Growth and calcium metabolism in horses fed varying levels of protein

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Abstract
The effect of level of protein intake on growth and calcium metabolism was studied in 24 foals. Starting at four months old, the foals were fed one of three diets containing all nutrients, with the exception of protein, at levels recommended by the United States National Research Council Subcommittee on Horse Nutrition for a 12 month period. The protein levels in the three diets were 9 percent (low protein) 14 percent (NRC recommended level) and 20 percent (high protein). The foals fed the low protein diet were changed to the high protein diet after 140 days when they were nine months old. There were no significant differences in the rates of growth in weight, height, cannon circumference or in hoof growth and feed utilisation of the horses fed the 14 or the 20 percent protein diets. However, growth, feed intake and feed utilisation by the foals fed the low (9 percent) protein diet were significantly depressed. The average daily gains for the first 140 days for the 9, 14 and 20 percent protein treatment groups were 64, 631 and 687 g in weight, 0.57, 0.83 and 0.87 mm in height and 0.04, 0.13 and 0.14 m in forescannon circumference, respectively. The average daily feed intakes for the 140 day period for the three groups were 2.7, 4.4 and 4.7 kg, respectively. After the change to the high protein diet the foals that had been fed the low protein diet maintained a higher rate of gain in bodyweight, height and cannon circumference, and utilised feed more efficiently than the other two groups throughout the second 140 days of the experiment. At the end of the second 140 days there were no longer differences in weight, height or cannon circumference among the treatment groups. Calcium and phosphorus balance studies were carried out using three foals from each treatment group when they were six months old and repeated with the same horses when they were 12 months old. Urinary calcium excretion averaged 23 to 30 percent of calcium intake and calcium absorption averaged 52 to 77 percent of intake. Neither was affected by the protein content of the diet. Bone turnover, as measured with 47Ca, was depressed in the foals fed the low protein diet. A concentration of dietary protein of 20 percent which is significantly greater than the NRC recommended level of 14 percent is neither helpful nor harmful to growing horses.