Effect of sward dry matter digestibility on methane production, ruminal fermentation, and microbial populations of zero-grazed beef cattle.

Increasing the digestibility of pasture for grazing ruminants has been proposed as a low-cost practical means of reducing ruminant CH(4) emissions. At high feed intake levels, the proportion of energy lost as CH(4) decreases as the digestibility of the diet increases. Therefore, improving forage digestibility may improve productivity as DM and energy intake are increased. A zero-grazing experiment was conducted to determine the effect of sward DM digestibility (DMD) on DMI, CH(4) emissions, and indices of rumen fermentation of beef animals. Twelve Charolais-cross heifers were assigned to 1 of 2 treatments, with 6 heifers per dietary treatment. Additionally, 4 cannulated Aberdeen Angus-cross steers were randomly allocated to each of these 2 treatments in a crossover design. Dietary treatments consisted of swards managed to produce (i) high digestibility pasture (high DMD) or (ii) pasture with less digestibility (low DMD), both offered for ad libitum intake. All animals were zero-grazed and offered freshly cut herbage twice daily. In vitro DMD values for the high and low DMD swards were 816 and 706 g/kg of DM. Heifers offered the high DMD grass had greater (P < 0.001) daily DMI of 7.66 kg compared with 5.38 kg for those offered the low DMD grass. Heifers offered the high DMD grass had greater (P = 0.003) daily CH(4) production (193 g of CH(4)/d) than those offered the low DMD grass (138 g of CH(4)/d). However, when corrected for DMI, digestible DMI, or ingested gross energy, there was no difference (P > 0.05) in CH(4) production between dietary treatments. For cannulated steers, intake tended (P = 0.06) to be greater for the high DMD grass (5.56 vs. 4.27 kg of DM/d), but rumen protozoa (4.95 x 10(4)/mL; P = 0.62); rumen ammonia (34 mg of N/L; P = 0.24); rumen total VFA (103 mM; P = 0.58), and rumen pH (6.8; P = 0.43) did not differ between treatments. There was no difference in total bacteria numbers, relative expression of the mcrA gene, and numbers of cycles to threshold for fungi when determined using quantitative PCR between dietary treatments with mean values of 73.0 ng/microL, 0.958, and 21.75 C(T), respectively. Results of this study demonstrate that there was no difference in CH(4) production when corrected for intake or rumen fermentation variables of beef cattle offered a high or low digestibility sward.