The nutrient intakes of brood mares grazing native and improved sub-tropical pastures were determined on five occasions during the winter, spring and summer of two breeding seasons. One half of the mares was supplemented with concentrates supplying energy and protein to determine whether this could improve their reproductive efficiency and production.
In the first experiment, 24 brood mares in good body condition (condition scores 5 to 7, scale 1 to 9) were allocated to two groups, supplemented and control. The supplemented mares consumed pasture and molasses ad libitum and 2 kg DM/head/d pellets containing 24% crude protein. The control mares consumed pasture and molasses.
The supplemented mares consumed less molasses and pasture than the control mares. The nitrogen requirements of the control lactating mares were not met in spring, however, the nutritional requirements of the mares appeared to be met in summer.
The protein supplement had no effect on the reproductive performance or plasma progesterone concentrations of the dry or lactating mares. The foals of the supplemented mares gained more weight and height during their first 90 days of life than the foals of the control mares. In the second year of the study, 29 brood mares in thinner body condition than in the first year of the study (condition scores 3 to 5) were allocated to two groups, supplemented and control. The supplemented mares consumed pasture and 2 kg DM/head/d pellets containing 24% crude protein and the control mares consumed pasture.
As in the first year of the study the supplement decreased the pasture intakes of the mares provided there was sufficient pasture available for ad libitum intake. The nutritional requirements of the pregnant and lactating mares were not met in winter and spring, but were met in summer. The protein supplement had no effect on the reproductive performance or plasma progesterone concentrations of the dry or lactating mares. Foals of the supplemented mares gained more weight during their first 90 days of life than the foals of the control mares. The protein supplement did not influence intakes of milk or milk nutrients by the foals or milk composition. Foal milk intakes increased during their first 2 months of life. Stage of lactation had a significant effect on milk composition.
A further study determined milk production and composition and growth rates of foals of mares fed low protein diets supplemented with urea. Six mares were handfed ad libitum for 71 days either a low protein diet or the same diet with added urea. The mares were unable to consume sufficient quantities of either diet to meet their nitrogen requirements and all lost weight. Adding urea to the diet raised urea concentrations in the milk, decreased the mares' feed intake and increased weight loss. Low concentrations of protein were measured in milk from mares on both diets. Milk intakes of all the foals were reduced when compared to the intakes of foals sucking grazing mares and they had poor growth rates. The dynamics of nitrogen metabolism in horses were also investigated. The horses were fed either a low nitrogen diet or the same diet supplemented with sufficient urea or soybean meal to meet their theoretical nitrogen requirements.
There were no differences in dry matter, organic matter or neutral detergent fibre digestibilities for the three diets. Nitrogen digestibilities and digestible nitrogen intakes were similar for the urea and soybean supplemented diets and very low for the low nitrogen diet.
Plasma urea was degraded in the digestive tract to ammonia for all three diets. Ammonia was utilised by the large intestinal microbes. Ammonia from the colon was absorbed into the blood and converted to urea. It was also incorporated into plasma proteins.
The horses fed the low nitrogen diet degraded a greater proportion of endogenous urea in the digestive tract than did horses fed the urea or the soybean supplemented diet. However, the horses fed the urea diet had the highest degradation rate of urea. The quantity of urea degraded in the digestive tract of horses fed the urea or low nitrogen diet could not compensate for a lack of dietary crude protein in these diets. The horses were in a negative nitrogen balance when fed the low nitrogen and urea supplemented diets and a positive nitrogen balance when fed the soybean supplemented diet. Dietary urea supplementation did not benefit the horses.