Abstract:
Proc. Aust. Soc. Anim. Prod. 1994 Vol. 20 THE INFLUENCE OF RATE OF CHANGE IN FEED INTAKE ON STAPLE STRENGTH D.W. PETERA, P.J. FORMANBC and S.K. BAKER AB ACSIRO Division of Animal Production, Private Bag, PO Wembley, W.A. 6014 *School of Agriculture, The University of Western Australia, Nedlands, W.A. 6009 CCurrcnt address: Dept of Agriculture, Merredin, W.A. 6415 SUMMARY Hogget wethers in condition score 2 were gradually or immediately returned (AL) to an ad libitum intake after 4 weeks of restricted feeding, or fed at maintenance throughout (C). The staple strength (SS) of the AL sheep over the experimental period was reduced when compared with the C sheep (P < 0.05). The SS in the AL group alone was significantly correlated with the rate of change in average fibre diameter and the coefficient of variation of fibre diameter. However, the point of break in wool of most AL sheep was not closely related to the time of change in feed intake. It is suggested that changes in fibre diameter may sometimes be associated with or reflect other alterations in fibre characteristics which in turn alter SS. Keywords: sheep, staple strength, fibre diameter, rate of intake. INTRODUCTION Staple strength (SS) and the point of break (POB) generally have been associated with the point of minimum fibre diameter, but the variability in and rate of change in fibre diameter (ED) appear to be the more important determinants of SS (Hansford and Kennedy 1988; Ritchie and Ralph 1990). The rate of change in FD is influenced by the body reserves of nutrients such that sheep with a low condition score show a greater rate of change in FD of their wool following sudden changes in nutrient supply (Mata et al. 1990; Revel1 1992) and a greater reduction in staple strength (Mata et al. 1990) than sheep with a higher body-condition score. Because changes in FD can be associated with changes in the nutrient supply, rapid alterations in nutrient availability may be expected to increase the rate of change in FD and thereby alter SS. The effects on SS of the rate of change in feed intake and nutrient availability following a period of restricted feed intake and liveweight loss was therefore examined in this study. MATERIALS AND METHODS Animals and treatments Twenty four 2 year old Merino wethers, with body-condition score 2, were selected from a CSIRO flock at Bakers Hill, Western Australia on the basis of uniformity of liveweight, the previous year' clean s wool weight (mean 2.65 kg) and FD (mean 19.8 ,um). Animals were randomly allocated to 1 of 3 treatment groups: control (C), fed to maintain liveweight, gradual (G), a 4 week period (Pl, days O-28) of progressively restricted feed intake to induce liveweight loss followed by a 5 week period (P2, days 29-63) in which feed on offer was gradually increased to ad libitum levels; and Ad lib (AL), the same period of restricted feed intake as group G followed by an immediate return to ad Zibitum levels. On a dry matter basis the diet consisted of 70% hammer-milled oaten hay, 13.2% oat grain, 14.2% lupin seed and 2.6% minerals (Siromin@) (in vitro dry matter digestibility 67.90/o, 1.56% nitrogen, 0.23% sulphur). The sheep were housed in individual pens, fed once daily at 0900 hours and weighed weekly before feeding. Sheep were allowed to adjust to the diet for 1 week prior to Pl. Average dry matter intakes are shown in Table 1. Table 1. Dry matter intake of sheep at maintenance (Control) or with a restricted intake for 4 weeks and then gradually (Gradual) or immediately (Ad lib) returned to an ad Zibitum intake (kg/hd.day) 273 Proc. Amt. Sot. Anim. Prod. 1994 Vol. 20 Wool measurements Dyebands applied to the midside on days 0, 29 and 63 and removed on day 91 were used in conjunction with greasy wool weight, recorded at shearing on day 91, and yield of a mid-side sample, to estimate the growth rates of clean wool (g/day). The strength of the staple over the experimental period was determined on staples clamped at the edges of the initial (day 0) and final dyebands (day 63) (Agritest, Sydney). The tex measurement was based on sequential measurements of staple diameters between these dyebands. The location of the POB was noted in relation to the dyebands. Prior to breaking the staples the length of the staple between dyebands was determined with vernier calipers. The SS of wool grown between shearing, 7 months previously (tip), and the first dyeband while sheep grazed together was used to provide a covariate of SS. Average FD (AFD) and the coefficient of variation in FD (CV-FD) were determined using a Fibre Diameter Analyser on scoured 2 mm snippets cut from dyebanded staples at the beginning and end of Pl (close to dyebands applied on days 0 and 29 respectively) and likewise at the beginning and end of P2. The CV-FD at these times were used to calculate the coefficient of variation within the period and along the staple over the entire experimental period. Another snippet was taken approximately 4 mm before the first dyeband on day 0 to provide a covariate estimate of AFD. All measurements were made on a minimum of 5 replicate staples per sheep. Rate of change of AFD in Pl or P2 was calculated as the change in AFD between the start and end of the period relative to the length of the staple grown in the period @m/mm). Statistical Analysis Data were analysed using analysis of variance for the covariate period or covariate analysis of variance for the experimental period with pairwise comparisons of means using the Tukey HSD test. Analysis of the rates of liveweight change and the correlations between SS and wool characteristics were carried out using regression analysis. RESULTS Liveweight During the experimental period there were small increases in liveweight (30 g/day) of sheep in group C (Figurel). In Pl and P2 the liveweight changes in groups G and AL differed from those in group C (P c 0.05). Sheep in groups G and AL lost liveweight at similar rates (-51 cf -75 g/day) in PI but in P2 sheep in group AL gained liveweight faster than did sheep in group G (298 cf 228 g/day) (P c 0.05). Figure 1. Liveweight (mean + s.e.m.) of sheep fed at maintenance (closed @ares), or fed to lose liveweight (Period 1) and then gradually (closed triangles) or immediately (open circles) returned to an ad Zibitum intake (Period 2) Rate of wool growth and variation in fibre diameter Growth rates of clean wool (mean + s.e.m.) in groups C, G and AL were 7.9 t 0.12, 6.9 + 0.12 and 7-O 2 0.12 g/day and 7.7 + 0.14, 8.6 IT 0.14 and 9.8 + 0.14 g/day respectively during Pl and P2. The rate 274 Proc. Aust. Sot. Anim. Prod. 1994 Vol. 20 Table 2. Average fibre diameter (AFD, pm), rate of change in average fibre diameter @m/mm) and staple strength (SS, N/ktex) in covariate and experimental periods of sheep fed at maintenance (Control) or fed a restricted intake (Period 1) before gradually (Gradual) or immediately (Ad lib) being returned to an ad Zibitum intake (Period 2) Table 3. Correlations (r2 (%)) between wool characteristics in periods Pl or P2 and staple strength of sheep fed at maintenance (Control) or fed a restricted intake during Pl before gradually (Gradual) or immediately (Ad lib) being returned to an ad Zibitum intake in P2, or the combined data of wool growth was constant in sheep in group C during the experiment and was higher in Pl (P c 0.05) and lower in P2 (P < 0.05) than the rates in groups G and AL. Sheep in groups G and AL had similar orowth rates of wool during Pl but wool growth was higher (P < 0.05) in group AL than in group G ZY during P2. There was a significant effect of treatment on AFD (P c 0.001; Table 2), and within periods the rates of changes in AFD were significantly correlated with changes in liveweight (P c 0.05). In group C sheep there was no change in AFD along the staple. After 4 weeks of progessively restricted feed intake the AFD in groups G and AL were similar and lower, at the end of Pl and start of P2, than in group C (P < 0.05). By the end of P2 AFD in all 3 groups were similar. There were no significant differences in the rates of change in AFD in groups G and AL in either Pl or F2 or in the CV-FD between groups within or between periods. The coefficient of variation along the staple in groups G and AL were higher than 275 Proc. Aust. Sot. Anim. Prod. 1994 Vol. 20 that in group C (P c 0.05). Point of break, staple strength and associations between staple strength and fibre diameter The POB in most staples (88%) from sheep in group AL occurred in the middle of P2 some time after the change in feed intake, while the POB in group C occurred at random. In group G the POB in staples from half of the sheep occurred at the dyeband associated with the change in feed intake while the POB in the remainder occurred either earlier in Pl or later in P2. The SS of all wool grown during the experiment exceeded 40 N/ktex with that of group C being similar to that of group G but higher than that of group AL (P c 0.05; Table 2). The variations in SS in wool from all 3 groups were similar but none of the wool characteristics that were measured accounted for the variation in SS in group G and only the CV-FD along the staple was associated with the variation in SS in group C (Table 3). This contrasts with the results with group AL or when data from all 3 groups were combined. DISCUSSION The reduction in SS of sheep fed ad Zibitum (AL) in P2 compared with those fed at maintenance (C), suggests that the rate of change in feed intake following a period of undernutrition can effect SS, but the changes in SS were small given the rate of change in AFD. Hansford and Kennedy (1990) also observed much lower reductions in SS with changes in diet quantity than with changes in diet quality. They postulated that these differences were due to the time taken by the rumen microbial population to adapt to dietary change, although it is unclear how this would have caused the higher rate of change in FD which was associated with the larger reduction in SS. From the present study it appears more likely that the availability of nutrients from body reserves counteracts changes in the dietary supply and balance of nutrients and has an effect on SS which, under some circumstances, is independent of the changes in FD. Mata et al. (1990) and Revel1 (1992) proposed that when body reserves of protein are depleted an increase in protein intake and absorption will result in the repletion of body reserves before responses in wool growth are maximised. Under such circumstances the protein:energy ratio in the nutrients absorbed may be critical to the rate of change in wool growth since it will affect the demand for amino acids by body tissues for growth. In this study the diet was formulated to provide a surplus of protein in the nutrients absorbed irrespective of dry matter intake. This may explain why the effect of rate of change in intake on SS was small. Alternatively, the period of underfeedin g may have been insufficient to deplete body reserves of protein to the point whereby they did not buffer against the changes in energy and protein absorbed. The positive correlations between SS and rates of change in AFD or CV-FD in the AL group and with the combined data from all 3 groups confirm previous findings (Hansford and Kennedy 1990; Ritchie and Ralph 1990). However, the rate of change in AFD or CV-FD per se may not be the primary cause of changes in SS, since they failed to account for most of the variation in SS in groups C and G. Furthermore, the POB as visually observed in most of the sheep in group AL and in 25% of sheep in group G occurred some time after the change in feed intake. Although Woods et al. (1990) concluded that nutritional stress had little effect on the intrinsic strength of wool from Romney sheep, Hunter et al. (1983) found a high positive correlation between SS and fibre intrinsic strength. The latter finding and the results of this study su ggest that under some nutritional conditions changes or variations in AFD may reflect other changes in the fibre, such as a change in intrinsic strength, which in turn alter SS. ACKNOWLEDGMENTS This study was supported in part by the WRDC. REFERENCES HANSFORD, K-A. and KENNEDY, J.P. (1988). Proc. Aust. Sot. Anim. Prod. 17: 415. HANSFORD, K.A. and KENNEDY, J.P. (1990). Proceedings 8th International Wool Textile Research Conference, Christchurch, 1: 590-8. HUNTER, L., LEEUWNER, W., SMUTS, S. and STRYDOM M. (1983). South African Wool Textile Research Institute Technical Report No. 514, pp. 1-15. MATA, G., PETER, D.W. and PURSER, D.B. (1990). Proc. Aust. Sot. Anim. Prod. 18: 132. REVELL, D.K. (1992). Ph.D. Thesis, The University of Western Australia. RITCHIE, A.J. and RALPH, I.G. (1990). Proc. Aust. Sot. Anim. Prod. 18: 543. WOODS, J.L., ORWIN, D. and NELSON, W.G. (1990) Proceedings 8th International Wool Textile Research Conference, Christchurch, 1: 557-68. 276