Low methane phenotype of feedlot steers changes throughout production and depends on diet.

A better understanding of the variability in methane (CH4; g/day) emissions from beef cattle throughout production is necessary to implement mitigation programs. One hundred Angus crossbred steers were fed diets including either barley silage (BS) or corn silage (CS) throughout a backgrounding (BG) and finishing (FN) phase feedlot study. The objectives were to characterize the variability in methane yield (CH4Y; g/kg dry matter intake) among and within steers during BG and FN phases of production, assess whether the variability in CH4Y differs by silage type, examine the stability of CH4Y ranking (Low, Intermediate, or High) of steers within and across phases, and evaluate relationships between range of CH4Y, CH4 production, and feeding behavior. Cattle were fed a BG diet (70% silage and 22%-26% barley grain on a DM basis) for 121 d followed by a FN diet (15% silage and 77%-78% barley grain on a DM basis) for 158 d, with each phase comprised of two measurement periods. Steers were assigned to one of the silage sources for the entirety of the experiment with n = 50 each. Enteric CH4 was measured using GreenFeed emissions monitoring systems (C-Lock Inc., USA) for 2 wks/period in BG and 2-3 wks/period in FN. Steers were ranked based on their CH4Y (MYR) in each period and phase and categorized as Low (<0.5 SD from the mean), Intermediate (within ±0.5 SD of the mean), or High (>0.5 SD of the mean) emitters. Statistical analysis was performed within a period or phase using PROC GLIMMIX of SAS as a mixed model with fixed effects of MYR, silage, MYR×silage, and period or phase. Steer was the experimental unit, and the random effect accounted for individual variability. Rankings of individuals were compared across periods and phases to determine whether rankings changed over time. The CH4Y of steers ranked Low in period 1 were not different from Intermediate steers in period 2 of BG, with both being lower than High steers (P < 0.01). Similarly, steers ranked as Low in BG overall were not different from those ranked as Intermediate in FN for CH4Y (P < 0.01). During FN, Low and Intermediate steers differed in period 2, whether using period 1 or 2 ranking (P < 0.001). During BG, Low and Intermediate steers had a lower (P < 0.01) proportion of caproate in rumen fluid than High steers, and cattle fed BS had increased (P ≤ 0.01) propionate and valerate and decreased (P ≤ 0.01) acetate and NH3-N. During FN, Low steers had lower (P = 0.03) acetate and higher (P ≤ 0.04) propionate and isobutyrate proportions than high steers. Steers were more likely to change MYR during BG (56%) and from BG to FN (42%), compared to during FN (20%). During BG, 14% of BS and 71% of CS steers ranked as low in period 1 changed rank in period 2. When steers were ranked as low during BG, 43% (BS) and 57% (CS) had a different MYR in FN. Pearson correlation coefficients revealed a moderate to strong positive relationship between the CH4Y range and CH4 and CH4Y during FN for both silage sources (P < 0.05). For CS steers, feed intake decreased as the CH4Y range increased (P < 0.001; r = -0.61). In conclusion, the variability in CH4Y was present throughout BG and FN, and MYR was most likely to change within BG and across phases. Therefore, these results suggest that selecting steers for their MYR based on a short-term measurement may not reflect lifetime MYR within and across phases and diets.