Development and evaluation of a model to predict sheep nutrient requirements and feed utilisation

A new feeding system for sheep, called MIPAF, was developed by integrating previously published equations with new
ones to predict energy and protein requirements as well as feed utilization of sheep. Special emphasis was given to
dairy sheep, whose specific needs are not considered by most sheep feeding systems, and to some of the environmental
factors that affect requirements. Original equations were added to predict fluxes in body energy reserves from body
weight (BW) and body condition score. The prediction of supply of nutrients was based on the discount system of Van
Soest. Thus, the MIPAF system predicts feed value as a function of the specific feeding level of the sheep that receive
the ration.
The ability of the MIPAF model to predict BW variations was evaluated using data from six studies with adult sheep (13
treatments with lactating ewes and 15 with dry ewes or wethers). The model predicted the variations of BW in sheep
with no bias, but with high rooted mean squared prediction error (RMSPE) (mean bias = -0.1 g/d; P > 0.1; RMSPE =
44.9 g/d; n = 28). Three extreme outliers were discarded because the treatment diets, made only of wheat straw and
supplied to mature wethers, had very low CP concentrations (less than 3.25%, DM basis).
After the outliers were removed, the prediction error improved but the mean bias became significantly different from
zero (mean bias = -12.3 g/d; P < 0.05; RMSPE = 29.6 g/d; n = 25). Prediction accuracy was different between lactating
and non lactating sheep. Variations of BW in lactating ewes were predicted with high accuracy (mean bias = 6.8
g/d; P > 0.1; RMSPE = 18.7 g/d; n = 13), while for dry ewes the model was less accurate, under predicting the variations
in BW (mean bias = -33.0 g/d, P < 0.001; RMSPE = 38.1 g/d; n = 12).
The evaluations included published experiments with sheep of diverse body sizes and physiological stages fed diverse
diets at various levels of nutrition. This suggests that the MIPAF model can be used to evaluate diets and animal performance
in a variety of production settings with good accuracy.