The use of n‐alkanes to estimate herbage intake and diet composition by dairy cows offered a perennial ryegrass/white clover mixture

The n ‐alkane technique for estimating herbage intake and diet selection in dairy cows fed perennial ryegrass ( Lolium perenne )/white clover ( Trifolium repens ) herbages was evaluated. Pairs of animals were offered either 8, 10, 12 or 14 kg dry matter (DM) d −1 of herbage alone or with 2 kg DM d −1 of barley. Fresh herbage was cut daily from a perennial ryegrass/white clover sward and the appropriate amount was fed in four feeds during the day. Individual intakes and the white clover proportion of the diet were estimated during a 12‐d period using the n ‐alkane technique. Animals were dosed twice daily with paper pellets containing dotriacontane (C 32 ‐alkane). Faecal grab samples were collected after the morning and afternoon milking. Three least‐squares optimization methods were compared in calculating the white clover proportion in the diet; then, total DM intake was calculated. The different least‐squares optimization methods gave similar predictions of the white clover content of the forage consumed. No significant ( P < 0.05 ) effects of sampling routine, concentrate (barley) fed or interactions between the two were detected with respect to the difference between calculated and actual intake, the difference as a proportion of the total intake and estimated white clover content of the diet. The difference between the calculated and actual intake ranged from 139 to 366 g DM d −1 , which resulted in a proportional difference ranging from 0.004 to 0.02 depending on sampling routine. The actual white clover content of the herbage mixture fed was 0.42 ± 0.008, whereas the estimated white clover content ranged from 0.41 ± 0.006 to 0.43 ± 0.008. The results suggest that accurate herbage intake estimates can be achieved in dairy cows grazing perennial ryegrass/white clover swards if representative samples from herbage consumed can be collected.