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The Effect of Weaning Age on Foal Growth and Bone DensityBy Kentucky Equine Research Staff · December 4, 2013

Weaning can be one of the most stressful events in a foal’s life, often resulting in a decreased growth rate, increased susceptibility to infectious disease, and an increased risk of self-induced injury. Foals may take several weeks to resume their rapid growth following weaning, which may lead to a competitive disadvantage in foals intended for show or sale in late summer and early fall. More importantly, loss of the mare’s milk may reduce the nutrients available for bone formation during the postweaning period.

A study was carried out at the University of Kentucky to assess the effect of the weaning process and weaning age on foal growth and bone density. Weaning ages of 4.5 and six months were chosen because they represented ages commonly used in the industry. Foals weaned at six months should be relying less on the dam’s milk and more on solid feed for their nutrients at the time of weaning. Therefore, the researchers hypothesized that the growth and bone density of foals weaned at six months should be affected less by weaning than in foals weaned at 4.5 months.

Four Quarter Horse (QH) foals and three Thoroughbred (TB) foals were weaned at 140 days of age (4.5 months), and eight foals (four QH, four TB) were weaned at 182 days of age (six months). Foals were weaned under identical management conditions, beginning in late June and continuing through mid-October. Body weight (BW), withers height (WH), and cannon circumference (CC) measurements were obtained from each foal at three-week intervals before and after weaning. Measurements were collected from 119 days of age through 224 days of age in foals weaned at 4.5 months and through 266 days of age in foals weaned at six months. Additional BW measurements were obtained from all foals at one week postweaning. Dorsopalmar radiographs of the right and left third metacarpals (MCIII) were also obtained at three-week intervals for the determination of radiographic bone density.

Average daily gain. Foals gained an average of 0.83 kg/d (1.8 lb) during the three-week interval prior to weaning. Average daily gain (ADG) decreased to 0.13 kg (0.28 lb) in the first week postweaning and remained lower than the preweaning ADG through three weeks postweaning. The decline between pre- and postweaning ADG was similar between foals weaned at 4.5 months and foals weaned at six months, indicating a similar reduction in ADG in response to weaning. When compared at the same age intervals, BW was similar between weaning groups.

Withers height. The gain in WH was similar before and after weaning, and no differences in WH were observed between foals weaned at 4.5 months and foals weaned at six months when compared at similar ages. Therefore, weaning did not appear to affect WH growth.

Cannon circumference. The postweaning gain in CC was less than the preweaning gain in foals weaned at 4.5 months, but not in foals weaned at six months. As a result, foals weaned at 4.5 months had smaller CC at 161 days and 182 days than foals weaned at six months. Thus, it appears that weaning may result in growth depression of CC in younger foals.

Bone density. The densities of the medial, medullary, and lateral areas of the right and left cannon bones continued to increase after weaning, indicating that weaning did not affect bone density. Medial bone density was greater at 140 days of age in foals weaned at six months than in foals weaned at 4.5 months; however, there were no differences in medial density between weaning groups at any age beyond 140 days. No differences were observed in medullary bone density between weaning groups. However, lateral bone density was greater at 140 days and 161 days of age in foals weaned at six months compared to foals weaned at 4.5 months. Because differences in bone density were noted before any of the foals had been weaned (i.e., the differences observed at 140 days of age), it is difficult to determine if weaning age influenced changes in bone density in response to weaning. In conclusion, weaning at six months of age may provide little growth advantage over weaning at 4.5 months of age.

Research summarized in this article was conducted by L.K. Warren et al. at the University of Kentucky. The report of this research was first published in proceedings of the Kentucky Equine Research 1997 Equine Nutrition Conference.