Growth parameters in the quarter horse

Various body measurements were collected on 136 horses from 22 leading Quarter Horse farms in the Southeast. Offspring, ranging in age from one month to maturity, from 43 stallions were included in the study. Factors developed by Cunningham and Fowler (1961) were used to adjust each measurement to a mature equivalent, sex constant basis. Several horses were measured twice at six months interval and coefficients of intraclass correlation between the two adjusted measurements ranged from .454 to .987. The mean values for all measurements at each age, except for width of chest, were larger for males than for females. Average mature values were larger for males except width of head, depth of neck, length of body, depth of hind flank and depth of fore flank. Males tended to be larger than females, for most measurements taken, from birth to three months of age. However, the females tended to grow faster from three to six months and at six months of age were about the same size as the males. Although males tended to increase in size faster from six months of age to maturity, there was little difference between males and females at maturity. Linear growth increased faster than width growth in both males and females. Males achieved 99.6 percent of their elbow to ground growth and 100 percent of their knee to ground and hock to ground growth by 12 months of age. Females developed slower in the foreleg area attaining only 97.2 percent of their mature elbow to ground growth and 98.7 percent of their knee to ground growth by 12 months of age. However, they achieved 100 percent of their hocktoground growth by six months of age. Both males and females reached 100 percent of their height at withers by 36 months of age. Heart girth was highly related (P < .01) to height at withers (r = .93), length of body (r = .95), width of quarters (r = .93), depth of fore flank (r = .95), circumference of cannon bone (r = .89) and body weight (r = .90). It was concluded that heart girth was a good indicator of many different measurements of development. The estimates of genetic parameters calculated in this study were biased due to the confounding of herd effects and sire effects. However, these results indicated that the procedures used in this study would be satisfactory for use in adjusting data from which genetic parameters are to be estimated.