EVALUATION OF GENETICS AND ENVIRONMENTAL FACTORS OF MILK PRODUCTION-RELATED TRAITS AND PREDICTED ENTERIC METHANE EMISSIONS IN DAIRY COWS OF DIFFERENT BREEDS AND CROSSBREEDS FROM NORTHERN ITALY

The composition and variation of milk yield in dairy cows can be attributed to several genetic and environmental factors. Therefore, the main objective of this thesis was to evaluate genetic and environmental factors with traits related to milk production and enteric emissions of methane from different dairy breeds and crossbred cows in northern Italy. In recent years, strategies have been developed to predict traits related to enteric methane (EME) emissions of dairy cows as they are credited as the most impactful source of greenhouse gases from food production worldwide. Among these strategies, very interesting are the predictions of EME from milk samples through Fourier-transform infrared spectrometry (FTIR). As a first chapter, variability was evaluated in the FTIRS predicted EME traits due to cow breed, farm intensiveness, the level of production of individual cows within the herds, and their interactions. EME predictions were affected by intensiveness of farm system, by breed of cows, by the level of production of individual cows within herd and breed and by their interactions. A better understanding of the complex interactions influencing the EME in dairy herds was achieved. This will be useful for genetic improvement, environmental certification of farms and for environmentally adjusting the price of milk in payment systems. An increasingly interesting productive system is the rotational crossbreeding of specialized dairy breeds, which has not been extensively studied in relation to EME of dairy cows, nor has the variation of emissions during lactation been modelled. As seen in the first chapter, milk infrared spectra could be used to predict EME in dairy cows. Therefore, the objective of the second chapter was to study the effects of the crossbreeding on the predicted infrared proxies of EME and their variation during lactation. Given the greater longevity of crossbred cows, and their lower replacement rate, rotational crossbreeding could be a way of mitigating the environmental impact of milk production. As we all know, it is important to estimate genetic parameters that allow the establishment of genetic strategies to reduce the impact of methane production on dairy systems. The objective of a third chapter was to estimate heritability (h2), additive genetic correlations (rg), residual correlations (re) and repeatability (REP) at population level of production traits, dairy fatty acid traits related to EME (FA), and EME. With these results, it is expected to demonstrate the viability of using prediction of EME traits Direct-IR and Indirect-IR in animal breeding programs and of calculating methane production with FTIR prediction using the FAs information. Finally, beyond the effect of cows on climate change, it is interesting to study also the effect of climate change on the cows. The objective of the fourth chapter was to distinguish clearly the long-term (circannual) seasonal pattern from acute, short-term changes in relation to both climatic traits (daytime and air temperature) and milk production traits (daily yield, and fat and protein percentages). As expected, the most important sources of variation were herd and stage of lactation, while parity was important only for milk yield. Among the calendar traits, the long-term sinusoidal function resulted more important than the year and the short-term monthly deviations. This seems to indicate that milk quality is affected by photoperiod (daytime). The results obtained through the comparison of different genetic and environmental effects associated with the production of milk from dairy cows, can be used for a better understanding of the behavior of lactation and factors-interactions that influence the expression of production traits of economic interest and therefore implement genetic improvement programs and create strategies that contribute to increase production and reduce greenhouse gases emission.