Long-Term Encapsulated Nitrate Supplementation Modulates Rumen Microbial Diversity and Rumen Fermentation to Reduce Methane Emission in Grazing Steers

Abstract

This study investigated the long-term effects (13 months) of encapsulated nitrate supplementation (ENS) on enteric methane emissions, rumen fermentation parameters, ruminal bacteria, and diversity of archaea in grazing beef cattle. We used a total of thirty-two Nellore steers (initial BW of 197 ± 15.3 kg), 12 of which were fitted with rumen cannulas. For 13 months, the animals were maintained in 12 paddocks and fed a concentrate of ground corn, soybean meals, mineral supplements, and urea (URS) or encapsulated nitrate (EN) containing 70 g of EN/100 kg of BW (corresponding to 47 g NO₃ ^-/100 kg BW). Encapsulated nitrate supplementation resulted in similar forage, supplement and total DMI values as URS (<i>P</i> > 0.05), but ENS tended to increase (+48 g/d; <i>P</i> = 0.055) average daily weight gain. Daily reductions in methane emissions (-9.54 g or 18.5%) were observed with ENS when expressed as g of CH₄/kg of forage dry matter intake (fDMI) (<i>P</i> = 0.037). Lower concentrations of NH₃-N and a higher ruminal pH were observed in ENS groups 6 h after supplementation (<i>P</i> < 0.05). Total VFA rumen concentration 6 h (<i>P</i> = 0.009) and 12 h after supplementation with EN resulted in lower acetate concentrations in the rumen (<i>P</i> = 0.041). Steers supplemented with EN had a greater ruminal abundance of <i>Bacteroides, Barnesiella, Lactobacillus</i>, <i>Selenomonas, Veillonella, Succinimonas, Succinivibrio</i>, and <i>Duganella</i> sp. (<i>P</i> < 0.05), but a lower abundance of <i>Methanobrevibacter</i> sp. (<i>P</i> = 0.007). Strong negative correlations were found between daily methane emissions and Proteobacteria, Erysipelotrichaceae, Prevotellaceae, and <i>Roseburia</i>, <i>Kandleria</i>, <i>Selenomonas</i>, <i>Veillonella</i>, and <i>Succinivibrio</i> sp. (<i>P</i> < 0.05) in the rumen of ENS steers. Encapsulated nitrate is a feed additive that persistently affects enteric methane emission in grazing steers, thereby decreasing <i>Methanobrevibacter</i> abundance in the rumen. In addition, ENS can promote fumarate-reducer and lactate-producer bacteria, thereby reducing acetate production during rumen fermentation.