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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. |
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