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/kg^0.75 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/kg^0.75 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.

Keywords: energy expenditure; grazing cattle; indirect calorimetry; maintenance energy cost.

The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

由于消费者对环保和动物福利的关注增加，放牧型乳牛生产系统越来越受到重视。然而，与在室内喂食TMR（Total Mixed Ration）的乳牛相比，放牧的乳牛饲料转化率较低，部分原因是放牧增加了维持能量成本以及行走活动。但是据我们所知，目前还没有对乳牛放牧行为的能量消耗进行量化研究。本工作的目的是评估放牧对氧气脉冲（O₂P；每心跳耗氧量; 实验1）的影响，并使用氧气脉冲-心率（O₂P-HR）技术来量化乳牛放牧行为的能量消耗（实验1和实验2）。在这两个实验中，通过同步记录动物活动数据并结合O₂P-HR技术测量热产生量（HP）。在实验1中，使用了15头乳牛来分别测定休息时的氧气脉冲(O₂P)，然后立即测定第一次放牧后的氧气脉冲，以评估放牧行为对O₂P的影响。与奶牛处于休息状态相比，放牧活动增加了心率和耗氧量。然而，这两个变量的变化是成比例的；因此，O₂P相似，表明心率变化是响应因放牧而增加氧气需求的主要因素。因此，可以使用O₂P-HR方法来测量奶牛在放牧行为中的热产生量。放牧行为的能量成本通过比较奶牛放牧时与不放牧时（即闲逛或反刍）的热产生量差异进行估计，并且这一能量消耗范围从6.63 ± 1.12 到7.85 ± 0.68 kJ/kg^0.75/小时以及从1.31 ± 0.28到1.59 ± 0.14 kJ/kg/小时。此外，行走的能量成本被计算为奶牛在往返牧场和挤奶厅之间与闲逛或反刍时热产生量的差异，即24.03 ± 1.12 kJ/kg^0.75/小时或者4.72 ± 0.28 kJ/kg/小时。每小时的能量支出（kJ/h）在放牧和在饲料槽内食用TMR之间相似，表明与圈养喂食TMR的乳牛相比，在放牧系统中饲养的乳牛所需额外能量主要是由于收割牧场时间更长以及往返牧场和挤奶厅之间的行走成本所致。最后，本研究结果表明O₂P不受放牧的影响，因此认为O₂P-HR技术是一种很有前景的方法，用于估算动物在生产系统中进行放牧行为时的能量消耗而不显著影响其行为。

关键词：能量支出；放牧牛；间接量热法；维持能量成本。