Milk urea as nutritional indicator in sheep grazing legume-based pastures
2009
Molle, G. | Giovanetti, V. | Cabiddu, A. | Cuccureddu, M. | Scanu, G. | Decandia, M.
A three-year study was carried out to assess the effects of different grass-legume mixtures on: (i) dietary CP and NEL (Net Energy for milk production) concentrations and milk urea level; and (ii) the relationship between milk urea and several dietary variables. Replicate groups of Sardinian sheep in early-mid lactation (January-May) rotationally grazed plots of three grass-legume mixtures consisting of common grass (Lolium rigidum, Gaudin) and different legumes, namely: Medicago polymorpha L. (burr medic, treatment BM), Trifolium subterraneum L. (subclover, treatment SC) and Hedysarum coronarium L. (sulla, treatment SU). Herbage dry matter intake and diet composition were estimated by the n-alkane method. Across the three grazing seasons the dietary CP and milk urea concentrations were consistently higher in BM than in the other treatment groups (P less than 0.05). Dietary NEL content was similar between treatments. Pooling all data (N equal to 72), a linear relationship between milk urea concentration (MUC, mg per 100 ml) and dietary CP (per cent DM) was found: MUC equal to 2.012 CP + 1.8411, R2 equal to 0.55, RMSE equal to 6.73. Significant but weaker linear relationships were also found between MUC and CP per NEL (g CP per Mcal NEL, r equal to 0.673, P less than 0.001) and tannic phenols (TP, per cent DM, r equal to -0.506, P less than 0.001). After stepwise regression analysis, besides CP, dietary TP concentration was kept in the model (R2 equal to 0.60, RMSE equal to 6.39). To conclude, the grass-legume mixture exerted a clear effect on milk urea concentration with higher values in groups having higher CP and lower tannic phenols concentrations in their diet
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