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Beyond the Bale : June 2013
Staple Strength genetic gain 35 35 on-farm June 2013 BeYoNd the BaLe fast facts l AWI funded work to examine the effect of Staple Length (SL) on the correlation between Staple Strength (SS) and Coefficient of Variation of Fibre Diameter (FDCV). l The results showed that where SS is an important breeding objective, direct measurement of SS is better than using FDCV and best on 9 month’s wool growth or more. l Genetic gain can still be made using FDCV but progress is much slower, and it is also better to use 9 month’s wool growth or more. In many wool-growing businesses, Staple Strength (SS) is an important profit driver affecting clean price. SS is heritable and good responses to direct selection have been shown over a reasonable number of years. However direct measures of SS on individual animals is expensive and for many years breeders have used Coefficient of Variation of Fibre Diameter (FDCV) as a proxy for direct measurement of SS, since FDCV is measured and reported automatically when Fibre Diameter is now measured. FDCV is genetically moderately strongly correlated with SS. This makes FDCV a useful indirect indicator of SS, without incurring the expense of measuring individual sheep directly for SS. In recent years, breeders have been measuring their sheep at earlier ages and in shorter wool. There have been questions raised regarding the effect Staple Length (SL) has on the accuracy of SS breeding values (e.g. MERINOSELECT ASBVs), based on the observation that shorter staples have a higher SS and lower FDCV than longer staples. In order to determine whether the relationships between SL, SS and FDCV are reducing the accuracy of the SS breeding values and therefore reducing the potential genetic gain being made by ram breeders, AWI funded work with the Department of Agriculture and Food Western Australia to carry out a trial to determine the genetic parameters of SS and FDCV measured on wool with a range of different staple lengths. Using 1,000 stored wool samples obtained from a staple strength resource flock run at Katanning, Western Australia, in the 1990s, SS and FDCV was measured on full length staples (100mm), staples that were cut at 80% of their original length (80mm), and on staples that were cut at 60% of their original length (about 60mm). In all cases when the staples were cut, the base of the staple was removed so that the remaining length would replicate a shorter growing time for both the 80mm and 60mm staples – effectively representing 7 month and 9.5 month growing periods, compared to 12 months for the untrimmed staples. The results of the trial demonstrated moderate to high genetic and phenotypic correlations between the measures of FDCV and SS, together with reasonably consistent heritability estimates of SS when measured across the different staple lengths, although importantly, these declined for the shortest SL group. Technically, using shorter staples reduces the observed SS and FDCV variation between sheep in a mob, reduces the correlations between FDCV and SS, and the heritability of the key traits – thus reducing the utility of FDCV as a proxy for direct SS measurement within flocks, by reducing the accuracy of selection. As a result of these changes, the researchers predicted that the rate of genetic progress towards SS, while selecting directly on SS measurements, would decline rapidly with progressively shorter growth periods – from 5.1 N/kt per generation for 12 month growth staples, to 3.3 N/kt per generation for 9.5 month samples (~a 36% decline) to 2.1 N/ kt per generation for 7 month samples (~ a 60% decline). Selecting for SS by using FDCV as an indirect trait will result in a predicted genetic gain in SS of 1.2 N/Ktex per generation using 12 month wool samples. This is significantly less than using the direct SS measurement, and using shorter staples will result in a further decline in genetic gain to between 0.7 to 0.9 N/Ktex per generation, depending on the staple length of the wool. Thus selecting directly on SS is the best strategy. This implies that where SS is an important breeding objective it is still recommended that key animals are also directly assessed for SS. In other scenarios where less emphasis is placed on SS, this research confirmed previous findings that FDCV can be used to estimate a breeding value for SS, such as the MERINOSELECT SS ASBV. It also clearly demonstrated that the usefulness of both SS and FDCV declines where breeders use wool samples of less than 12 month’s growth. A final component of the trial was to investigate suggestions that FDCV measured between the fibres within a staple may be a better indicator of SS than FDCV measured along the staple, where the along-staple variation primarily reflects environmental influences, such as diet change or pregnancy impacts. Previous work in this area had shown that the genetic correlation between SS and FDCV measured across the fibres within a staple was much stronger than FDCV measured along the staple. It had also already been shown that heritability of FDCV measured across the fibres within a staple was also higher than FDCV measured along the staple. However the recent results demonstrated that to focus only on FDCV measured across the fibres of the staple ignores the significant contribution that FDCV measured along the staple makes to SS. Accordingly, it is suggested that overall FDCV, which contains both the variation across -fibres and along- fibres should continue to be used as the best proxy for SS. More information: Geoff Lindon, AWI Program Manager, Productivity and Animal Welfare firstname.lastname@example.org measurement of staple strength is best on 9 month’s growth or more.