Botanical and chemcial composition of a Townsville stylo-spear grass pasture in relation to conception rate of cows

Abstract

A previously described experiment designed to study the effect of stocking rate, superphosphate, and conservation of hay on the fertility of cows had shown that only superphosphate significantly increased the conception rate. The present paper discusses the nutritional adequacy of the pastures for breeding cows.

All experimental factors significantly influenced yield and botanical composition. Pastures at the lower of two stocking rates remained dominated by perennial grasses while those at the higher rate became dominated by Townsville stylo and annual grasses. Conservation of hay increased annual species, and superphosphate increased Townsville stylo in lightly grazed pastures. Pasture yield was adequate at the lower stocking rate but became limiting at the higher rate. Superphosphate increased pasture yields during the third and fourth year.

Superphosphate increased the phosphorus and sulphur contents of the three major components —Townsville stylo, perennial grasses, and annual grasses, the nitrogen content of Townsville stylo, and the sodium content of perennial and annual grasses. Stocking rate affected chemical composition in the third and fourth year: the high stocking rate increased the nitrogen contents of the three major components, the phosphorus and sulphur contents of Townsville stylo and perennial grasses, and the sodium contents of perennial and annual grasses.

Cow conception rate was significantly and positively correlated with the phosphorus contents of the three major components, the sulphur content of Townsville stylo, and three yield attributes of annual grasses (dry matter, phosphorus, and nitrogen yield). The phosphorus content of all three components was less than 0.15% for much of the year at the two lower rates of superphosphate; only at the highest rate did the pastures approach an adequate phosphorus content for pregnant and lactating cows. Phosphorus and sulphur deficiency could have limited the utilization of Townsville stylo protein by cattle on unfertilized pastures.

Superphosphate at a rate of 126 kg/ha a year was adequate for maintaining Townsville stylo above the critical plant value for phosphorus, but inadequate to satisfy the phosphorus requirements of pregnant and lactating cows.