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.

Eighty-four autumn (ACS, n = 45)- and spring (SCS, n = 39)-calved multiparous early lactation Holstein cows were assigned to groups of either: (a) grazing + mixed ration (MR) during partial confinement in outdoor soil-bedded pens with shade (OD-GRZ); (b) grazing + MR during partial confinement in a compost-bedded pack barn with cooling (CB-GRZ); or (c) total confinement fed a totally mixed ration (CB-TMR) in a compost-bedded pack barn. Data were analyzed using the SAS MIXED procedure with significance at p ≤ 0.05. In both seasons, despite behavioral differences (p < 0.05) between the OD-GRZ and CB-GRZ groups (i.e., standing, first grazing meal length, bite rate), the milk and component yields, DM intake, microbial CP output (MCP) and NE efficiency were unaffected by the housing conditions, possibly due to mild weather conditions. The milk yield was substantially higher in the CB-TMR group versus the OD-TMR and CB-TMR groups (p < 0.01) in both ACS (~35%) and SCS (~20%) despite there being no intake differences, without any impact on milk component levels. In ACS, this was associated with a higher MCP, likely due to the higher nutritional value of TMR compared to pasture, which was not the case in SCS. In conclusion, the OD-GRZ group achieved the same milk production as the CB-GRZ group through behavior adaptation, under mild weather conditions, in both calving seasons. The CB-TMR group outperformed the grazing systems in both calving seasons, regardless of the MCP.

Primiparous Holstein cows (n = 18; 528 ± 40 kg BW, 3.2 ± 0.2 BCS) calved in fall were used in a randomized block design to study the effect of feeding strategy on production, metabolic, and endocrine responses in early lactation. At calving, cows were assigned within block to 1 of 2 feeding strategies during the first 65 d postpartum (DPP). Feeding strategies were either (G0) total mixed ration (TMR) ad libitum (17 kgDM/d offered; 70% forage, 30% concentrate) or (G1) grazing of alfalfa (Medicago sativa; 6-h am grazing in 3-d strips; pasture allowance = 20 kgDM/d) + TMR (70% of ad libitum TMR; 12 kgDM/d offered). Both groups consumed 2.2 kgDM/d of a commercial ration at each milking. Cows were milked twice a day, milk yield was recorded daily and samples were collected weekly for milk composition. Cow BW and BCS were recorded every 2 wk from −40 to +65 DPP. Blood samples were collected for metabolite and hormone analyses at −7 ± 2 and +42 ± 3 DPP. Data were analyzed as repeated measures with a mixed model that included: feeding strategy, DPP, and its interaction as fixed effects, block as random effect and calving date as a covariate. Milk yield (26.7 vs. 25.1 ± 0.58 kg/d), total solids (3.38 vs. 3.1 ± 0.09 kg/d) and NEL output (20.9 vs. 19.2 Mcal NEL/d) tended (P < 0.07) to be greater for G0 than G1 cows, being differences more marked from +30 to +60 DPP. Cow BW and BCS did not differ (P > 0.30) between feeding strategies. Concentrations of plasma NEFA decreased (P < 0.01) at +42 DPP when compared to −7 DPP and at +42 DPP tended (P = 0.10) to be greater for G0 than G1 cows (0.34 vs. 0.25 ± 0.03 mmol/L. Plasma BHB concentration at +42 DPP was greater (P = 0.02) for G0 than G1 cows (0.46 vs. 0.27 ± 0.05 mmol/L) as it decreased from −7 to +42 DPP only in the latter group. In contrast, plasma insulin was reduced (P = 0.05) in G0 than G1 cows at +42 DPP (11.7 vs. 7.2 ± 1.3 uU/mL) as it increased from −7 to +42 DPP only in the latter group. Concentrations of cortisol, leptin and adiponectin were not different (P > 0.20) at −7 than +42 DPP and neither differed between feeding strategies at +42 DPP. Metabolic and endocrine profile would indicate a greater lipolysis in early lactation in G0 than G1 cows which would be probably associated to their greater milk production. Copyright © 2016. American Society of Animal Science .