Estimation of the pasture intake of individual yearlings by controlled supplementation with natural 13C or alkanes and alcohols

• A system for automatically dispensing a supplement with a different stable isotope profile than that of the pasture has been successfully used as a more cost effective way of estimating pasture intake than using alkanes and/or alcohols as markers.
• Where δ13C values are similar amongst grazed plants, the mixture of plants consumed may have minimal impact on total intake solutions or ranking of animals.
• Intakes determined by alkanes and/or alcohols may not always be more credible than intakes determined by 13C due to the analytical variability that can occur with their use.

This study compared measuring the stable isotope, 13C, in the faeces with measuring alkanes and alcohols, using a system for automatically dispensing supplement, to determine the proportions of C3 (temperate legumes and grasses) and C4 (maize grain supplement) dietary components. These proportions enable the estimation of the total pasture intake of individual animals if the intake of one dietary component (the supplement) is known. Pasture intakes of 32 Hereford yearling bulls that had been fed known amounts of maize grain mixed with paraffin wax, while grazing C3 pasture, were estimated. Intake estimates from 13C results corrected for both diet discrimination and organic matter digestibility, using a C4 supplement, were more credible than solely correcting for diet discrimination. 13C values were similar for the C3 plants in the study so the assumed mixture of C3 plants in the diet had minimal impact on the ranking of animals for total intake. Intakes determined by alkane and alcohol concentrations in the faeces did not appear to be more credible than intakes determined by 13C due to the analytical variability that can occur with measuring alkanes and alcohols. The rank correlations of the bulls’ feed efficiencies (estimated as liveweight gain over 76 days / estimated intake) determined by different intake marker methods and assumed pasture mixes in the diet were all 0.95–0.96. When total pasture intake estimates are required, rather than intakes of all dietary components, the most cost- effective method studied was to use 13C as a marker and a C4 plant, such as maize grain, as a supplement.