Proc. 57th Southern Pasture and Forage Crop Improvement Conference, Athens, GA April 23-25, 2002
Utilizing Grazing in Development of Forage Germplasm
R.N. Gates
USDA, ARS
Coastal Plain Experiment Station, P.O. Box 748
Tifton, GA 31793-0748
Several state and federal programs have made important contributions to improved forages for the humid Southeast. With the exception of a few operations which produce hay exclusively, most forages used in the South will be grazed at some time during the life of a stand. “Improved” forages must contribute to the reliability of the forage supply. Little can be done to ameliorate the impact of environmental conditions, particularly drought, which interrupta forage supply. Germplasm improvement programs must, however, consider reliability as a most important trait. Consequently, for perennial forages, grazing tolerance and persistence are the characteristics most critical to improving forage germplasm.
Two approaches have been employed in developing persistent germplasm in the Southeast: 1) Species with exceptional persistence under grazing have been selected to improve complementary traits, such as yield or forage quality; 2) Species with desirable traits, such as high forage quality or desirable seasonal distribution, that are not inherently persistent have been selected to improve persistence.
Evaluation of plant response to grazing is important to either approach. Persistence must be verified if selection is made within a grazing tolerant species. In attempting to improve grazing tolerance with a non-persistent species, persistence must be demonstrated in elite germplasm. Exposure to grazing may also be important in the selection process.
Bermudagrass and bahiagrass are both inherently persistent and therefore have been widely distributed throughout the Southeast. The USDA-ARS forage breeding effort at Tifton, GA has successfully released improved cultivars of both species. In every case, it was important to demonstrate that grazing tolerance had not been compromised during selection.
Tifton 85 bermudagrass has been widely planted since its release in 1989. Initial grazing experiments verified high potential production, due to increased yield and forage quality, and good persistence (Hill et al., 1989). More recently, Tifton 85 has been stocked as heavily as 9 steers (940 lb) per acre from July through September without compromising persistence (Gates and Hill, 2001). Tifton 85 has also proved to be very tolerant of overseeding supporting animal performance superior to Coastal, without any stand loss (Hill and Gates, 2001).
Tifton 9 bahiagrass was selected for increased yield. When evaluated with continuous stocking, Tifton 9 supported greater carrying capacity and demonstrated persistence comparable to Pensacola. However, a more advanced selection, RRPS Cycle 14, was not as persistent. Selection resulted in substantial changes in morphology: taller, longer-leaved plants with fewer rhizomes (Werner and Burton, 1991; Pedreira and Brown, 1996). Inferior persistence has been detected with the most elite populations, RRPS Cycles 18 and 23 (Gates, 2002).
Grazing has also been important in evaluating tall fescue germplasm, attempting to develop a winter productive perennial grass for the lower South (Bouton et al., 1993a; 1993b). Using grazing animals to select persistent individual plants has provided limited improvement to new populations (Bouton et al., 2001). However, using small plots to closely control exposure to grazing with electric fence has provided a useful tool in developing recommendations for fall deferment of tall fescue planted in warm-season grass sods (Gates et al., 1999).
Alfalfa is recognized for outstanding properties as a forage crop. Non-dormant alfalfa germplasm can provide substantial cool-season growth in the lower South. Use of continuous stocking to identify persistent individuals and combining these to produce a non-dormant, grazing-tolerant alfalfa germplasm has verified the usefulness of this procedure (Bouton et al., 1998). Most recently (Bouton and Gates, unpublished) utility of this selection procedure has been demonstrated for alfalfa, even when managed with rotational stocking or mechanical harvest.
Response to grazing will remain a vital criteria for the evaluation of new forage germplasms intended for use in the Southeast. Recent research has proven convincingly that exposure to grazing can be exploited in plant selection to improve grazing tolerance. Extending this strategy to red and white clover is underway. It might also be a useful procedure to increase the grazing tolerance of desirable native plants. Verifying persistence will also be critical with inherently grazing tolerant species. The evidence resulting from sustained selection for yield with bahiagrass provides a caution that grazing tolerance can never be assumed.
REFERENCES
Bouton, J.H. and R. N. Gates. 2002. Grazing tolerant alfalfa tested under different management conditions. Agron. J. (unpublished).
Bouton, J.H., R.N. Gates, D.P. Belesky, and M. Owsley. 1993a. Yield and persistence of tall fescue in the southeastern coastal plain after removal of its endophyte. Agron. J. 85:52-55.
Bouton, J.H., R.N. Gates, G.M. Hill, M. Owsley, and D.T. Wood. 1993b. Registration of ‘Georgia 5’ Tall Fescue. Crop Sci. 33:1405.
Bouton, J.H., R.N. Gates, and C.S. Hoveland. 2001. Selection for persistence in endophyte-free Kentucky 31 tall fescue. Crop Sci. 41:1026-1028.
Bouton, J.H., R.N. Gates, and P.R. Utley. 1998. Grazing persistence and yield among nondormant alfalfas selected for grazing tolerance. J. Prod. Agric. 11:314-318.
Gates, R.N. 2002. Response of high yielding bahiagrass populations to continuous stocking. Agronomy Abstracts. ASA, Madison, WI. (In Press)
Gates, R.N. and G.M. Hill. 2001. Late season response to stocking rate of Coastal and Tifton 85 bermudagrass. c06-gates043746-O. In Agronomy Abstracts. ASA, Madison, WI.
Gates, R.N., G.M. Hill and J.H. Bouton. 1999. Wintering beef cattle on mixtures of ‘Georgia 5’ tall fescue and warm-season perennial grasses on Coastal Plain soils. J. Prod. Agric. 12:581-587.
Hill, G. M., and R. N. Gates. 2001. Coastal and Tifton 85 spring recovery following ryegrass sod-seeding. J. Anim. Sci. 79 (Suppl. 2):25. (Abstr.)
Hill, G.M., R.N. Gates, and G.W. Burton. 1993. Forage quality and grazing steer performance from Tifton 85 and Tifton 78 bermudagrass pastures. J. Anim. Sci. 71:3219-3225.
Hill, G.M., R.N. Gates, and J.W. West. 2001. Advances in bermudagrass research involving new cultivars for beef and dairy production. J. Anim. Sci. 79(E. Suppl.):E48-E58. http://www.asas.org.jas/jas0940.pdf
Pedreira, C.G.S., and R.H. Brown. 1996. Physiology, morphology, and growth of individual plants of selected and unselected bahiagrass populations. Crop Sci. 36:138-142.
Werner, B.K., and G.W. Burton. 1991. Changes in morphology and yield of Pensacola bahiagrass due to recurrent restricted phenotypic selection for yield. Crop Sci. 31:48-50.