Genetic and environmental relationship among calving interval, survival, persistency of milk yield and somatic cell count in dairy cattle
Introduction
Milk yield (MY), fertility and health are the most important traits that influence profitability of dairy production. Generally selection for increased MY reduces reproductive efficiency (Grosshans et al., 1997, Dematawewa and Berger, 1998, Castillo-Juarez et al., 2000), increases susceptibility to some diseases and the risk of culling due to diseases and other abnormalities (Simianer et al., 1991, Dematawewa and Berger, 1998). Although the genetic correlation between fertility and milk production traits is generally established to be antagonistic, their association is influenced by level of production and management (Nebel and McGilliard, 1993, Castillo-Juarez et al., 2000, Lucy and Crooker, 2001). Furthermore, most literature estimates are based on data from dairy production systems that use silage and relatively large amount of concentrates (Garcia and Holmes, 1999). In pasture-based production systems in New Zealand (McDougall et al., 1995) and in Tasmania, Australia (Fulkerson, 1985) a positive phenotypic relationship between MYs and fertility was reported. However, others in Australia (Jonsson et al., 1999) and New Zealand (Grosshans et al., 1997) have reported unfavourable genetic relationship between fertility and MY.
The main reason for the antagonistic relationship of MY with reproduction and health is assumed to be that cows produce at a maximum level at the time when they are expected to show oestrous and conceive. The period between calving and peak yield also coincides with the time when incidences of most health problems, including mastitis, are high. Early in lactation, cows are usually in negative energy balance, which means they need to mobilise body reserves to meet the increased nutrient demand for MY (Tamminga, 2000). At the same total yield, cows with lower peak yield and greater persistency may experience less energy imbalance and thus less reproductive and health problems than cows with higher peak yield.
Thus, in addition to including functional traits such as fertility, health and longevity, in the breeding goal, the effort to minimise the negative effect of selecting for MY on fertility and health traits might be helped by considering persistency of MY. A related advantage of improved persistency may be that more persistent cows with lower peak yield can be fed on cheaper roughage (Sölkner and Fuchs, 1987) than cows with higher peak yield. In a pasture-based production system, where an annual calving cycle is considered necessary, an optimum level of fertility is crucial for the sustainability of the production system. Thus far, however, information on the association among important traits such as fertility, longevity, mean MY and persistency of MY in pasture-based dairy production systems is lacking. The objective of this study is, therefore, to estimate heritability (h2) as well as genetic and environmental correlations among mean MY, persistency of MY, mean loge somatic cell count (LnSCC), slope of LnSCC, CI, lactation length (LL) and survival (Surv) in Australia Holstein–Friesian dairy cattle using data from the first two parities.
Section snippets
Data
Test-day MY, fat yield (FY) and SCC and Surv, lactation termination date and calving data of Holstein–Friesian cows were extracted from the Australian Dairy Herd Improvement Scheme (ADHIS) database. Cows that calved between January 1993 and June 1999, sired by bulls in the artificial insemination service, were included in this study. Initially, large herds with over 1000 test records were selected from the ADHIS database. Records in this dataset were edited on the following criteria: (1) tests
Analyses of MY, LnSCC, CI and Surv
The 305-day mean MY in the first parity was 5558 kg (Table 1). The genetic correlation between mean MY and the slope of MY was −0.58 and the environmental correlation was −0.42 before the slope was made independent of the mean. Similarly the genetic correlation between mean LnSCC and the slope of LnSCC was 0.50 and the environmental correlation was 0.14, before the adjustment. Before making the slope of MY or LnSCC independent of their corresponding means, the h2 of the slope of MY was 0.12
Heritability
Heritability of mean MY in the first parity (0.32) is similar to that reported for mean test-day deviation for MY (0.33) by Visscher and Goddard (1995), who also reported h2 of 0.03 for Surv and 0.08 for LL for the Holstein–Friesian cattle in Australia. The h2 of persistency of MY (per MY1) (0.1) and FY (per FY1) (0.06) in the first parity are slightly lower than those reported (0.15 for MY and 0.1 for FY) for Dutch Holstein cattle (Van der Linde et al., 2000) but are higher than those reported
Concluding remarks
The economic advantages of increasing persistency in dairy cattle have been suggested to be reduced feed, health and reproduction cost and increased revenue from sale of milk produced beyond the standard lactation (Dekkers et al., 1998). The current result shows that the genetic correlations of persistency of milk with Surv and CI are near zero (∼−0) suggesting that any reduction in health and reproduction cost is likely to be small. Saving on feed cost by replacing concentrate with roughage (
Acknowledgements
The Australian Dairy Research and Development Corporation (Melbourne) funded this project and the ADHIS kindly provided the data. We thank Dr. Arthur Gilmour for providing the asreml program.
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