Comparison of techniques to assess the concentration of nutrients consumed by dairy cows grazing pasture in Victoria

Abstract

Two experiments were undertaken to compare the use of pre- and post-grazing sampling of herbage and a cage technique for estimating the concentrations of nutrients consumed by dairy cows grazing pasture in Victoria. The two techniques were compared through spring, summer and autumn in an onfarm monitoring project (experiment 1) and in a controlled experiment where grazing intensity was varied in spring (experiment 2). In experiment 1, there were no significant (P>0.05) differences between techniques in relation to the nutritive characteristics of the herbage consumed, with the exception of herbage calcium. The calcium concentrations of the cage herbage samples were significantly (P<0.05) higher than those calculated from pre- and post-grazing measurements. In experiment 2, when grazing was hard, both methods of estimating the in vitro DM digestibility of the pasture consumed produced similar (P>0.05) estimates. When grazing was lax, the estimated digestibility using the pre/post method was 13 digestibility units higher (P<0.05) than that estimated using the simulated grazing cuts from herbage protected from grazing by cages. In contrast, there were no differences (P>0.05) in crude protein content of the pasture eaten. In conclusion, the cage technique will generally give the most acceptable results, but the sampling of pre- and post-grazed herbage seems to be suitable in all situations except under lax grazing conditions where the removal of small amounts of herbage necessitates more extensive sampling than was undertaken here.