Interpretive Summary: Use of gallic acid and hydrolyzable tannins to reduce methane emission and nitrogen excretion in beef cattle fed a diet containing alfalfa silage.
By: Jackie Walling
Ruminants struggle to efficiently utilize dietary nitrogen (N) and it is thought enteric methane (CH4) and nitrous oxide (N2O) emitted from manure may be contributing to greenhouse gases. A new article, coming soon in the Journal of Animal Science determines the effects different sources and forms of hydrolyzable tannins (HT) have on apparent total-tract digestibility, CH4 production, and nitrogen utilization in beef cattle fed alfalfa silage diets.
Eight ruminally cannulated heifers fed a high protein forage diet were separated into two groups by weight. A double 4x4 Latin Square design was used consisting of four 28-day periods with a seven-day washout between periods. HT are secondary compounds in plants that have been shown to improve N utilization and reduce CH4 production. It is hypothesized that source, unit structure, and molecular weight of tannins may cause different effects. Four dietary treatment groups evaluated one subunit and two sources of tannins. The groups were Control (no HT), GA (gallic acid subunit; 1.5% of diet), TA (tannic acid source; 1.5% of diet), and CN (chestnut source; 2.0% of diet).
For apparent digestibility, only crude protein (CP) digestibility decreased for TA and CN treatment groups compared to the other two groups. There were no differences for restricted dry matter, nutrient intakes, or ruminal pH. Total volatile fatty acid concentrations varied by time. Most notably, total VFA concentration increased the most for GA. Plasma urea N concentration was lower for all treatment groups compared to the control.
For CH4 production, dry matter intake (DMI) and the amount produced did not differ among groups, but the ratio of CH4 to DMI decreased in GA. Also, gross energy and digestible energy emitted as CH4 decreased.
For nitrogen utilization, treatments did not affect intake, but the excretion of N output was altered. Fecal output and fecal N increased for TA and CN which was a result of the CP digestibility and N in urine moving over to feces. For TA, CN, and GA, urea N to urinary N was reduced compared to controls, with GA having the highest reduction of urea and uric acid.
Overall, HT treatment groups did not negatively affect palatability or energy availabilities in cattle diets, but the form of HT can affect the way cattle respond. More complex and heavier molecular weights are thought to decrease CP digestibility as indicated by the effect of TA and CN compared to GA. Because of this, GA is thought to be the most effective tannin for reducing CH4 and N2O emissions without impacting CP digestibility.