The use of High Volume Instruments (HVI) for classing cotton (Gossypium spp.) and the increased use of rotor spinning frames by the textile industry has resulted in fiber strength being the most important component of cotton fiber quality. An accurate measure of this trait is essential if; 1) the producer is to receive appropriate compensation for the quality of the fiber he delivers, 2) the spinning mill is to be efficient in its selection of bales, and 3) the breeder is to effectively select material with improved fiber and yarn strength.

While progress has been made in improving the precision (repeatability) of HVI strength measurements, the accuracy in terms of its relationship with yarn strength remains of concern. Two different instrument manufacturers supply HVI equipment for USDA-AMS Classing Offices. The two instruments employ different procedures in determining fiber strength and there is concern over whether they generate the same results.

Also, there is evidence that some cotton varieties test differently on HVI relative to other varieties, but that the differences are not reflected in yarn strength. This is of particular concern to plant breeders who are attempting to develop new varieties with improved fiber and yarn strength through selection.

Research was initiated in 1992 with the following objectives: 1) to characterize the fiber and yarn strength relationships in varieties commonly grown in the Southeast; 2) to evaluate the effectiveness of the different HVI strength instruments in determining the fiber strength of different cotton genotypes; 3) to determine the most effective method of evaluating fiber strength of selections in a breeding program.

Eleven cotton varieties, selected to represent adaptability in the Southeast and a range of fiber properties, were planted in 1992 and 1993 at the Pee Dee Research and Education Center in Florence, SC using a randomized complete block design with four replications. Plots were mechanically harvested and ginned on a modified commercial gin with no lint cleaning. Fiber from these plots was analyzed at the USDA-AMS lab in Clemson, SC using the stelometer, MCI-HVI, and Spinlab/Uster HVI. In 1993, fiber strength measurements were also determined using a universal strength tester under development by engineers with USDA-ARS at Clemson. Remaining portions of each sample were spun into 22's yarns using both ring and rotor spinning systems. Skein Strength and single end tenacity were determined from each yarn.

Analysis of variance revealed highly significant differences among varieties for all of the fiber and yarn strength measurements. A significant interaction between varieties and fiber strength instruments was detected in both years. The effect was not only in magnitude, but in the ranking of varieties. The ranking of varieties by stelometer was in much better agreement with yarn strength rankings than either HVI ranking. Stelometer consistently produced higher correlations with yarn strength than the HVI instruments with an average r² of 76% while the correlations for HVI strength were not statistically significant. The stelometer measurement correlated equally well with the strength of ring and rotor spun yarns. The strength measurements from both HVI instruments correlated better with strength of rotor spun yarns than with ring spun yarns. The universal strength tester produced an average correlation with yarn strength equal to that of the stelometer, but showed a better relationship with rotor spun yarns than with ring spun yarns.

It appears from these results that the fiber strength measurement as determined by the two HVI instruments is of little value in predicting strength of a ring or rotor spun yarn. While the stelometer is vastly superior to HVI in the accuracy of its measurement, it is too slow and tedious to be adapted to HVI. A universal tester is being developed that offers hope for addressing the needs for accuracy, precision, and adaptability to HVI. However, until this instrument becomes available, breeders should make their selections on the basis of stelometer data in order to have a reasonable expectation of improving yarn strength.