A study of the efficiency of wool growth and its relation to fleece weight of grazing sheep

Fleece weight differences between individual sheep within a flock is a common feature. Extent of this variation has been calculated in Merino and allied breeds in terms of coefficient of variation and found to be 10 to 17 per cent for greasy fleece weight and 12 to 17 per cent for clean fleece weight (Morley 1951, 1955). Of the total variation in wool production in Romneys 10 to 15 per cent is due to heredity and the rest is due to environment (Rae, 1947). Environmental causes influencing the variation in fleece weight on breeding ewes are health, age, intake, pregnancy and lactation. Some will be true in case of non-breeding animals except for pregnancy and lactation. The most important factor affecting the environmental variation in wool production is nutrition (Henderson, 1953). The major factor affecting the variation in wool production between individuals in a flock will be due to animal’s capacity to eat and parts of this variation can be accounted for by individual’s efficiency of the conversion of available nutrients into wool. So for the maximum production of wool per acre, we not only want the highest producer but the most efficient converter also. The measurement of efficiency of wool production under grazing condition is difficult because easy and accurate methods for the estimation of their intake is lacking. To overcome this difficulty Australian workers (Weston 1959, Schinckel 1960) have measured the efficiency of wool production under controlled condition (in the pens) and correlated with total production of the same animals in the field. Their results show that efficiency of wool production measured in pens in highly correlated with total wool production in the field, showing thereby that efficient animal under pen condition is also efficient in the field. They also found that the relative efficiencies between the animals do not change with the change in quality or quantity of feed (Dolling and Moore 1961). All of this work showed the validity of selection based on the efficiency of the animals measured under controlled conditions. High correlation values between wool production in the field and controlled condition so far has been reported in case of Merino sheep. This breed has higher values for the repeatability of wool production (Morley 1951) than British breeds (Blackwell and Henderson 1955). The present experiment was undertaken to try and establish whether the correlation values – between efficiency measured under controlled conditions and total wool production in the field – will be high enough in case of Romneys and Corriendales also which have lower repeatability of wool production. This work also included the study of the main causes of variation in wool production which have been reviewed in the text.