Vital statistics for an experimental flock of Merino sheep. III. Factors affecting wool and body characteristics, including the effect of age of ewe and its possible interaction with method of selection

Abstract

Estimates were made of the effects of the following factors on 10 fleece and body characteristics measured on breeding ewes aged 1½ to 10½ years in three mating groups over a period of 15 years: age of ewe, single or twin birth, age of dam, the ewe's own lambing performance, the year in which measurements were made, and the year in which each set of ewes was born. Two groups (S and MS) were under selection for high clean wool weight at 15–16 months, with a ceiling on wrinkle score and fibre diameter, while the third (C) was a random control. Changes with age were present in all characteristics and were similar in the three groups. The finding that selection on wool weight at an early age had no effect on subsequent age changes in any characteristic is of considerable importance. Greasy and clean wool weight reached a maximum at 34 years, then declined by 0.3–0.2 1b per year. Percentage clean yield, fibre diameter, body weight, and wrinkle score had maxima at 5½ to 6½ years. Staple length fell consistently by approximately 0.2 cm per year, while face cover rose consistently but slightly. Crimp number rose, fell, and rose again, while fibre number rose, fell, and remained constant from 4½ years. The chief source of increase in wool weight from l½ to 3½ years was an increase in the total number of fibres. The chief source of the subsequent fall was a decrease in fibre volume, with a minor contribution from a fall in total fibre number after 6½ years. Twin-born ewes cut 0.21 lb (4.2% of the mean) less clean wool per year over their lifetime than single-born ewes, while the progeny of 2-year-old ewes cut 0.32 lb (6.4%) less than the progeny of adults. The main source of lower weight in each case was a lower total fibre number. Pregnancy lowered clean wool weight more than lactation, the separate effects being 0.87 and 0.38 lb respectively (17.4 and 7.7% of the mean) and the combined effect 1.25 1b or 25.1%. Pregnancy lowered total fibre number but lactation had no further effect. Mean clean wool weights over all ages in the C group varied from year to year, the range being from 1.08 lb (21.6%)below the mean to 0.97 lb (19.4%) above. Differences in total fibre number contributed between one-third and two-thirds of the variation. Ewes born in consecutive years in the S and MS groups showed marked upward trends in clean wool weight, fibre number, and staple length, with a marked downward trend in crimp number and a slight upward trend in body weight. These trends demonstrate direct and correlated responses to the strong selection for high clean wool weight at 15–16 months of age, and the associated slight selection against fibre diameter and wrinkle score. The mean annual increases in clean wool weight were 0.15 and 0.11 Ib (3.0 and 2.2%) in the S and MS groups, approximately 40% of the increase arising from increased total fibre number and 40% from increased staple length. The effects of age and lambing performance can be used to predict productivity in flocks of differing age structures. As the casting age rises to 54 years changes in productivity are negligible. With a rise in casting age to 7½ years the average clean wool weight of the flock would fall by 0.14 lb, with a slight decrease in staple length and crimp number. These changes need to be balanced against any increased lambing percentage or decreased annual genetic gain due to increased generation interval. Comparison with other available figures indicates that age changes may vary from one area to another.

Wool growth of pregnant and lactating Merino ewes

“…This would be partly due to change in diameter being less easy to detect than changes in clean wool weight, for volume (weight, assuming density is constant) of wool is proportional to diameter 3 . It could also reflect a reduction in the number of active follicles (Brown et al 1966). There was no indication, however, that the reduction in wool growth during pregnancy was due to any factor other than decreased production per follicle.…”

On-farm factors affecting physical quality of Merino wool. 1. Nutrition, reproduction, health and management

“…The accepted view for Merino sheep is that as they increase in age fibre diameter (FD) increases and staple length (SL) decreases (Brown et al, 1966;Black and Reis, 1979;Hatcher et al, 2005). Such analyses did not determine if variations in liveweight were more important than age in determining FD as exemplified by the well documented effects of stocking rate on Merino wethers and ewes, where increases in liveweight with age may not occur at higher stocking rates and FD may not change (e.g.…”

Nutrition during fetal life alters annual wool production and quality in young Merino sheep

“…Jackson et al (1975) reported negative phenotypic and genetic correlations between follicle measurements (number of follicles and S:P ratio) and mean fibre diameter to 15-16 months of age in Merino sheep, and that both follicle characteristics explained a major portion of the genetic variation in clean wool weight. With respect to the effect of birthrank on wool production, it is often observed that twin-born animals produce about 4% less wool which is up to 0.2 p m broader than contemporary single-born animals (Turner 1961;Brown et al 1966;Turner et al 1968;File 1981).…”