METHANE MITIGATION, FEED EFFICIENCY, AND HEAT STRESSIN REPLACEMENT HEIFERS: NUTRITIONAL ANDMANAGEMENT APPROACHES FOR SUSTAINABLE DAIRYPRODUCTION

The 21st century is characterized by profound social and environmental changes. Global population is expected to reach 10 billion people by 2050, generating a great demand for food production. At the same time, climate change has already increased the average global surface temperature by approximately 1.5 °C compared to the pre-industrial era, posing significant threats to human, animals, and environmental health. Agriculture and livestock farming play a central role in this scenario, as they are essential for food supply but are also required to drastically reduce their greenhouse gas emissions. In addition, global warming exerts a direct negative impact on animal health and welfare, leading to important economic losses in recent years. The increase in average temperatures has also been associated with a higher frequency of extreme weather events, such as droughts and heatwaves, which further challenge farm management and livestock productivity. The overall aim of this PhD thesis was to investigate nutritional and management strategies to better address the challenges faced by livestock production in the 21st century. In particular, the research focused on improving feed efficiency, mitigating methane emissions, and alleviating the detrimental effects of heat stress in Italian Holstein dairy heifers. The first study (Chapter 3) describes an in vivo trial evaluating the inclusion of increasing doses of lysozyme in the diet of dairy heifers to assess the effects of this antimicrobial enzyme on enteric methane production. The results showed a 12% reduction in methane emissions in the group receiving the highest lysozyme dose (10.000 FPI U/Kg DM of lysozyme), without significant effects on animal performance or welfare. The exact mechanisms by which lysozyme acts in the rumen to decrease methane production are still not fully understood; therefore, further analysis on ruminal fermentation patterns and microbial populations need to be carried out to clarify this aspect. The second trial (Chapter 4) aimed to evaluate different energy sources (corn and hydrogenated fat) in the diet of dairy heifers with respect to feed efficiency, energy metabolism, and methanogenesis. The study showed that feed efficiency, assessed through residual feed intake (RFI), appeared promising as it remained moderately consistent and repeatable across the two diets, as confirmed by a Pearson correlation (r=0.64; p=0.0072). As expected, the different energy sources influenced energy metabolism and nutrient partitioning, affecting acid–base balance, ruminal fermentation, and the physiological patterns of fat storage. The third trial (Chapter 5) evaluated two different strategies for mitigating heat stress in dairy heifers: a control group (C), a conduction-based micro-cooling system applied in the resting and feeding areas (F) and shifting feed delivery to the cooler hours of the day (N). The study showed that, although modifying the feeding schedule altered feeding behavior by increasing intake during night and early morning, the cooling system was more effective overall. In particular, it reduced respiratory rate (F: 41.51± 1.1 vs C: 48.02 ± 1.1; N: 48.66 ± 1.1 breath/min; P<0.05) and rectal temperature (F: 38.77 ± 0.07 vs C: 38.99 ± 0.07; N: 39.10 ± 0.07 °C; P<0.05), while improving average daily gain (ADG) the hottest week of the trial (F: 0.76 ± 0.24 vs C: -0.66 ± 0.24; N: -0.42 ± 0.24 Kg/day; P<0.05). Building on the effectiveness of the cooling system, a subsequent trial (Chapter 6) compared the same cooling strategy (F) with the dietary supplementation of rumen-protected calcium gluconate (T) and a control group (C). Once again, the cooling system proved more effective in mitigating the effects of heat stress during hot periods. While calcium gluconate supplementation improved total tract apparent diet digestibility (NDF total tract digestibility: T: 57.26 ± 0.93 vs C: 51.50 ± 0.93; F: 51.61 ± 0.93; P<0.05), it was not sufficient to counteract the negative effects of heat stress on animal performance.