Sex ratios have been the subject of numerous investigations, particularly in studies with humans. In general, however, the literature relating to sex ratio in sheep is limited (Chapman & Lush, 1932;Henning, 1939;Rasmussen, 1940; Karam, 1957).Population studies have generally been based on an expected sex ratio of 0\m=.\5, and any factor causing marked divergence from this ratio is important. Although the proportion of entire males used in domestic animal populations in very much smaller than for females, production from castrated males is a significant proportion of the total production of wool and meat from sheep. Hence, studies of population dynamics and flock structure need to take account of factors causing variation in sex ratios. This paper examines the influence of some factors, breed and strain, age of mother, year of birth and type of birth (single or twin) on the sex ratio of sheep at birth and weaning.The data used in these analyses were collected as part of a wider study, which has been previously reported (Mullaney, 1966; Mullaney 8c Hyland, 1967). Sex ratio at birth was defined as the proportion of males born, either dead or alive; sex ratio at weaning was the proportion of males present at weaning (approximately 150 days of age). Each ewe for each breed (Merino and Corriedale) was classified according to year of birth of offspring, age of ewe, birth type of offspring (single or twin) and strain (property). The method of least squares was used to obtain estimates of the effects of each of these factors on sex ratio at birth and weaning.The overall sex ratio at birth was 0-508 (11,962 lambs) and at weaning was 0-501 (9746 lambs) (Table 1). These estimates are slightly higher than those quoted previously (Chapman & Lush, 1932;Henning, 1939;Rasmussen, 1940; Karam, 1957) and are based on larger numbers.There was a trend for the sex ratio in single births for both breeds to be higher than in twin births (0-516 versus 0-489, Merino; 0-503 versus 0-486, Corriedale), but this trend was not significant. This supports previous work which suggested a preponderance of females among twin births, probably resulting from differential intrauterine mortality of males in multiple births.The sex ratio of single lambs decreased at weaning because of the higher survival rate of females (85-6%) than of males (82-4%) ( Table 2). The survival rate of the two sexes was similar for twin lambs (both 65-7 %) and the sex ratio of this group remained constant between birth and weaning. Differential 391

A technique is described for assessing the profitability of selecting sheep for wool production. A model of the breeding system incorporating three submodels is developed: one to represent the flock from which the selection intensities for rams andewes are calculated; a second to determine gains made in wool production and the year in which they accrue; and the third uses discounting procedures to examine the economics of selecting for wool production. Flock and genetic parameters were set at 'average' values for an Australian Merino wool producer and gains were determined for a range of lambing percentages. Three measures were used to evaluate profitability; the net present value of the programme which shows overall profits, the internal rate of return, which gives the earning rate of the programme, and the payback period, which identifies the number of years before early costs are repaid. A sensitivity analysis was used to examine changes in the three measures of profitability resulting from changes in interest rate, wool price, costs for labour and computer analysis, wool testing costs and lambing percentage. Profitability was sensitive to changes in all parameters except wool testing charges, and of the remainder the producer has a degree of control only over labour costs. A programme can be profitable, however, if wool price and lambing percentage are adequate and costs can be kept at reasonably low levels. Simple breeding programmes should be studied in which labour inputs are kept to a minimum or where costs are confined to studs (nuclei) and genetic gain disseminated by the sale (migration) of rams.

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The effects of age, dam age, type of birth and rearing on weaning weight, clean fleece weight, hogget weight, fibre diameter and yield were estimated for 751 Corriedale rams aged 16-19 months, over three years. Twin born and reared rams were 20% lighter at weaning and 4% at hogget weighing than singles and cut 4% less wool. All characters except yield increased with the age of the ram, the effect being greatest for weaning weight with a regression coefficient of 0.17 kilogram per day of age. Of particular interest was fibre diameter, which increased with age; the regression coefficient was 0.026 pm per day of age. Clean fleece weight also increased with age; the regression coefficient was 0.0061 kilogram per day of age. The phenotypic correlation between clean fleece weight and fibre diameter was very high (0.45), other correlations were within previously published estimates. The relation between eight qualitative characters (head type, face cover, back cover, belly cover, wool quality, standard of feet, body size and classer grade) and three productive characters (clean fleece weight, weaning weight and hogget weight) was studied. Clean fleece weight was not correlated with face cover but was negatively correlated with belly cover (-0.30) and positively correlated with classer grade (0.41).