Assessing enteric methane emissions in feedlot beef heifers: effects of seaweed supplementation and individual variation in methane yield

Abstract

Enteric methane (CH4) emissions from ruminants is an important greenhouse gas (GHG) and contribute to global warming, as well as representing energy loss for the animal. Consequently, there is growing interest in developing mitigation strategies that reduce CH4 emissions without compromising cattle health and productivity. This body of work was carried out to investigate whether dietary intervention or selecting low CH4-producing cattle technologies may potentially reduce enteric CH4 in feedlot systems across backgrounding (high-forage diet) and finishing (high-grain diets) phases. Initially, this study evaluated the effect of adding 0.60% dry matter (DM) of a multispecies seaweed blend (SWblend) in a total mixed ration (TMR) on growth performance, feeding behavior, ruminal fermentation, protozoa count, ruminal pH, GreenFeed (GF) system variables, and hematological parameters of feedlot heifers fed backgrounding and finishing diets. One hundred and twelve beef heifers (100 intact heifers + 12 cannulated heifers) were enrolled in a completely randomized block design with two treatments (control and SWblend) across two blocks. Supplementation with SWblend did not affect (P > 0.05) dry matter intake (DMI; kg/d), average daily gain (ADG; kg), feed efficiency (gain:feeding; G:F), feeding behavior patterns, or hematological indicators in either backgrounding or finishing phases. Although the seaweed blend altered (P < 0.05) ruminal fermentation during the finishing phase, including total volatile fatty acids (VFA) and ruminal ammonia (NH3) concentration, it did not reduce CH4 production (g/d), CH4 yield (CH4y; g CH4/kg DMI), or CH4 intensity (Mcal/% GEI). These results suggest that the tested multispecies seaweed blend may be safe for inclusion in feedlot diets but has limited potential to mitigate enteric CH4 emissions under the conditions of this study. Subsequently, the 100 intact feedlot beef heifers were ranked into low (<0.5 SD from the mean), intermediate (within ±0.5 SD of the mean), or high (>0.5 SD of the mean) CH4y groups (MYG) in each period by phase, to evaluate how MYG affects growth performance, feeding behavior, GF system visitation patterns, CH4 emissions, and compare the rumen microbiota of heifers in low and high MYG. As treatment did not affect CH4 emissions; treatment and pen (within block) were used as random effects. Throughout both backgrounding and finishing phases, considerable variability was observed in the GF system visitation and CH4 emissions both within groups and by individual heifers. Low MYG had lower (P < 0.05) CH4 production, CH4y, and CH4 intensity in both dietary phases. Heifers classified as low MYG demonstrated greater (P < 0.05) DMI in contrast to other MYG during both phases. There were no differences (P ≥ 0.12) between low and high MYG in rumen microflora alpha- diversity estimated by observed ASVs or in β-diversity based on Bray-Curtis dissimilarity. However, a temporal difference (P ≤ 0.04) between low and high MYG was found in alpha diversity of the last period (period 4) of backgrounding and generalized β-diversity based on UniFrac distance (α = 0.5) in the last period of the finishing phase. These findings emphasize the complexity and temporal variability of CH4 traits in feedlot systems and demonstrate the importance of evaluating CH4 emissions across and within productive phases to identify consistently low-emitting cattle. Overall findings underscore that sustainably mitigating enteric CH4 in beef production will likely require a combination of nutritional strategies, genetic selection, and improved measurement technologies, anchored in a deep understanding of rumen microbial ecology and animal behavior.