Evaluation of average daily gain prediction by level one of the 1996 National Research Council beef model and development of net energy adjusters.

Two data sets were developed to evaluate and refine feed energy predictions with the beef National Research Council (NRC, 1996) model level 1. The first data set included pen means of group-fed cattle from 31 growing trials (201 observations) and 17 finishing trials (154 observations) representing over 7,700 animals fed outside in dirt lots. The second data set consisted of 15 studies with individually fed cattle (916 observations) fed in a barn. In each data set, actual ADG was compared with ADG predicted with the NRC model level 1, assuming thermoneutral environmental conditions. Next, the observed ADG (kg), TDN intake (kg/d), and TDN concentration (kg/kg of DM) were used to develop equations to adjust the level 1 predicted diet NEm and NEg (diet NE adjusters) to be applied to more accurately predict ADG. In both data sets, the NRC (1996) model level 1 inaccurately predicted ADG (P < 0.001 for slope = 1; intercept = 0 when observed ADG was regressed on predicted ADG). The following nonlinear relationships to adjust NE based on observed ADG, TDN intake, and TDN concentration were all significant (P < 0.001): NE adjuster = 0.7011 x 10(-0.8562 x ADG) + 0.8042, R2 = 0.325, s(y.x) = 0.136 kg; NE adjuster = 4.795 10(-0.3689 x TDN intake) + 0.8233, R2 x = 0.714, s(y.x) = 0.157 kg; and NE adjuster = 357 x 10(-5.449 x TDN concentration) + 0.8138, R2 = 0.754, s(y.x) = 0.127 kg. An NE adjuster < 1 indicates overprediction of ADG. The average NE adjustment required for the pen-fed finishing trials was 0.820, whereas the (P < 0.001) adjustment of 0.906 for individually fed cattle indicates that the pen-fed environment increased NE requirements. The use of these equations should improve ADG prediction by the NRC (1996) model level 1, although the equations reflect limitations of the data from which they were developed and are appropriate only over the range of the developmental data set. There is a need for independent evaluation of the ability of the equations to improve ADG prediction by the NRC (1996) model level 1.