Intestinal digestible protein balance and milk urea in lactating goats

Aim of this study was to evaluate, in lactating goats, the effects of diets with two different balances of intestinal digestible pro- tein (PDI), on milk production and milk urea level (MUL). Twenty four lactating Saanen goats, milked once a day, were divided into four pens and fed in a change-over design two diets with different concentration of PDI with Nitrogen (PDIN) or Energy (PDIE) as limiting factors: dietary treatments L (low) and H (high) had, respectively, 102 and 125 g PDIN/kg DM and 102 and 114 g PDIE/kg DM. The diets had the same forage:concen- trate ratio (45:55) but different amounts (on as fed) of maize grain and soya-bean meal (maize: 27.3 vs 17.9%; SBM: 1.6 vs 11.0%, for L and H, respectively). Chemical composition (g/kg DM) of L and H diets were: CP, 149 vs 181; NFC, 360 vs 323; aNDFom, 398 vs 400. Milk yield (MY) and MUL were individually measured, whilst DM intake (DMI) was determined for each pen. Individual DMI was calculated on the basis of MY and body weight according to INRA (2007) and adjusted for intake of the related pen. Data were statistically analysed by SAS-GLM proce- dure and linear regression analysis was performed between indi- vidual MUL and PDIN balance (PDIN intake – PDI requirement). Dry matter intake (overall mean: 2716 g/d), MY (overall mean: 2539 g/d) and milk composition were not influenced by dietary treatments. On the contrary, MUL (mg/dL) resulted higher (P<0.001) for diet H (58.3) as compared to diet L (37.1). Dietary N efficiency (milk N/intake N) was greater for the diet with lower CP concentration (0.21 vs 0.18) (P<0.001). The regression equa- tion between MUL and PDIN balance resulted as follows: MUL (mg/dL) = 0.151 x PDIN bal. (g/d) + 31.2; (n=46; r2=0.20; P=0.002). This equation shows that when the PDIN supply is equal to requirements, MUL is close to 30.