Joe H. Bouton¹ and Roger N. Gates²

1Dept. of Crop and Soil Sciences, University of Georgia, Athens, GA 30602; and ²USDA-ARS, Coastal Plain Experiment Station, Tifton, GA 31793

A standard test to screen alfalfa cultivars for grazing tolerance has been included in the NAAIC Standard Tests to Characterize Alfalfa Cultivars manual (Bouton and Smith, 1996). The test relies on overgrazing during screening and is not intended to be used as a grazing recommendation for producers. Moreoever, re-selection and intermating of surviving plants from non-tolerant germplasms in a range of fall dormancy groups after overgrazing showed increased grazing tolerance as measured by procedures identical to the standard test (Smith and Bouton, 1993). However, the question arose of whether cultivars selected and tested under the specificity of overgrazing would be useful in management systems recommended for alfalfa. Therefore, the objective of this study was to test non-dormant populations selected under overgrazing against their parental germplasm in the following management conditions: grazing with continuous stocking, grazing with rotational stocking, and standard hay harvesting.

The experimental design was a split plot arranged in 6 blocks with management conditions of continuous grazing, rotational grazing, and hay harvest as main plots and alfalfa entries as sub-plots. ‘Florida 77', ABT 805, a grazing tolerant cultivar essentially derived from Florida 77 (Bouton et al. 1997), ‘Diamond’, Diamond GZ2, a grazing tolerant germplasm from Diamond, along with Alfagraze and Apollo, the grazing tolerant and intolerant checks from the standard test, respectively, were the alfalfa entries. The experiment was sown within a 0.5 ha grazing paddock containing 2-4 beef steers. Land preparation, liming, fertilization, and pest control for establishment and maintenance are the same as those used for an alfalfa yield trial. Step-in electric fence was used around each main plot to control the desired management conditions with all animal excluded from the hay main plots at all times, animals allowed access to the rotational grazing area for 5-7 days then excluded for 28 days, and animals allowed access to the continuous main plots all the time.

As expected, cultivars developed with selection after overgrazing (Alfagraze, ABT 805, and Diamond GZ2) showed the highest survival rate after 2 years in the grazed plots with continuous stocking (essentially the standard test). These same entries also showed the highest survival rate in the rotational grazing and hay harvest treatment areas during the same time period. Therefore, in all management treatments, the grazing tolerant population possessed significantly better survival than the parental population from which it was derived. In the hay management area, yield was not different among all entries during the first year, but the grazing tolerant populations showed higher yields during the second growing season. These studies indicate that grazing tolerance achieved through overgrazing results in alfalfa germplasm with better performance in any recommended management regime.

References

Bouton, J.H., and S.R. Smith, Jr.. 1996. Standard test to characterize alfalfa cultivar tolerance to intensive grazing with continuous stocking. p. A-8. In C.C. Fox, R. Berberet, F.A. Gray, C.R. Grau, D.L. Jessen, and M.A. Peterson (ed.) Standard Tests to Characterize Alfalfa Cultivars (Third Edition). North American Alfalfa Improvement Conference, Beltsville, MD.

Bouton, J.H., R.N. Gates, D.T. Wood, and P.R. Utley. 1997. Registration of 'ABT 805' alfalfa. Crop Sci. 37:293.

Smith, S.R., Jr., and J.H. Bouton. 1993. Selection within alfalfa cultivars for persistence under continuous stocking. Crop Sci. 33:1321-1328.

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