Factors contributing to the variation in staple strength of reproducing ewes

Abstract

Proc. Aust. Soc. Anim. Prod. 1996 Vol. 21 FACTORS CONTRIBUTING TO THE VARIATION IN STAPLE STRENGTH OF REPRODUCING EWES S.M. ROBERTSONA, G.E. ROBARDSA and E.C. WOLFE B * Dept of Wool and Animal Science, University of New South Wales, Sydney, N.S.W. 2052 JJ School of Agriculture, Charles Sturt University, Wagga Wagga, N.S.W. 2678 SUMMARY Correlations with staple strength of reproductive, wool and management parameters were determined in 3 flocks of reproducing Merino ewes. The strongest associations were with wool factors, particularly the co-efficient of variation of fibre diameter (CV) and ewe weight change. Reproductive parameters, including the number of lambs born and reared, and lamb weights, had generally low or negligible correlations with staple strength. Except for CV, all other correlations were inconsistent in both magnitude and direction between flocks and lambing classes. The study indicates more detailed work is needed to define the management variables influencing the staple strength of reproducing ewes under a range of grazing conditions. Keywords: staple strength, wool growth, reproduction, management, Merino ewes INTRODUCTION Reproduction in ewes is well recognised as a factor reducing wool growth and staple strength (Corbett 1979). However, the effect of reproduction on staple strength is inconsistent, both between and within mobs. Pen and grazing studies have shown effects ranging from a dramatic reduction in staple strength (Hall 1987; Thomberry et al. 1988; Masters et al. 1993a), to the maintenance of strength at levels which do not attract price discounts (Williams and Butt 1989; Masters et al. 1993b; Butler et al. 1994). It is not clear what reproductive, nutritional or management strategies are successful in maintaining staple strength in reproducing ewes. Identification of such factors could assist the development of management strategies aimed at improving wool quality and, so, in 1994 observations were taken on 3 different flocks of Merino ewes to examine variability in staple strength within mobs, and to define the relative importance of the parameters associated with staple strength. MATERIALS AND METHODS Sheep and management Three flocks of ewes were grazed under commercial management conditions at 3 different locations. Observations were taken under severe drought conditions during 1994. Flock 1 was located at the University of New South Wales (UNSW) Hay Field Station. It comprised 450 mixed age ewes, the progeny of Merino sire evaluation trials. The ewes had been shorn 2 weeks pre-lambing, and lambed during July/August. Lambs were tagged and identified with ewes at birth. The lambs were weighed at birth (LBW), and at marking (LMW) in late August and weaning (LWW) in late September. Ewes were weighed at lamb marking (EMW) and weaning (EWW) and 2 weeks prior to the start of lambing (EPW). Two hundred and twenty-one ewes were randomly selected for wool sampling from all lamb bearing/rearing classes. Midside samples of wool were collected in mid-October. Flock 2 was located at UNSW Wellington Field Station. Two hundred and forty medium wool October shorn ewes lambed in July/August. Lambs were identified with their dams, and weighed at birth (LBW), at marking (LMW) in mid-August, early September (LXW) and weaning (LWW). Ewes were weighed prelambing (EPW), at marking (EMW) and shearing (EWW) in early September, prior to weaning. Midside samples were collected and greasy fleece-weights (GFW) recorded. Flock 3 comprised 505 medium wool ewes on a commercial property at Junee. The ewes lambed in ApriYMay. Lambing status was determined by udder examination in mid-June. Dry ewes were marked with raddle for later identification. Lambs were weaned in late July, and midside samples and fleeceweights were taken at the annual shearing in mid-August. Fleece samples were taken from all 34 dry ewes and 42 randomly selected wet ewes. No other measurements were possible. 166 Proc. Aust. Sot. Anim. Prod. 1996 Vol. 21 Wool testing The average fibre diameter (FD) and co-efficient of variation of FD (CV) were measured using an Optical Fibre Diameter Analyser (OFDA). Greasy wool staples were conditioned, and 10 staples per sample measured for length (SL) using a ruler, tested for staple strength (SS) using the Agritest system, and tip and base portions weighed to determine the position of break (POB). Statistical analysis Correlations between staple strength and other variables were determined using a standard spreadsheet software package (Excel 5). RESULTS The average wool test results for each bearing/rearing class are presented in Table 1. Compared with ewes which did not lamb, ewes which lambed had a reduction in staple strength of 7 N/Ktex in flock 1, but there was no difference in flocks 2 or 3 between ewes which did not lamb and bearing/rearing ewes. Table 1. Average staple strength of lambing classes in 3 flocks The correlations between staple strength and other variables are shown in Tables 2 and 3. Wool characteristics were associated with SS across all lambing classes within flock 1 except l/O. However, within flocks 2 and 3, wool characters were not consistently related to SS, except for CV, which was negatively associated with SS in all flocks, although the correlation did not reach significance in all categories due to small numbers of ewes. Micron, length, POB and GFW were more strongly associated with SS in flock 1 than 2 or 3. In flock 1, liveweight of ewes was not associated with SS, although there was an association with weight change over time. Greater loss between pre-lambing and marking increased SS, while loss between marking and weaning reduced SS in all classes except l/O. As lambing date progressed in flock 1, staple strength declined. This may have been because the weakest point of the staple occurred close to lambing and the ewes were shorn very close to commencement of lambing, so in earlier lambing ewes the wool grown during this period was beneath the clamp during the SS test, the results then reflecting the next weakest position along the staple. Correlations of staple strength with the number of lambs born and reared were negligible in both flocks 1 and 2, so are not shown. In flock 2, only pre-lambing ewe weight was related to SS, strongly but not significantly. Change in ewe weight was relevant only in twin-bearing and non-lambing ewes. Tine direction of change had an inconsistent effect between pre-lambing and marking, but weight gain between marking and shearing was associated with reduced staple strength. Overall, the effect of lamb weight on SS was important only in twin-bearing ewes, higher weights being associated with greater SS in both flocks 1 and 2. 167 Proc. Aust. Sot. Anim. Prod. 1996 Vol. 21 Table 2. Correlations between staple strength and various wool characteristics and production variables for flocks land 2 Table 3. Correlations between staple strength and various wool characteristics for flock 3. DISCUSSION The study indicates that reproduction need not reduce the staple strength of commercial Merino ewes, even under sub-optimal nutritional conditions. However, variation occurs between and within flocks. In all three flocks studied staple strength was more strongly associated with wool characteristics than with any reproductive parameter. This does not necessarily mean that reproduction had no effect on staple strength in these flocks, as strength was reduced in flock 1, but rather that the position of break may have been caused by a stress unrelated to reproduction, or that the individual reproductive factors measured did not reflect the whole reproductive influence. In flock 1 a greater effect of reproduction on staple strength could well have been masked by both the clamping procedure of the staple strength testing and by only a very short period of the reproductive period, mostly lactation, being measured. The former is indicated by quite strong negative correlations between staple strength and lambing date. This could also suggest that the pattern of quantitative or qualitative aspects of nutrition, even over the relatively short period of reproduction studied, could be quite important for the maintenance of staple strength. The causes cannot be determined from this study. However, the data from flock 1 indicate that shearing ewes pre-lambing may not eliminate the effects of reproduction on staple strength. 168 Proc. Aust. SW. Anim. Prod. 1996 Vol. 21 One factor consistently correlated to staple strength was coefficient of variation of fibre diameter. The observed decrease in staple strength as variation increased is consistent with other studies (Ritchie and Ralph 1990; Lewer and Ritchie 1993). Other wool measurements such as fibre diameter, staple length, greasy fleece weight and position of break were quite strongly correlated with staple strength within the data from flock 1 but not the other two flocks, although correlations were inconsistent between lambing classes within these flocks. The only other parameter with a consistent moderate to high correlation with staple strength was ewe weight change, although the time and direction of change had an inconsistent effect between flocks and lambing classes. This variation between and within flocks and between measured parameters indicates a need to more closely examine the underlying factors and management variables which may be consistently associated with staple strength, particularly those affecting the extent and direction of ewe weight change. ACKNOWLEDGEMENTS The assistance of staff at UNSW Hay and Wellington Field Stations is gratefully acknowledged, also the co-operation of flock owner Mr W. Turner who made his sheep available. Part funding for this work was provided by Australian woolgrowers and the Australian Government as S.R. was in receipt of a postgraduate scholarship from the International Wool Secretariat. REFERENCES BUTLER, L.G., GIBSON, W.R. and HEAD, G.M. (1994). Proc. Aust. Sot. Anim. Prod. 20: 269-72. CORBETT, J.L. (1979). In 'Physiological and Environmental Limitations to Wool Growth', (Eds J.L. Black and P.J.Reis) pp.79-98, (The University of New England Publishing Unit: Armidale). HALL, D.G. (1987). 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