Phenotypic and genetic parameters for fertility in sheep, in terms of number of lambs born and number weaned, have been estimated in a flock of medium Peppin Merinos. Repeatability of fertility traits over all ages is low, but there is an age effect, the record at 3 years of age having a higher value than those at 2 or 4 years. The regression of subsequent performance on a difference of 1 lamb at the initial lambing was higher for the difference between 1 and 2 lambs than between 0 and 1 lamb, which indicated that selection for twins is likely to raise fertility in the current flock more rapidly than selection against barrenness. The estimate of heritability for 2 years of age was negligible for each fertility trait, but the estimates for the 3-year-old record were higher, the value for lambs born being over 0.3. Mass selection for number of lambs born at 3 years of age would thus be expected to lead to appreciable genetic progress, while there would be at least some progress in number of lambs weaned. Phenotypic and genetic correlations were also estimated between fertility at different ages and 10 sheep and wool traits measured at 15–16 months of age. Phenotypically, fertility is positively correlated with body weight and negatively with skin wrinkle score. Genetically, it is suggested from the estimates that fertility is positively correlated with body weight and staple length and negatively correlated with fibre diameter, clean scoured yield, and wrinkle score. No phenotypic or genetic correlation was found between greasy or clean wool weight and either measure of fertility. The application of the findings to breeding for higher fertility is discussed. In an appendix, a maximum likelihood method of estimating heritability for all-or-none traits in half-sib data is presented.

The variations in fleece characters and the dependence of wool production per unit area of skin on these characters were studied with 15 sheep in both a medium and a strong-wool strain of Merino. Small but significant differences in staple length and fibre diameter were found between regions on the body, whereas differences in density were large. The variation in density was about three times as large as those in staple length and fibre diameter. Distinct dorsoventral and anteroposterior gradients over the body existed for fibre density, but not for staple length and fibre diameter. The influences of the fleece characters on wool production per unit area were somewhat different in the two strains, and changed with level of production. Among the medium-wool sheep, fibre density had the largest effect on production, with staple length less and mean fibre cross-sectional area least. Among the strong-wool sheep, length was more important than density, which in turn was more important than fibre cross-sectional area. The combined data indicated that as mean wool weight per unit area increased, the influence of density rose to a maximum and then diminished, whereupon mean fibre volume became the main contributor to wool weight. For different positions on the body of individual sheep, the dependence of wool production per unit area on the fleece characters was found to be similar in the two strains. Fibre density had the major effect in determining the level of production, whereas the influences of staple length and fibre area were negligible.

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.

Aflock of New Zealand Halfbred hoggets and three flocks of Merino hoggets were shorn and wrinkle score determined. There was a small (0.2 µm) decrease in fibre diameter with each unit of increase in wrinkle score (P = 0.003) in one Merino flock (Flock C), but this trend was not significant in the other three flocks. The coefficient of variation of fibre diameter increased with wrinkle score (0.9% per unit increase) in the Halfbred flock (Flock A, P < 0.001) and two of the Merino flocks (P = 0.004 in Flock C, P = 0.005 in Flock D) but the trend was not significant in Flock B. Greasy fleece weight increased significantly with increasing wrinkle score in all four flocks (P = 0.012, < 0.001, < 0.001, and < 0.013 for Flocks A, B, C, and D respectively). Yield was not measured for Flock B, did not change with wrinkle score in the Halfbreds (Flock A), and tended to decrease with wrinkle score in the other two flocks (by 0.4% per unit increase), although the relationship was significant in Flock C only (P = 0.040). The proportional change in yield was small, so the estimated clean fleece weight increased with wrinkle score in the Halfbreds (P = 0.011) and Flock C (P < 0.001) but not Flock D (P = 0.076). Staple length was measured in the Halfbreds, where it did not change with wrinkle score (P = 0.476), and in one flock of Merinos (Flock C) where it decreased by 3.4 mm per unit increase in wrinkle score (P < 0.001). In a second experiment, a flock of adult A03053; Online publication date

Estimates are given of the change in the level of production of 10 fleece characteristics of four Merino, one Polwarth, and three Corriedale flocks, in Victoria associated with variation in property, breed, season, property x season, age, and reproductive status. There was a decrease with age of characteristics associated with wool weight, and a decline in general wool quality. The decline was approximately linear for most characteristics from ewes of 3 years of age. Lactation was generally more severe than pregnancy in reducing wool production. The estimates of the effect of age on wool characteristics are used to assess the likely quantity and quality of wool produced by flocks of different age structures. These estimates may be used in conjunction with reproductive performance data to plan for maximum wool production.