In Australia, dairy farming is carried out in environments that vary in many ways, including level of feeding and climate variables such as temperature and humidity. The aim of this study was to assess the magnitude of genotype x environment interactions (GxE) on milk production traits (milk yield, protein yield, and fat yield) for a range of environmental descriptors. The environment on individual test days was described by herd size (HS), average herd protein yield (AHTDP), herd test-day coefficient of variation for protein yield (HTDCV), and temperature humidity index (THI). A sire random regression model was used to model the response of a sire's daughters to variation in the environment and to calculate the genetic correlation between the same traits measured in two widely different environments. Using test-day records, rather than average lactation yields, allowed exploitation of within-cow variation as well as between-cow variation at different levels of AHTDP, and led to more accurate estimates of sire breeding values for "response to environment." The greatest GxE observed was due to variation in AHTDP, with a genetic correlation of 0.78 between protein yield when AHTDP = 0.54 kg and protein yield when AHTDP = 1.1 kg (the 5th and 95th percentile of the distribution of AHTDP). The GxE was also observed for THI, with a genetic correlation of 0.90 between protein yield at the 5th and 95th percentile of THI. The use of response to environment estimated breeding values to improve the accuracy of international sire evaluations is discussed.

Estimates of covariance components and genetic parameters were obtained for birth, weaning, 200-d, and 400-d weight for a herd of Polled Herefords and a herd of a multibreed synthetics, the so-called Wokalups. Data originated from an experiment in Western Australia selecting for preweaning growth rate. Analyses were carried out by REML fitting an animal model including genetic and permanent environmental maternal effects. Wokalups showed consistently more phenotypic variation, partly due to a scale effect, and higher direct and lower maternal heritabilities than Herefords. Maternal environmental effects were more than twice as important in Herefords than in Wokalups. Estimates of both genetic and environmental correlations among weaning and subsequent weights were essentially unity, identifying maternal effects found for postweaning weights as a "carry over" of those on weaning weight.

The existence of a genotype x environment interaction (G x E) for fertility traits, survival, and milk yield traits was examined by considering performance recorded in different calving systems (seasonal, split, and year round) or regions as different traits. For fertility traits and survival, G x E were also investigated by applying a random regression model using continuous environmental variables, such as level of herd milk production, temperature-humidity index, and herd size as environmental descriptors. The traits considered were calving interval, calving to first service interval (CFS), 25-d nonreturn rate at first service, pregnancy rate, survival, milk yield, fat yield, and protein yield and percentage. Data on Holstein-Friesian cows that calved between 1997 and 2005 were analyzed. The number of cows included in the analyses ranged from approximately 21,000 for pregnancy rate to approximately one-half million for survival. For all traits, heterogeneity in additive and phenotypic variances was observed. For example, for CFS the additive genetic and phenotypic variance in seasonal calving herds was only 9 and 15% of that in year-round calving herds, respectively. Genetic correlations among calving systems for milk yield traits were greater than 0.96. For calving interval, the lowest genetic correlation, of 0.83, was between split and year-round calving herds, but for CFS and pregnancy rate, genetic correlations as low as 0.37 were observed, although these estimates were associated with large standard errors. Genetic correlations between traits recorded in different Australian regions were greater than 0.89. Heritability and phenotypic variance for milk yield traits were the greatest in region 1, which consisted of Queensland, West Australia, South Australia, and New South Wales, and were least in region 3, which included Gippsland and Tasmania, in accordance with mean milk yield levels. Genetic correlations as low as 0.5 for some fertility traits between the 5th and 95th percentile of the distribution of the environmental descriptors, such as herd size and average herd milk production, were also observed. However, these estimates had large standard errors. Regardless of the environmental descriptor used, there was no evidence for the presence of a large G x E that resulted in economically significant reranking of bulls.

Estimates of covariance components were obtained for milk production of beef cows, measured by the weigh-suckle-weigh method, and weaning weight and preweaning growth of their calves for a herd of Polled Herefords and a herd of a multibreed synthetics, so-called Wokalups. Analyses were carried out by restricted maximum likelihood fitting an animal model incorporating both direct and maternal genetic and permanent environmental effects and allowing for the direct-maternal covariances. Average 14-h milk yield was 3.6 kg for Herefords and 4.9 kg for Wokalups and heritabilities were .12 and .08, respectively. Treating preweaning gain of the calf as a trait of the cow, estimates of the additive genetic and permanent environmental correlations between preweaning gain and milk yield were higher for Herefords (.73 and 1.00, respectively) than for Wokalups (.53 and .77). Bivariate analyses of milk production (treated as a trait of the cow) and weaning weight (treated as a trait of the calf) showed virtually no direct additive genetic, direct permanent environmental, or phenotypic association between the two traits but identified strong correlations between direct effects for milk yield and maternal effects for weaning weight. The estimate of the direct-maternal genetic correlation between milk yield and weaning weight was .80 for both breeds, and the estimated correlation between direct, permanent environmental effects for milk yield and maternal, permanent environmental effects on weaning weight was unity for Herefords and .89 for Wokalups. Results indicate that milk production is the main determinant of maternal effects on the growth of beef calves with breed differences in the importance of maternal effects largely attributable to differences in milk production.