Energy relations in cattle can be quantified using open-circuit gas-quantification systems

Abstract

This study was conducted to evaluate the relationships between metabolisable energy (ME) intake and outputs of methane (CH₄), rumen-derived carbon dioxide (rCO₂), lung-derived carbon dioxide (lCO₂), and total carbon dioxide output (tCO₂) measured using an open-circuit gas-quantification system (GQS). Three trials were conducted to produce a wide range of energy intake and gas emissions to allow relationships between gas outputs and ME intake to be quantified. Gas emissions and ME intake were measured in eight Angus steers (455 ± 24.6 kg initial bodyweight; Trials 1 and 2), and in eight pregnant Angus heifers (503 ± 22.0 kg initial bodyweight; 5 months pregnant; Trial 3). Animals were fed twice daily to allow ad libitum intake in Trial 1, whereas in Trials 2 and 3, feed intake was restricted and energy density was varied to provide a wide range of ME intakes. Animals were allocated to individual pens during a 20-, 19- and 15-day experimental periods, and total faecal output was measured for the last 8, 4 and 4 days in Trials 1, 2 and 3 respectively. Gas emissions were measured for 16, 8 and 8 days after the adaptation period (4, 11 and 7 days) and each animal was allowed to visit the GQS every 2 h. Total CO₂ in breath (tCO₂) was separated into CO₂ arising from rumen fermentation (rCO₂) and CO₂ in expired air from the lungs (lCO₂) by manually identifying the eructations from normal breaths using the GQS gas-output trace. All CO₂ outputs (lCO₂, rCO₂ and tCO₂) were highly correlated with each other (r = 0.74–0.99; P < 0.01). Measurement of CO₂ output was more repeatable with fewer days of measurement than was CH₄ output. Metabolisable-energy intake was closely related to all three measures of CO₂ output (rCO₂, r = 0.69, P < 0.001; lCO₂, r = 0.70, P < 0.001; and tCO₂, r = 0.73, P < 0.001). Heat production was estimated from lCO₂ output by assuming a value of 0.85 for the respiratory quotient of metabolised products. The heat production estimated at the extrapolated zero ME intake (0.52 MJ/kg⁰·⁷⁵) was 60% higher than previous estimates of fasting heat production in cattle. However, our estimate was made under non-fasting, non-sedentary, non-thermoneutral conditions, so it may be a realistic estimate of maintenance energy requirement excluding heat increment of feeding. In conclusion, the open-circuit GQS can be used to provide estimates of the ME intake and heat production of cattle, and, as such, provides a valuable opportunity to describe the energy relations and efficiency of beef cattle in the field, with minimal interference to normal grazing patterns and behaviour.