Intake and milk production of goats grazing sulla forage under different stocking rates

This investigation aimed to individuate the dietary factors affecting the milk urea nitrogen (MUN) concentrationin goats grazing herbaceous pasture and, particularly, to verify the relationship linking the dietcrude protein (CP) content to MUN.A total of 205 individual observations regarding dietary and milk variables of 37 Girgentana goats involvedin two experiments were used. Goats, averaging 154±14 days in milk and 38.1±5.4 kg of live weight,grazed on swards and received 500 g/d of barley meal. Sward biomass, herbage selected by goats andindividual milk yield were measured and sampled weekly. The herbage intake and diet digestibility wereestimated by the n-alkane method. Milk urea content was determined by an enzymatic method and transformedin MUN (MUN=urea*0.4665).The MUN concentration (9.7-35.4 mg/dl) was positively correlated with diet CP content (13.7-26.0% ofdry matter (DM); r=0.76; P<0.001), pasture allowance (39-151 kg DM/goat; r=0.42; P<0.001), diet netenergy for lactation concentration (NEL) (1.5-1.9 Mcal/kg DM; r=0.37; P<0.001) and milk yield (320-2250 g/d; r=0.25; P<0.001), and negatively related with NDF (18.7-37.4% DM; r=-0.69; P<0.001) anddiet digestibility (72.6-92.5%; r=-0.33; P< 0.001).The stepwise selection from dietary variables and milk yield showed dietary CP percentage to be the singlevariable explaining the most variation in MUN (R2=0.56; P< 0.0001). The other variables entering into themodel were diet NDF, 3.5% fat-corrected milk (FCM), DM intake and NDF intake (total R2=0.66). Includingthe CP/NEL and CP/NDF ratios of diet in the stepwise regression model, the CP/NDF ratio alone explained60.1% of MUN variability, followed by barley proportion in the diet, FCM and diet CP concentration, absorbingan extra 4.6% of MUN variability.A linear regression, fitting mean feeding treatment per time data of MUN and dietary CP concentration(n=28)[CP(% of DM)=6.91±1.42+0.61±0.06*MUN (mg/dl); R²=0.79; P<0.0001], suggests that MUNcould be used for predicting the CP content of the diet, as a tool for developing feeding strategies aimedat balancing the rations of grazing goats through adequate supplementation. Further data from experimentson grazing goats in different environmental conditions are required in order to define a more robustrelationship by which to predict the dietary CP content by MUN.