in calf. One strategy of interest to the dairy industry is for high-producing cows being mated out of season and then carried over and milked for an extra 6 months (18 month calving interval). In Australia this is regarded as the most optimal and economical method on pasture-based systems. Limited information was available about the potential for genetic improvement of extended lactation under the Australian dairy system, which led to the objectives of this thesis. A very large amount of data with over 158 million test-day records was available to examine the scope for genetic improvement in extended lactation in the Australian dairy industry. A data handling pipeline was developed to assist in analysing and editing large scale industry test day records. Variation in Australian dairy cattle in respect to their lactation curves over an extended lactation was explored and derived traits obtained that were then used in the genetic analysis using mixed model methodology. The Wood model was fitted to milk yield records divided into 8 random subsets of 1 million pure Holstein cows with 20 million test-day records. Two traits of interest, namely persistency and extended lactation, were quantified and relevant descriptive traits derived. Variation among cows in their ability to maintain high production over a longer period of time was evident and a representation of the shape of average lactation curves in Australian dairy cows was presented. Findings showed that milk production during the extended lactation phase (from day 305 to day 610 of lactation) was on average equivalent to 40% of the production of the first 305 days of lactation (standard lactation) with an average milk yield of 8,887 L over the extended lactation. In terms of lactation persistency, 50% of peak production is maintained up to day 305. Estimates of heritability for milk, fat and protein yield under extended lactation (0.09-0.06±0.004) and lactation persistency (0.06-0.1±0.004) were low, while the cows’ repeatability for extended lactation and lactation persistency milk traits (0.20) was higher by comparison. Such findings indicate the potential for early cow performance to be used as an indicator for a cow’s successive lactation performance. Extended lactation and lactation persistency were found to be highly correlated to each other (genetic 0.60-0.98 and phenotypic 0.68-0.93), but not with the other economically important traits of milk, fat and protein yield (-0.05-0.1). This suggests that selection for increased extended lactation will not adversely impact on economically important and highly selected traits, namely cumulative 305 day yields and the Australian Selection Index (ASI). Extended lactation measured as a standardised ratio trait automatically adjusts for high peak yield. Producers selecting on this trait do not favour selecting cows with higher peak yields yet maintaining peak production over another 300 days in milk. In addition, extended lactation can be incorporated with cumulative 305 day milk yields and lactation persistency in a selection index in a designed breeding program. Estimated breeding values (EBVs) were generated for extended lactation and lactation persistency for milk, fat and protein yields, to assist with the selection of cows and bulls. Genetic trends showed no change till early 1990, followed by a steady increase (1genetic SD) in EBVs for extended lactation characteristics in the last 20 years. This increase is potentially a reflection of the increased selection for milk production and introduction of new germplasm from North American Holstein Friesian cattle into Australian herds. The research presented in this thesis will provide the Australian dairy industry with the breeding tools to enable selection of bulls to produce cows that are best suited to extended lactation in pasture-based systems. It has also contributed to the understanding of the genetics of extended lactation and lactation persistency that can be applied to other breeds in the Australian herd, and opened avenues for genomic predictions for these traits.