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© 2006 Plant Management Network. Grazing-Tolerant Alfalfa Cultivars have Superior Persistence under Continuous and Rotational Stocking E. Charles Brummer, Raymond F. Baker Center for Plant Breeding, Department of Agronomy, Iowa State University, Ames 50011 Corresponding author: E. Charles Brummer. brummer@uga.edu Brummer, E. C. 2006. Grazing-tolerant alfalfa cultivars have superior persistence under continuous and rotational stocking. Online. Forage and Grazinglands doi:10.1094/FG-2006-0825-01-RS. Abstract Grazing-tolerant alfalfa (Medicago sativa L.) cultivars are developed and evaluated under continuous stocking, but this does not reflect the grazing method used by most farmers. The objective of this experiment was to compare persistence of grazing-tolerant and intolerant cultivars grown in pure stands under continuous and rotational stocking in central Iowa. Small-plot trials were established in 2001 using the tolerant cultivars ‘Alfagraze’ and ‘Samurai’ and the intolerant cultivars ‘Apollo’ and ‘5454.’ Plots were not grazed in 2001, but continuous or rotational stocking was applied from mid-May through mid-September in 2002 and 2003. All cultivars had similar stands at the end of the establishment year. Under continuous stocking, the tolerant cultivars had superior survival after 1 year compared with the intolerant cultivars, but no differences among cultivars were seen under rotational stocking after the first year. After the second year of grazing, tolerant cultivars had superior persistence compared to the intolerant cultivars under both stocking methods. This supports a conclusion that evaluation under continuous stocking accurately reflects the performance of cultivars under rotational stocking. Given that continuous stocking allows faster separation of cultivars, it should be the preferred method of evaluation. Introduction Alfalfa is the most important forage legume in the USA, with a large concentration of production in the upper Midwest (1). Although alfalfa is used primarily for hay or silage, the development of dual-purpose cultivars, such as Alfagraze (4), gave forage producers the option of either making hay or grazing their stand. Grazing-tolerant cultivars are tested under continuous stocking with high animal density in order to maximally stress the plants, enabling the identification within a selection nursery of those superior in grazing tolerance (8). A standard test that uses continuous stocking to assess grazing tolerance has been developed and approved by the North American Alfalfa Improvement Conference (3). Although the idea of using high-density continuous stocking to develop and evaluate cultivars for grazing tolerance has clearly worked, most producers who grow alfalfa for grazing will not manage their stands under continuous stocking and overgrazed conditions. Is a grazing-tolerant cultivar necessary if pastures will be rotationally stocked? If grazing-tolerant cultivars are not more persistent under rotational stocking, then variety selection could be based on yield (or other traits of importance) rather than on grazing tolerance. From a cultivar evaluation perspective, knowing whether performance under continuous stocking reflects that under rotational stocking would be a useful guide in designing experiments that will produce timely data for cultivar recommendations. Two experiments have been conducted to compare the performance of grazing-tolerant cultivars under high-density continuous stocking as well as the rotational stocking typical of high-production farms. In Manitoba, Canada, grazing-tolerant cultivars performed better than intolerant cultivars under continuous stocking and to a lesser extent under rotational stocking (7). In contrast, grazing-tolerant cultivars performed better than intolerant cultivars under rotational stocking in southern Georgia, but no differences were noted in central Georgia (2). These experiments were conducted in environments with more severe summer or winter stresses than those in the Midwestern USA. Therefore, the objective of this experiment was to test the hypothesis that grazing-tolerant cultivars are more persistent than non-grazing-tolerant cultivars under both continuous and rotational stocking in central Iowa. Procedures for Assessing Grazing Tolerance under Continuous and Rotational Stocking Two grazing experiments were established on 27 April 2001 at the Iowa State University Rhodes Research Farm in central Iowa (41°52'N, 93°10'W) on a Downs silt loam soil (fine-silty, mixed, mesic Mollic Hapludalf). The two experiments were planted approximately 20-m apart, separated by a fence to allow one to be continuously stocked and the other rotationally stocked by beef cattle (Bos taurus). Four cultivars of alfalfa, representing two grazing-tolerant cultivars (Alfagraze and Samurai) and two grazing intolerant cultivars (5454 and Apollo) were included in each experiment. Of these cultivars, Alfagraze is the tolerant check and Apollo the intolerant check for the standard test for alfalfa grazing tolerance (3). Samurai and 5454 were released more recently and are higher yielding than the other cultivars. The fall dormancy ratings (10) for these cultivars are Alfagraze = 2; Samurai = 3; and 5454 and Apollo = 4. These cultivars are resistant or highly resistant to most major diseases prevalent in Iowa, with the exception of Alfagraze, which has low resistance to Phytophthora root rot [Phytophthora medicaginis (Hansen and Maxwell)]. For each experiment, seed was drilled at 17 kg/ha into plots consisting of five rows spaced 15 cm apart and 3.7 m long. Plots were arranged in a randomized complete block design with six replications in each experiment. All seed was treated with the correct Sinorhizobium species. The entire plot area was bordered by alfalfa. No herbicides were used for weed control during establishment. Plots were mown twice during the summer of 2001 to control weeds; no grazing was imposed in the establishment year. Soil tests showed both P and K to be above recommended levels at planting and they remained high throughout the experiment. For the continuous stocking experiment, 112 kg K per ha was applied in April 2003, but no other fertilizer was applied to either experiment. The climatological conditions during the experiment did not exhibit any severe departures from those normally encountered in central Iowa, as suggested by variety test data gathered at Ames, IA during the period of this study (6). Grazing management treatments were imposed as follows. For continuous stocking, beef cattle were stocked to maintain the available forage at or below 5 cm throughout the grazing season in order to maximally stress the plants. Plots were grazed from mid-May until mid-September (approximately 120 days) in both 2002 and 2003. Sufficient cattle were introduced into the paddock at the onset of grazing to remove all forage to a short stubble within a week, after which stocking rate was adjusted to maintain the sward at the desired height. Cattle were supplemented with hay as needed to maintain their body condition. For the rotational stocking experiment, cattle were introduced on the same day as the continuously stocked experiment but removed after approximately 5 days, when the standing vegetation had been reduced to a 10- to 15-cm stubble. Cattle were rotated onto this experiment throughout the grazing period, with the typical length of rotation being 5 days on the paddock followed by a 30-day rest period. The length of rest period varied from 30 to 40 days depending on regrowth. Cattle were removed from the rotational stocking experiment on approximately the same date as the continuously stocked experiment each year. After each rotational stocking period, plots were mowed to a uniform height of approximately 10 cm. Based on visual observations, cattle did not preferentially graze particular cultivars under either stocking method. Stands were rated from 0 to 5 using a visual estimate of percent canopy cover as described previously (5). Because this method was highly correlated with actual plant counts (5), only visual estimates were made. In practice, canopy cover estimates can be affected by regrowth height, but by scoring plants within 2 weeks after removing cattle in autumn, regrowth was generally short (< 10 cm) and similar for all entries. Initial stands of alfalfa were scored on 3 October 2001. In subsequent years, stands were evaluated approximately 2 to 3 weeks after the conclusion of grazing each year on 1 October 2002 and 14 October 2003. Stand percentages were computed by multiplying the stand score by 20. Data from the rotational and continuous stocking experiments were analyzed separately. The effects of cultivar and year on stand percentage were evaluated using the PROC GLM procedure in SAS (SAS Institute Inc., Cary, NC). Mean separations were based on Fisher’s protected LSD (9). Statistical significance was assessed at the 5% probability level. Stand Persistence under Contrasting Grazing Managements All cultivars established well in both the continuous and rotational stocking experiments and no differences were evident among cultivars (Table 1). Thus, the ability of plants to establish and compete with weed pressure, as assessed by canopy cover at the end of 2001, was not affected by the grazing tolerance classification of the cultivars. Table 1. Visual estimates of stand percentages for four cultivars differing in grazing tolerance across three years under two stocking methods near Rhodes, IA.
NS = not significant. After a single year of grazing, the stands of all cultivars in the continuous stocking management declined, but the grazing-tolerant alfalfa cultivars were superior to the grazing intolerant cultivars (Table 1). However, under rotational stocking, no differences in stands were evident among the alfalfa cultivars. A second year of continuous stocking did not further diminish stands of any cultivar except Samurai. Under rotational stocking, grazing-tolerant cultivars did not show any stand loss, but the two intolerant cultivars declined approximately 30% in the second year (Table 1). These cultivars all have similar and acceptable disease and pest resistance profiles, with the exception of Alfagraze, which has low resistance to Phytophthora root root. Thus, the stand declines would not be expected based on differential disease susceptibilities; if anything, Alfagraze would have been expected to show more stand decline. However, the cultivars differ in terms of fall dormancy, with the grazing-tolerant cultivars being more than the intolerant cultivars. Cultivars in the fall dormancy class 4, including Apollo and 5454, do not have winter persistence problems in Iowa as shown by variety trial results (6), and none of these cultivars has had persistence problems related to winter in other trials in Iowa (E. C. Brummer, personal observations). Therefore, winter hardiness is unlikely to be the cause of the differential survival. These results have two important implications. First, they show clearly the benefit of using cultivars selected under high-density continuous stocking for grazing systems employing both rotational and continuous stocking in the Midwestern USA. Although the differentiation among cultivars was not observed until one year later under rotational stocking, stands markedly thinned in the grazing intolerant cultivars after just two years of grazing. Given that the rotational stocking treatment imposed in this experiment was likely favorable to alfalfa, these results strongly suggest to farmers that they choose grazing-tolerant cultivars for any grazing operations. Second, the results show that using continuous stocking can identify grazing-tolerant cultivars in less time than rotational stocking, and the results after a single year of continuous stocking were congruent with those found after two years of rotational stocking. Thus, continuous stocking is a useful tool to assess alfalfa grazing tolerance, even if the intended grazing method is the less stressful rotational stocking. Conclusions This experiment showed that in central Iowa, a very clear distinction in persistence is seen between grazing-tolerant and intolerant cultivars after one year under continuous stocking and after two years under rotational stocking. Grazing-tolerant alfalfa cultivars are clearly superior to intolerant cultivars under practical rotational stocking systems used by the best graziers. Acknowledgments I thank Mark Smith for technical help and the whole staff at the Iowa State University Rhodes Research Farm for assistance with the cattle. This journal paper of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA, Project No. 6631, was supported by Hatch Act and State of Iowa funds. Literature Cited 1. Barnes, D. K., Goplen, B. P., and Baylor, J. E. 1988. Highlights in the USA and Canada. Pages 1-24 in: A. A. Hanson, D. K. Barnes, and R. R. Hill, Jr., eds. Alfalfa and alfalfa improvement. Amer. Soc. Agron. Monograph 29. ASA, CSSA, and SSSA, Madison, WI. 2. Bouton, J. H., and Gates, R. N. 2003. Grazing-tolerant alfalfa cultivars perform well under rotational stocking and hay management. Agron. J. 95:1461-1464. 4. Bouton, J. H., Smith, S. R., Jr., Wood, D. T., Hoveland, C. S., and Brummer, E. C. 1991. Registration of 'Alfagraze' alfalfa. Crop Sci. 31:479. 5. Brummer, E. C., and Moore, K. J. 2000. Persistence of perennial cool-season grass and legume cultivars under continuous grazing by beef cattle. Agron. J. 92:466-471. 6. Brummer, E. C., and Smith, M. 2003. 2003 Iowa Crop Performance Test — Alfalfa. Iowa State Univ. Ext. Bull. AG-84. 7. Katepa-Mupondwa, F., Singh, A., Smith, S. R., and McCaughey, W. P. 2002. Grazing tolerance of alfalfa (Medicago spp.) under continuous and rotational stocking systems in pure stands and in mixture with meadow bromegrass (Bromus riparius Rehm. syn. B-biebersteinii Roem & Schult). Can. J. Plant Sci. 82:337-347. 8. Smith, S. R., Jr., Bouton, J. H., Singh, A., and McCaughey, W. P. 2000. Development and evaluation of grazing-tolerant alfalfa cultivars: A review. Can. J. Plant Sci. 80:503-512. 9. Steel, R. G. D., and Torrie, J. H. 1980. Principles and Procedures of Statistics: A Biometrical Approach, 2nd Ed. McGraw-Hill, New York, NY. |
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