An introduction to the special issue for the 2020 Australasian Dairy Science Symposium.

Circular economy principles are being adopted to design out waste, use resources more efficiently and maintain ecosystem health. A framework was developed to assess the degree that individual pastoral farms adhere to these principles and that was adaptable to a range of farming systems. Further development and testing of this framework will provide a tool to benchmark farms and identify areas where they could improve sustainability.

The effects of extreme heat events on the nutritive characteristics of pastures were studied in three farms in northern Victoria, Australia. Nutritive quality decreased at high temperatures in all species, with implications for milk production. Further research is recommended.

This study investigated changes in milk constituents and behaviour of cows following exposure to Coxiella burnetii infection. Coxiella burnetii infection was quarter specific and associated with subclinical mastitis, with changes in somatic cell count, total immunoglobulin G, lactate dehydrogenase, serum albumin, milk protein and daily rumination. Combined signals of important mastitis markers and behaviour might be helpful to better understand the impact of C. burnetii on the mammary gland and milk production parameters.

Pharmaceutical industries have been making efforts to find alternative methods to animal tests. A review was made of the available bibliography about the factors influencing the quality of bovine oocytes that can contribute to increase the success of embryos in in vitro cattle production. The bovine model using slaughterhouse material (according the 3R principle) is a valuable instrument to unravel reproductive events in humans due to the similarities in ovarian follicular dynamics and endocrine control between those species.

Interest in grazing dairy systems has increased because of benefits related to production cost, product quality and animal welfare. However, not all dairy cows are equally suitable to be managed under grazing systems. For the Holstein breed, energy efficiency, a key factor in the sustainability of the grazing systems, was greater for New Zealand than North American Holstein cows.

A reference population of Holstein cows was selected for milk yield production, by using lactation records from Tunisian dairy herds and BLUP modelling. Following the steps described in this study, researchers in developing countries will be able to identify a reference population toward genomic evaluation even without a progeny testing scheme.

Meeting the protein needs of high-yielding cows for optimum production is critical, with soybean meal being the most important protein source used to feed dairy cows. Its nutritional value can be increased by protecting proteins against rumen degradation, and to this end, we developed a new method of fat coating of solvent-extracted soybean meal. The results show that the technique was successful in protecting soybean meal and increasing milk fat yield of dairy cows.

Threshold models were used to genetically evaluate sexual precocity (PREC) and subclinical mastitis incidence (SCM), while linear models were used to analyse milk yield (MY) and casein percentage . Both models allowed the precise estimation of the high genetic influence on PREC and CP (high heritability) and the important environmental contribution on SCM and MY (low heritability and high permanent environmental variance). These differences in heritability estimates imply that different genetic selection strategies should be considered for each trait.