The objective of this study was to explore hepatic metabolic adaptations in mid-lactation Holstein cows managed under feeding strategies with different levels of pasture inclusion. Sixteen multiparous North American Holstein cows were assigned from calving to 180 days in milk (DIM) to either a fixed pasture strategy (FixP; n = 8), in which grazed pasture represented approximately one-third of dry matter intake and the remainder was provided as total mixed ration (TMR), or a maximum pasture strategy (MaxP; n = 8), in which pasture intake was maximized and cows were supplemented with concentrate and conserved forage according to pasture availability. At 180 ± 20 DIM, plasma samples and liver biopsies were collected for biochemical analyses, quantitative PCR, and targeted liver metabolomics by gas chromatography time-of-flight mass spectrometry. Milk yield, milk components, and body condition score did not differ between feeding strategies. Plasma urea nitrogen was greater in MaxP than FixP cows (6.64 vs. 4.96 mmol/L; P = 0.01). In liver, FixP cows showed greater abundance of metabolites related to carbohydrate metabolism, including phosphoenolpyruvate, fructose-6-phosphate, glucose-6-phosphate, and sucrose, together with greater expression of genes related to the pentose phosphate pathway and fatty acid synthesis, including glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, ribose-5-phosphate isomerase A, acetyl-CoA carboxylase alpha and fatty acid synthase (p ≤ 0.10). In contrast, MaxP cows showed greater abundance of metabolites associated with nitrogen metabolism, including citrulline, ornithine, glutamine, and creatinine. These results indicate that, during mid-lactation, greater pasture inclusion is associated with enhanced hepatic nitrogen metabolism, whereas partial replacement of pasture with TMR is associated with greater hepatic carbohydrate-related metabolism and fatty acid synthesis.

Residual sward height management is a key factor influencing milk production and grazed forage utilization per cow and per hectare in pasture-based dairy systems. This study evaluated the effects of postgrazing sward height on forage growth rate and DMI, milk production, and BCS in supplemented, high-producing dairy cows. The study captured cumulative treatment effects on animal performance across nearly an entire lactation, as well as sward dynamics over most of the tall fescue growing season. Thirty-two multiparous Holstein cows were assigned to 2 treatments: the control treatment, with a postgrazing sward height of 5.0 to 7.0 cm (CT), and the high treatment, with a postgrazing sward height of 12.0 to 15.0 cm (HI). The study spanned 215 d of grazing on a Lolium arundinaceum sward in a randomized complete block design, with 4 blocks, each measuring 3.2 ha, and consisted of 2 paddocks, each measuring 1.6 ha. Cows had one grazing session between milkings and were supplemented with a mixed ration in the opposite session. Rotational grazing was employed, with new strips allocated once the postgrazing sward height for each treatment was achieved. A new grazing cycle was initiated when plants reached the 2.5 to 3.0-leaf phenological state. Any extra or unused areas within a grazing cycle were always recorded. The postgrazing sward heights were 7.7 and 14.4 cm for the CT and HI treatment, respectively. Forage growth rate tended to be 21% higher in the HI treatment (40 vs. 33 kg DM/ha per day), and cows stayed longer per strip (3.9 vs. 2.9 d) and had 2.5 times more grazing cycles (8.3 vs. 3.3). Total forage disappearance was similar between treatments. Cows in the HI treatment had greater individual grazed forage DMI (+2.0 kg/cow) and BCS throughout lactation compared with CT cows. The HI treatment also increased the individual milk production by 10% (31.5 vs. 34.5 kg milk/d) while reducing concentrate intake per unit of milk by 10%, resulting in a tendency for higher income over feed costs at cow level. However, although we observed a tendency for a higher forage growth rate, this approach required a larger grazing area (1.18 vs. 1.60 ha/4 cows), leading to a trend toward 21% lower milk output per hectare (23,598 vs. 18,528 kg milk/ha). In contrast, the CT treatment supported a higher stocking rate but resulted in lower grazed forage DMI, higher supplemental feed input per hectare and slower BCS recovery. These findings highlight the importance of aligning grazing strategies with system-specific goals. Farmers seeking to maximize land productivity may favor lower postgrazing sward heights, although these must be managed carefully to prevent adverse effects on animal health, reproduction, and long-term productivity due to slower BCS recovery during lactation and lower BCS at the onset of dry off. Conversely, systems aiming to reduce feed costs may benefit from a higher residual sward height, which enhances individual cow performance and BCS while decreasing reliance on supplemental feed.

Grazing dairy production systems have gained interest due to the increasing consumer preferences associated with environmental care and animal welfare perception. Nonetheless, grazing dairy cows have lower feed conversion than those fed indoors with a TMR, partly because of increased maintenance energy costs associated with grazing and walking activity. However, to our knowledge, the energy cost of grazing has not been quantified for dairy cows. The objectives of the present work were to evaluate the effects of grazing on the oxygen pulse (O2P; mL O2 consumed per heartbeat; experiment 1) and to quantify the energy cost of grazing in dairy cows using the oxygen pulse-heart rate (O2P-HR) technique (experiments 1 and 2). In both experiments, heat production (HP) was measured using the O2P-HR technique and synchronized with animal activity records. In experiment 1, 15 dairy cows were used to measure the O2P when resting and then immediately after the first grazing session to assess the effect of grazing on O2P. Grazing activity increased HR and oxygen consumption compared with when the cows were resting. However, the increase in both variables was proportional; therefore, O2P was similar, indicating that the change in HR was the main component in the response to an increased O2 demand due to grazing. Hence, it is possible to use the O2P-HR method to measure HP during grazing in dairy cows. The energy cost of grazing was estimated by the difference in HP between when the cow was grazing and when it was not grazing (i.e., idling or ruminating) and ranged from 6.63 ± 1.12 to 7.85 ± 0.68 kJ/kg0.75 per hour and 1.31 ± 0.28 to 1.59 ± 0.14 kJ/kg per hour, respectively. In addition, the energy cost of walking was calculated as the difference in HP when the cow was walking to and from the pasture and the milking parlor versus when it was idling or ruminating. The energy cost of walking was 24.03 ± 1.12 kJ/kg0.75 per hour or 4.72 ± 0.28 kJ/kg per hour. Hourly energy expenditure (kJ/h) was similar between grazing and consuming TMR at the feed bunk, indicating that the extra energy required by dairy cows when fed in a grazing system is mostly explained by the longer time spent to harvest the pasture and the energy cost of walking between the pasture and the milking parlor compared with confined cows fed TMR. Finally, the results of this study suggest that O2P is not affected by grazing and therefore that the O2P-HR technique is a promising methodology to estimate the energy cost of grazing animals without substantially affecting their behavior within the production system.

Walking requires energy that cannot be used to produce milk. This aspect is especially important in grazing systems, where the cow must travel daily from the pasture to the milking parlour, which leads to a reduction in productive efficiency. Twenty-four indoor-fed Holstein cows were used to quantify the energy cost of walking and to evaluate the impact of walking activity on milk production and the energy partitioning. Twelve cows were subjected to a daily walking regimen of 8 km, divided into two sessions of 4 km (WALK), while another group of 12 cows remained in the barn without access to feed during these walking periods (NO_WALK). Heat production (HP) was measured using the oxygen pulse-heart rate technique, while retained energy in milk and tissue were calculated based on milk yield and composition and body weight change, respectively. Moreover, residual HP was calculated by the difference between the measured HP and the predicted HP. Walking 8 km/day did not affect feed intake, milk production or composition and therefore feed-to-milk conversion efficiency was similar between treatments. However, NO_WALK cows exhibited 12% greater energy efficiency than WALK cows when retained energy in tissue was considered. In contrast, the energy cost of walking-estimated either as the difference in HP between traetments during walking periods or by residual HP-was 0.98 or 1.92 kJ/kg per km, respectively. The results of this experiment demonstrate that walking activity have a negative impact on energy efficiency, although this is not necessarily reflected in a short-term decrease in milk production.

The objective of the study was to characterize adaptations of hepatic metabolism of dairy cows of two Holstein strains with varying proportions of grazing in the feeding strategy. Multiparous autumn calving Holstein cows of New Zealand (NZH) and North American (NAH) strains were assigned to a randomized complete block design with a 2 x 2 factorial arrangement with two feeding strategies that varied in the proportions of pasture and supplementation: maximum pasture and supplementation with a pelleted concentrate (MaxP) or fixed pasture and supplementation with a total mixed ration (FixP) from May through November of 2018. Hepatic biopsies were taken at - 45 ± 17, 21 ± 7, 100 ± 23 and 180 ± 23 days in milk (DIM), representing prepartum, early lactation, early mid-lactation and late mid-lactation. The effects of DIM, feeding strategy (FS), strain and their interactions were analyzed with mixed models using repeated measures. Cows of both strains had similar triglyceride levels, mitochondrial function and carnitine palmitoyltransferase activity in liver during lactation. However, there was an effect of DIM and FS as liver triglyceride was higher for the MaxP strategy at 21 DIM and both mitochondrial function and carnitine palmitoyltransferase activity in liver were lower for the MaxP strategy at 21 DIM. Hepatic mitochondrial function and acetylation levels were affected by the interaction between strain and feeding strategy as both variables were higher for NAH cows in the MaxP strategy. Mid-lactation hepatic gene expression of enzymes related to fatty acid metabolism and nuclear receptors was higher for NZH than NAH cows. This work confirms the association between liver triglyceride, decreased hepatic mitochondrial function and greater mitochondrial acetylation levels in cows with a higher inclusion of pasture and suggests differential adaptative mechanisms between NAH and NZH cows to strategies with varying proportions of grazing in the feeding strategy.

A linear relationship between heart rate (HR) and oxygen consumption (VO2) has been reported in homeothermic animals, indicating that is possible to estimate heat production through HR measurements. This relationship may depend on the animal activity and environmental conditions. The main objective of the present study was to evaluate the effect of the air temperature and animal posture and activity on heat production and VO2 in relation to HR. In addition, as a secondary objective, the energy cost of eating and ruminating versus idling and standing versus lying down was determined. Twelve Holstein lactating cows were housed inside climate-controlled respiration chambers for 8 d, where the air temperature was gradually increased from 7 to 21°C during the night and from 16 to 30°C during the day with daily increments of 2°C for both daytime and nighttime. During the 8-d data collection period, HR and gaseous exchange measurements were performed, and animal posture and activity were recorded continuously. The oxygen pulse (O2P), which represents the amount of oxygen that is consumed by the cow per heartbeat, was calculated as the ratio between VO2 and HR. Results showed that heat production and VO2 were linearly and positively associated with HR, but this relationship largely varied between individual cows. Within the range tested, O2P was unaffected by temperature, but we detected a tendency for an interaction of O2P with the temperature range tested during the night versus during the day. This indicates that the effect of air temperature on O2P is nonlinear. Standing and eating slightly increased O2P (1.0 and 2.5%) compared with lying down and idling, respectively, whereas rumination increased O2P by 5.1% compared with idling. It was concluded that the potential bias introduced by these effects on the O2P for the application of the technique is limited. The energy cost of eating and ruminating over idling was 223 ± 11 and 45 ± 6 kJ/kg0.75 per day, respectively, whereas the energy cost of standing over lying down was 53 ± 6 kJ/kg0.75 per day. We concluded that O2P in dairy cows was slightly affected by both animal posture and activity, but remained unaffected by air temperature within 8 to 32°C. Nonlinearity of the relationship between the O2P and air temperature suggests that caution is required extrapolating O2P beyond the temperature range evaluated in our experiment.

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.

The objective was to evaluate the effect of the Holstein genotype (North American Holstein vs. New Zealand Holstein; NAH vs. NZH, respectively) in a pasture-based system on heat production (HP), energy partitioning between maintenance and production (milk and tissue) and energy efficiency during two different stages of lactation. Twenty-eight Holstein dairy cows (14 cows of each genotype) with similar calving date (May 5, 2018 +/- 23 days) were managed in a pasture-based system and supplemented with one third of the predicted total dry matter intake as concentrate. Heat production, retained energy in milk and tissue, metabolizable energy intake (MEI) and the proportion of MEI retained in milk + tissue (RE/MEI) were measured at 115 and 192 +/- 19 days in milk and residual HP was estimated by the difference between measured HP and predicted HP based on NRC (2001) model according to body weight, body condition score and milk production. The NAH cows were 60 +/- 15 kg heavier and produced 4.7 +/- 1.0 kg/d more milk with lower percentages of fat and protein than NZH cows. However, there were no differences in fat or protein yield per day between genotypes. Metabolizable energy intake, retained energy in milk and tissue, HP and RE/MEI were not different between genotypes at 115 days in milk. Nevertheless, at 192 days in milk the MEI, HP and residual HP were lower in NZH than NAH, whereas RE/MEI was not different when both genotypes were managed under a pasture-based system with one third of the consumed diet as concentrate. The capacity of NZH cows to maintain the same RE/MEI than NAH cows at 192 days in milk despite of the lower MEI, was due to a lower metabolizable energy requirement for maintenance (853 vs. 729 kJ/body weight(0.75) per day for NAH and NZH, respectively). Indeed, the lower energy requirement for maintenance in NZH was associated with a lower fasting heat production since k(l) were not different between genotypes. Thus, NZH cows could have a lower proportion of their body as protein mass or a lower relative mass of the internal organs involved with digestive and circulatory functions. However, further investigation is necessary to understand the differences in maintenance energy requirements between the Holstein genotypes.

The objective of this study was to estimate energy efficiency and energy partitioning of the metabolizable energy intake (MEI) into milk production, body reserves and heat production (HP) for multiparous dairy cows (autumn calving) of two different Holstein genotypes under two different strategies during mid-lactation. New Zealand Holstein (NZH) and North American Holstein (NAH) cows were assigned to two feeding strategies: 48% pasture, 18% forage reserves and 34% concentrate (PMAX) or 32% pasture and 68% total mixed ration (P30). The NZH cows had greater retained energy (RE) in tissue and energy efficiency (RE/MEI) than NAH whereas MEI, HP or total RE were not affected for the cow genotype. Total RE did not differ due to feeding strategy but HP and MEI were greater for PMAX than P30 cows and therefore energy efficiency higher for the P30 cows. Although the interaction between Holstein genotype and feeding strategy did not affect energy partitioning between RE and HP or energy efficiency, a feeding strategy that maximized pasture intake (PMAX) allowed cows to consume more ME but decreased energy efficiency as more MEI was lost as heat.