Triiodothyronine influences digesta kinetics and methane yield in sheep

Abstract

Methane yield from ruminants is positively correlated with mean retention time (MRT) of digesta, which is known to be influenced by the hormone triiodothyronine (T₃). We hypothesised that a decrease in the MRT in the rumen in response to administration of a T₃ solution to sheep would reduce their methane yield. To test this hypothesis, 10 mature Merino wethers were injected with T₃ (300 mg) on two different protocols (daily; n = 5 and every second day; n = 5) and the effect on daily methane yield, digesta MRT, DM digestibility, rumen volatile fatty acid concentrations, microbial protein output and plasma T₃ concentrations studied. Compared with when injected with saline (control), injection of sheep with T₃ every second day resulted in decreased methane yield (P < 0.05) and lower acetate (P < 0.001), butyrate (P < 0.001) and propionate (P < 0.01) concentrations in the rumen. MRT of digesta, derived from faecal excretion of CoEDTA and Cr-mordanted fibre, were reduced in the total tract (P < 0.001) and hindgut (P < 0.01) but not in the rumen (P > 0.05). DM digestibility was not affected by injection of T₃ every second day but water intake (P < 0.05) and urine output (P < 0.01) were increased. When sheep were injected with T₃ daily, changes were only observed in plasma T₃ concentration (P < 0.001) and volume of CO₂ produced (P < 0.05). The results indicate that increasing plasma concentration of the thyroid hormone T₃ within physiological levels reduces digesta retention time, especially retention time in the hindgut and leads to a reduction in enteric methane yield. Further work is warranted to assess whether plasma T₃ concentrations may be indicative of enteric methane yield.