Ryegrass varieties in relation to dairy cattle performance

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Abstract
Tetraploid Western Wolths, "Grasslands Paroa", and "Grasslands Ruanui" perennial ryegrasses were compared in terms of their effects on the yield and composition of milk. A winter and a spring experiment were carried out. During the winter experiment, when groups of Friesian cattle were used, the cows on Western Wolths and Paroa ryegrasses produced a higher yield of milk than those on the Ruanui ryegrass, but the milk from the Western Wolths group was significantly (P<0.05) lower in butter-fat percentage than that from the others. This reduction was associated with a decreased proportion of acetic acid and an increased proportion of propionic acid in the rumen liquor. In the spring experiment, when lactating monozygous twin cattle were used, milk yields and butterfat percentages were similar for the 3 groups, but the solids-not-fat content of the milk produced by the cows grazing Ruanui ryegrass was significantly depressed (P< 0.01). Possible reasons for the differences in nutritive value of the ryegrasses are discussed in relation to the chemical characteristics of the respective varieties.