Growth and water metabolism of cows and progeny on fertilized and unfertilized tropical pastures

Abstract

Growth of Brahman x Shorthorn cows and their calves grazing unfertilized and fertilized pasture was studied in relation to body water content and water turnover; the main plant species in the pasture were spear grass (Heteropogon contortus) and Townsville stylo (Stylosanthes humilis). During the dry season, prior to the birth of calves, there was little difference in body weight or body composition between cows from the unfertilized and fertilized areas, despite the greater yield of pasture on the latter. The increase in body weight of both groups after the onset of the new season was due to expansion of the body water space. Body solids did not increase until c. 2 months later. The cattle from the fertilized pasture continued to increase in weight over the entire wet season at a significantly faster rate (P < 0.001) than those in the unfertilized pasture, and this latter group ceased growing a month before the end of the season. There was no significant difference in birth weight between the two groups of calves, although those from the fertilized pasture were significantly heavier at 2 weeks of age; during these 2 weeks they had received 45 % more milk, and until about 20 weeks they continued to grow more rapidly and turn over more water than the group from the unfertilized pasture. From 20 weeks the quality of the forage in the unfertilized pasture became superior, owing to the relatively high proportion of legume (Stylosanthes humilis), and the calves from the unfertilized pasture grew more rapidly until the end of the wet season. Plant nitrogen content was reflected in plasma urea nitrogen levels. The relationship between pasture quality and growth of cows and their calves is discussed.