An evaluation of contrasting C 32 alkane dosing and faecal sampling regimes to estimate herbage dry matter intake by beef cattle

Two experiments were carried out to evaluate different dosing and sampling regimes for estimating the dry matter intake (DMI) of vegetation by grazing beef cattle. Experiment 1 compared the DMI of a perennial ryegrass pasture by 48 grazing beef cattle dosed with C32 n-alkane and faecally sampled at different time intervals. The cattle, of dairy and suckler origin, were balanced by origin and allocated to one of three alkane-dosing treatments (n=16): T1, dosed once daily (10.30 h) with two 500 mg boluses of C32 alkanes; T2, dosed twice daily (09.00 and 16.00 h) with a 500 mg bolus of C32 n-alkane; and T3, dosed twice daily (07.00 and 19.00 h) with a 500 mg bolus of C32 n-alkane. Faecal samples were collected concurrently with dosing in each treatment. Dosing interval had no effect on estimated DMI, when calculated using the n-alkane content of faeces sampled concurrently with dosing, suggesting that once-daily dosing with concurrent faecal sampling is adequate to estimate DMI by beef cattle. The objective of Expt 2 was to compare the accuracy of herbage DMI estimated using a once-daily C32 alkane dosing regime and n-alkane concentrations in faeces sampled at 09.00, 13.00, 17.00 and 21.00 h, against DMI measured directly. Twelve Holstein-Friesian bulls (mean body weight 265 kg) were housed individually in digestibility stalls and offered harvested perennial ryegrass or harvested semi-natural upland vegetation, both ad libitum. The DMI estimated by once-daily dosing with n-alkane and concurrent per rectum faecal sampling did not differ significantly from the directly measured DMI for either herbage type. It is concluded that a once-daily alkane dosing and concurrent faecal sampling protocol is adequate to reliably estimate the DMI of both upland and lowland pasture vegetation by beef cattle.