Context Improving the partitioning of the energy consumed by dairy cows towards milk-solid production is a priority in grazing diary systems because energy efficiency has been associated with sustainability. Different selection criteria in the Holstein breed have led to divergent Holstein strains with different suitability to grazing systems. Aim The objective of this work was to quantify and evaluate the energy partitioning between maintenance and milk production of two divergent Holstein strains (New Zealand Holstein and North American Holstein) in a grazing system without supplementation of concentrate. Methods New Zealand Holstein and North American Holstein cows, nine of each, in mid-lactation (183 ± 37 days in milk, mean ± s.d.) were allocated in a randomised block design and evaluated under grazing conditions. The cows were managed under a daily strip-grazing system and grazed perennial ryegrass as the only source of nutrients. After an adaptation period of 14 days, heat production, retained energy in milk and metabolisable energy intake were measured over 7 days, and animal behaviour was simultaneously recorded. Key results Milk yield did not differ between Holstein strains, but fat and protein content were greater for New Zealand than North American Holstein cows; consequently, retained energy in milk was 13% greater for the former strain. Heat production did not differ between Holstein strains, but metabolisable energy intake (kJ/bodyweight0.75.day) was greater for New Zealand than North American Holstein cows, which was associated with a greater pasture dry matter intake relative to their body weight. Both feed and energy efficiency were greater for New Zealand than North American Holstein cows. Conclusions The results supported that the New Zealand Holstein strain has greater energy and feed efficiency, demonstrating that it could be more suitable to be managed under a grazing dairy system without supplementation than the North American Holstein strain. Implications The New Zealand Holstein strain may be suited to selection as a dairy cow with the capacity to fulfil energy requirements from pasture, which is a key factor to improve production efficiency of grazing dairy systems.
The objective of this study was to assess hepatic ATP synthesis in Holstein cows of North American and New Zealand origins and the gluconeogenic pathway, one of the pathways with the highest ATP demands in the ruminant liver. Autumn-calving Holstein cows of New Zealand and North American origins were managed in a pasture-based system with supplementation of concentrate that represented approximately 33% of the predicted dry matter intake during 2017, 2018, and 2019, and hepatic biopsies were taken during mid-lactation at 174 ± 23 days in milk. Cows of both strains produced similar levels of solids-corrected milk, and no differences in body condition score were found. Plasma glucose concentrations were higher for cows of New Zealand versus North American origin. Hepatic mitochondrial function evaluated measuring oxygen consumption rates showed that mitochondrial parameters related to ATP synthesis and maximum respiratory rate were increased for cows of New Zealand compared with North American origin. However, hepatic gene expression of pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and pyruvate dehydrogenase kinase was increased in North American compared with New Zealand cows. These results altogether suggest an increased activity of the tricarboxylic cycle in New Zealand cows, leading to increased ATP synthesis, whereas North American cows pull tricarboxylic cycle intermediates toward gluconeogenesis. The fact that this occurs during mid-lactation could account for the increased persistency of North American cows, especially in a pasture-based system. In addition, we observed an augmented mitochondrial density in New Zealand cows, which could be related to feed efficiency mechanisms. In sum, our results contribute to the elucidation of hepatic molecular mechanisms in dairy cows in production systems with higher inclusion of pastures.
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