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 selectio n in dairy cows fed perennial ryegrass (Lolium perenne)/white clover ( Trifolium repens) herbages was evaluated. Pairs of animals were offere d 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 peren nial ryegrass/white clover sward and the appropriate amount was fed in four feeds during the day. Individual intakes and the white clover pr oportion of the diet were estimated during a 12-d period using the n-a lkane technique. Animals were dosed twice daily with paper pellets con taining dotriacontane (C-32- alkane). Faecal grab samples were collect ed after the morning and afternoon milking. Three least-squares optimi zation methods were compared in calculating the white clover proportio n in the diet; then, total DM intake was calculated. The different lea st-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 calculat ed and actual intake, the difference as a proportion of the total inta ke and estimated white clover content of the diet. The difference betw een 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 t he 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 resu lts suggest that accurate herbage intake estimates can be achieved in dairy cows grazing perennial ryegrass/white clove swards if representa tive samples from herbage consumed can be collected.