A critique of the Cornell Net Carbohydrate and Protein System with emphasis on dairy cattle. 3. The requirements model

The Cornell Net Carbohydrate and Protein System (CNCPS) adopts energy requi rements as Mcals of metabolizable energy (ME) largely as NRC (1988), which are to be used with feed ME values as measured at the maintenance level of feeding. However, the model inserts calculated production level ME values i nto these NRC standards, equivalent to an upwards correction of about 5% to the estimates of ME requirements adopted. The energy accounting of the mod el is therefore flawed. The maintenance requirement of all breeds of dairy cattle, other than Holstein, are increased by a factor of 1.2, based on wor k with beef suckler cows. The efficiency of ME use for milk synthesis is al so raised to a constant 0.65 from the normal range of 0.62 to 0.64 specifie d in NRC (1988). The net effect on the adopted ME requirements at milk yiel ds of 30 kg/d is small, but both slope and intercept of the equation differ from NRC (1988). The model includes no effect of diet amount and compositi on upon nutrient partitioning between milk and body. Neither is there any e ffect of diet composition upon predicted milk composition, which is either an input to the model or a function of day of lactation. The body compositi on of growing heifers and cows up to four years of age is predicted by a fu nction which has a maximum body fat content of 22.5%, only 0.86 of the tota l body fat recorded in recent body composition measurements in Friesian dai ry cows. Associated estimates of maximum mobilizable body fat are one half or more below recent measurements with dairy cows. The handling of energy l osses and gains during lactation uses condition score as a measure and igno res liveweight change. This is also based on research with suckler beef cow s, and shown to over-estimate the energy equivalent of a unit change in con dition score of Holstein dairy cows. Prediction of dry matter intakes is cl oser to actual than other prediction functions available and the lag in int ake in early lactation is also accommodated well. The consequence of using mismatched energy requirements (as ME) is that the model predicts significa ntly lower milk yields at zero energy balance (or lower energy balances if milk yield is given) compared to both NRC (1988) and AFRC (1993). However, field tests of the model have shown that predicted milk yields are closer t o actual than those from other models.