Comparison of Thoroughbred Growth Data to Body Weights Predicted by the NRC

The National Research Council (NRC) recently published a method for calculating expected body weight (BW) in growing horses. The objective of this study was to compare foal BW data obtained from commercial Thoroughbred (TB) farms with BW estimates obtained using the 2007 NRC equation. A total of 9,589 observations on 885 growing TB horses were obtained. Data were sorted by age, and a subset was used for statistical analysis. Comparisons between actual BW and 2007 NRC-predicted BW at each month (0–18 months of age) were made using paired t-tests. The 2007 NRC closely predicted BW of growing TB horses at birth and between 6 and 9 months of age (within 3 kg). However, the equation underestimated BW between 1 and 5 months of age (by as much as 10 kg) and overestimated BW between 10 and 13 months of age (by as much as 9 kg). The 2007 NRC also underestimated BW by as much as 9 kg between 15 and 16 months of age. In the current study, the 2007 NRC equation did not precisely predict BW of horses at all ages. The 2007 NRC equation predicts that daily gain will decrease over time, whereas foals in the current study had an increase in daily gain between 12 and 14 months of age. Foals in the current study were raised in pasture-based management systems in which variations in forage availability and quality could have resulted in fluctuations in growth rate.