The data set used in the present study was obtained from 20 energy metabolism studies involving 579 lactating dairy cows (511 Holstein-Friesian, 36 Norwegian Red, and 32 Jersey-Holstein crossbreds) varying in genetic merit, lactation number, stage of lactation, and live weight. These cows were offered diets based on grass silage (n=550) or fresh grass (n=29), and their energy intake and outputs, including methane energy (CH(4)-E), were measured in indirect open-circuit respiration calorimeter chambers. The objective was to use these data to evaluate relationships between CH(4)-E output and a range of factors in animal production and energetic efficiency in lactating dairy cows under normal feeding regimens. The CH(4)-E as a proportion of milk energy output (E(l)), E(l) adjusted to zero energy balance (E(l(0))), or intakes of gross energy (GE), digestible energy (DE), or metabolizable energy (ME) was significantly related to a wide range of variables associated with milk production (E(l) and E(l(0))) and energy parameters (energy intake, metabolizability, partitioning, and utilization efficiencies). Three sets of linear relationships were developed with experimental effects removed. The CH(4)-E/GE intake (r(2)=0.50-0.62) and CH(4)-E/E(l) (r(2)=0.41-0.68) were reduced with increasing feeding level, E(l)/metabolic body weight (MBW; kg(0.75)), E(l(0))/MBW, GE intake/MBW, DE intake/MBW, and ME intake/MBW. Increasing dietary ME/DE decreased CH(4)-E/E(l) (r(2)=0.46) and CH(4)-E/GE intake (r(2)=0.72). Dietary ME concentration and ME/GE were also negatively related to CH(4)-E/GE intake (r(2)=0.47). However, increasing heat production/ME intake increased CH(4)-E as a proportion of E(l) (r(2)=0.41), E(l(0)) (r(2)=0.67) and energy intake (GE, DE, and ME; r(2)=0.62 and 0.70). These proportional CH(4)-E variables were reduced with increasing ratios of E(l)/ME intake and E(l(0))/ME intake and efficiency of ME use for lactation (r(2)=0.49-0.70). Fitting CH(4)-E/E(l) or CH(4)-E/E(l(0)) against these energetic efficiencies in quadratic rather than linear relationships significantly increased r(2) values (0.49-0.67 vs. 0.59-0.87). In conclusion, CH(4)-E as a proportion of energy intake (GE, DE, and ME) and milk production (E(l) and E(l(0))) can be reduced by increasing milk yield and energetic efficiency of milk production or by reducing energy expenditure for maintenance. The selection of dairy cows with high energy utilization efficiencies and milk productivity offers an effective approach to reducing enteric CH(4) emission rates.

Ninety autumn-calving Holstein dairy cows [45 primiparous and 45 multiparous (mean parity, 3.1)] were allocated to 1 of 3 dietary crude protein (CP) concentrations: 173, 144, or 114 g of CP/kg of DM, from calving until d 150 of lactation. On d 151, half of the animals in each treatment were allocated an alternative dietary protein concentration. Half of the animals receiving 114 g of CP/kg of DM went onto 144 g of CP/kg of DM; half of the animals receiving 144 g of CP/kg of DM went onto 173 g of CP/kg of DM; and half of the animals receiving 173 g of CP/kg of DM went onto 144 g of CP/kg of DM, with the remaining animals staying on their original treatment. This resulted in 6 treatments in the mid to late lactation period: 114/114, 144/144, 173/173, 114/144, 144/173, and 173/144 g of CP/kg of DM. An increase in dietary CP concentration significantly increased milk, fat, and protein yield in early lactation (d 1 to 150). Dry matter intake was also increased with increased dietary protein concentration; however, this was not significant between 144 and 173 g of CP/kg of DM. Increased dietary CP significantly increased plasma urea, albumin, and total protein concentrations but had no significant effect on NEFA, leptin, or IGF-1 concentrations. Decreasing the dietary CP concentration in mid-late lactation (d 151 to 305) from 173 to 144 g/kg of DM had no significant effect on milk yield, dry matter intake, or milk fat and protein yield, compared with animals that remained on 173 g of CP/kg of DM throughout lactation. Increasing dietary CP concentration from 144 to 173 g/kg of DM significantly increased dry matter intake compared with animals that remained on the 144 g of CP/kg of DM throughout lactation. There were no significant dietary treatment effects on live weight or body condition score change throughout the experiment. Results of this study indicate that high protein diets (up to 173 g of CP/kg of DM) improved feed intake and animal performance in early lactation (up to d 150), but thereafter, protein concentration can be reduced to 144 g of CP/kg of DM with no detrimental effects on animal performance.

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Thirty-six Holstein/Friesian cows were used in a 3 × 2 factorial design randomized-block experiment to evaluate the production and nutrient utilization responses of animals of three genetic indices (cow genetic index 90 (CGI); 950, 650 and 550), each given either a complete diet (CD) or concentrate separate from grass silage through out-ofparlour feeders (OPF). The experiment included days 11 to 160 of lactation. On the CD treatment the diet was offered ad libitum with a concentrate proportion of 0.64 (dry matter (DM) basis), while on the OPF treatment the grass silage urns offered ad libitum and the allowance of concentrate was made equal to the mean concentrate intake of the CD treatment during the previous week. The concentrate was based on barley, maize gluten, molassed sugar-beet pulp, citrus pulp, soya-bean meal, fish meal and protected fat. During the experiment eight blocks each of six animals were used in metabolism studies to determine total ration digestibility, nitrogen balance and energy utilization.No significant feeding method × genetic index interactions were found in terms of food intake, milk production or nutrient utilization. Although CGI had no significant effect on total DM intake, silage DM intake increased linearly as the cow CGI increased (P < 0.01) across the CD and OPF treatments. The high CGI cows produced respectively 6.60 and 8.25 kg/day more milk fP < 0.001) than the medium and low CGI animals without altering milk concentrations of fat and protein, but with on average a negative live-weight change with the high CGI cows. Although nitrogen digestibility was significantly higher with the low than medium CGI cows (P < 0.05), cow CGI had no significant effects on DM and energy digestibilities, daily methane energy output, heat production or the efficiency of utilization of metabolizable energy for lactation fk,) in the metabolism study. The results indicated that higher milk production with the high CGI cows was mainly attributed to an alteration in nutrient partitioning between milk energy and body energy retention.Across the three genetic indices, feeding method had no significant effect on total DM intake, although silage DM intake was 0.46 kg/day higher (P < 0-05) on the OPF treatment. However feeding the complete diet resulted in 3.04 kg/day more milk CP < 0.05) than feeding concentrate separate from silage without altering milk concentrations of fat and protein. In the nutrient metabolism study whole tract digestibilities of DM (F < 0.001), nitrogen (P < 0.05) and energy (P < 0.01) were higher on the OPF treatment, but methane energy output and heat production were similar between the two treatments.