Sex ratio in sheep

Autor: P. D. Mullaney, K. M. Napier
Rok vydání: 1974
Předmět:
Zdroj: Journal of reproduction and fertility. 39(2)
ISSN: 0022-4251
Popis: 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
Databáze: OpenAIRE
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