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© 2004 Plant Management Network. Performance of Rhizobial Inoculant Formulations in the Field Fran Walley, Department of Soil Science, University of Saskatchewan, Agriculture and Agri-Food Canada, Saskatoon, SK, S7N 5A8 Canada; George Clayton, Tillage and Crop Agronomy, Lacombe Research Centre, Agriculture and Agri-Food Canada, Lacombe, Alberta, T4L 1W1 Canada; Yantai Gan, Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Swift Current, SK S9H 3X2; and Guy Lafond, SPARC, Indian Head Research Farm, Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Indian Head SK S0G 2K0 Corresponding author: Fran Walley. walley@sask.usask.ca Walley, F., Clayton, G., Gan, Y., and Lafond, G. 2004. Performance of rhizobial inoculant formulations in the field. Online. Crop Management doi:10.1094/CM-2004-0301-03-RV. Introduction Over the years, a number of studies have been conducted in western Canada to examine the impact of rhizobial inoculant formulation on the success of various Rhizobium/legume associations. The introduction of granular inoculant formulations in the 1990s stimulated a new flurry of research activity and throughout the latter portion of the decade, a number of projects were conducted to compare the efficacy of this new formulation to liquid and peat-based powders. Coincidentally, at the same time that the new granular formulations were being introduced into the prairie market, the introduction and rapid expansion of desi- and kabuli-type chickpea production in Saskatchewan served to further stimulate inoculant research activity. As a consequence, a considerable body of research data has been amassed for a variety of pulse crops in which performance comparisons have been made between various inoculant formulations. Historical Perspective Inoculation for legumes has long been a topic of considerable interest in the prairie region of western Canada for farmers and researchers alike. Indeed, the first agricultural extension bulletin of the University of Saskatchewan, College of Agriculture (6), written by Professor Roy Hansen and published in 1922, was entitled “Inoculation for Legumes”. In this bulletin, Professor Roy provided guidance for farmers regarding inoculation strategies: “Collect soil from a field that was known to have had well inoculated plants, as evidenced by the presence of nodules on the roots. Thoroughly air-dry this soil, though not in sunlight, and run through a sieve to remove lumps and trash. Spread out the seed on a smooth floor and moisten (not wet) with a five percent glue solution (one-half pound of furniture glue to one gallon of water). Then sift on to the moist seed, the dried, pulverized soil, using two to three quarts of soil per bushel of seed, meanwhile shoveling over the seeds until they are uniformly dirty.” Clearly, we have moved well beyond inoculating seeds with soil and furniture glue. However, with the increasing inoculation strategy options currently available to farmers, including new formulations and delivery systems, there is a continuing need to evaluate the efficacy of these different strategies from both agronomic and economic standpoints. Current Legume Inoculation Practices in Western Canada Until the late 1980s and early 1990s seed-applied, peat-based inoculants dominated the commercial inoculant market in western Canada. However, although peat was recognized as a very good carrier of rhizobia, there was interest in developing alternate formulations, largely because some farmers were frustrated with application procedures that were viewed as time-consuming, messy, and somewhat impractical (7). As an alternative, liquid formulations were introduced, along with new packaging that allowed farmers to treat seed directly from the packaged product as seed was passing through the grain auger into the seeding equipment. Liquid inoculants soon gained considerable popularity with many growers, largely due to the ease of application that allowed growers to single-handedly apply the inoculant treatment. Perceived ease of application was clearly an important consideration. Consequently, when soil-applied granular inoculants were introduced to the commercial market in the latter portion of the 1990s, it was not surprising that this new formulation similarly gained popularity with growers. Along with these formulation innovations came questions regarding efficacy. Given the difference in cost per acre between the different formulations (ranging from approximately $3.00 an acre CDN for a peat-based powder and some liquid products to as much as $12.00 per acre CDN for a granular, soil implant product) farmers wanted to know which product was right for their operation. Hand-in-hand with the growing need for reliable, unbiased information came a number of field studies comparing the efficacy of the liquid and granular products to the more traditional peat-based carriers. Research Results Hynes et al. (7) conducted a 2-year (12 site-year) study using commercial scale farm equipment to compare liquid and peat carriers for Rhizobium leguminosarum strains 99A1 and 128C56G for lentil (Lens culinaris Medik.) and pea (Pisum sativum L.), respectively. Of the eight lentil trials, six indicated that the liquid and peat formulations were equally effective at enhancing final yields relative to the control: in one trial the liquid outperformed the peat and in the remaining trial there was no significant difference between any of the treatments, including the control. Of the four pea trials conducted, significant seed yield responses relative to the control were achieved only at one site and yields of the inoculated treatments did not differ significantly. The authors concluded that the liquid inoculant for lentil and field pea was as effective as the traditional peat inoculant. Moreover, they suggested that the ease of application of the liquid inoculant might actually encourage more farmers to diversify and consider including legumes in their regular cropping rotations. In contrast, Clayton et al. (4) conducted a 2-year study at Fort Vermilion and Beaverlodge, Alberta (6 site-years) to compare the efficacy of liquid, peat, and granular formulations, both with and without additional application of urea-N. They reported that in terms of seed yield, the peat-based powder typically out-performed the liquid inoculant, which did not differ significantly from the uninoculated control (Table 1). Indeed, the greatest biomass, final seed yield, and harvest index (HI) values typically were associated with the soil-applied peat granular formulation as compared to the seed-applied formulations, i.e., peat and/or liquid inoculant applications. Furthermore, they attributed the enhanced seed yield to improved nitrogen nutrition and reported that the effects of inoculant formulation on nodule number, nitrogen accumulation, and nitrogen fixation were in the order: granular > peat powder > liquid = uninoculated. Table 1. Effect of inoculant formulation averaged over 6-site years (1995-96) in the Peace River (Alberta) region on biomass production, seed yield, and harvest index of field pea.
Adapted from Clayton et al. (4). *, ** Significant at P < 0.05 and 0.01, respectively. Others working in western Canada similarly have reported that the effects of inoculant formulation on various plant growth parameters were in the order: granular > peat powder > liquid = uninoculated for pea (10) and chickpea (Cicer arietinum L.) (9). When the impact of inoculant formulation on both desi- and kabuli-type chickpea in the Brown and Dark Brown soil zones of Saskatchewan was examined, seed protein concentration and nitrogen fixation parameters generally were lower for the liquid than for the peat and granular inoculant, which typically did not differ (9). It was observed that inoculant formulation influenced the position of the nodules on the root systems, with granular inoculants encouraging a greater proportion of nodules on the lateral roots as compared to the crown root region (Table 2). Strong correlations between these lateral root nodules and various yield parameters (Table 3) led to the conclusion that the lateral root nodules contributed significantly to nitrogen fixation at a time when the plant required nitrogen to enhance seed production. Rice et al. (11) similarly reported that nodule distribution patterns in field pea reflected different formulations, with peat and liquid inoculants producing nodules clustered around the crown while the granular inoculant produced nodules distributed throughout the root system. Clustering of nodules around the position of initial inoculant placement is particularly evident when indigenous rhizobial populations are low or absent (Fig. 1). Table 2. Effect of inoculant formulation on the nodule dry weight of desi chickpea averaged over two sites in Saskatchewan, 1998.
Adapted from Kyei-Boahen (9). Table 3. Correlations between nodule dry weight at early pod-fill stage and yield parameters of desi chickpea averaged across various locations in Saskatchewan, 1997 and 1998.
Adapted from Kyei-Boahen (9). *, ** Significant at P < 0.05 and 0.01, respectively.
In experiments where granular inoculants out-perform other formulations, enhanced performance likely is related, in part, to increased numbers of rhizobia at seeding and/or reduced mortality rates once applied to the soils (2). Hynes et al. (8) studied rhizobial population dynamics in the rhizosphere of field pea inoculated with different formulations. Although little difference was detected at three of four study sites, at one site soil rhizobial populations from a liquid formulation declined to near zero within 28 days of seeding whereas populations from powdered peat and granular formulations were maintained. The difference in survival was attributed to the impact of adverse environmental conditions. Rice et al. (11) compared granular, powdered peat and liquid inoculant formulation for field pea at various levels of soil pH and concluded that low soil pH stress (i.e., pH 4.4) was best alleviated by the use granular inoculant. Moreover, they suggested that rhizobia formulated in powdered peat and granular inoculants were more effective than when a liquid formulation was used in terms of competing with indigenous, ineffective rhizobia, particularly in the acidic soils used in their study. Environmental conditions during inoculation and seeding influence rhizobial survival, and infectivity and exposure to high temperature and dehydration of inoculated seed have been identified as major factors limiting nodulation success (5). Any inoculant formulation, such as peat, which provides physical protection to rhizobia (3), or inoculant application technique that improves protection for the rhizobia, is likely to enhance subsequent nodulation. Thus, it is particularly important to read and follow the directions supplied with the inoculant. Many inoculants now come with a specified time limit between seed treatment and placement of the inoculated seed in the soil. If the directions are followed and care is taken to reduce the exposure of the inoculant or inoculated seed to heating or desiccation, ample viable rhizobia should be present on the seed to ensure good nodulation. Although some conflicting reports have emerged from western Canada regarding the efficacy of different inoculant formulations (e.g., 4,7), recent research which has included granular formulations has consistently shown that these soil implant delivery systems are at least as good as and often better than either the seed-applied, peat-based powders or the liquid formulations. This is in keeping with the early observations of Brockwell et al. (1) who summarized the results of experiments with several legumes. They reported that when conditions were stressful and generally unfavorable to rhizobial survival, or when germination was delayed due to unfavorable environmental conditions, soil inoculation (such as granular soil implants) typically resulted in better nodulation and often better plant growth and/or yield than seed-applied inoculants. However, despite the apparent agronomic advantage of using granular soil implants, economic considerations also play an important role in inoculant formulation decisions for the farmer who will therefore need to weigh the apparent advantages and disadvantages of each formulation choice. Although some formulations may provide physical protection to rhizobia, there are definite limits as to what the bacteria can tolerate, irrespective of formulation. Clearly, it is always in the best interests of the farmer to avoid exposing any rhizobial product, irrespective of formulation, to direct sunlight, heat, and desiccation. Any reduction in the number of rhizobia surviving in the product can potentially translate to reduced nodulation and nitrogen fixation. There is no doubt that whether a liquid or a peat-based powder inoculant is used, the sooner the inoculated seed gets into the ground after treatment, the better. Moist, cool soil is an ideal environment for rhizobia to grow and multiply. Thus, it is essential that the seed be placed into moisture in good contact with the soil -- not only to promote germination, but also to ensure rhizobial survival and multiplication. Where moisture limitations exist, i.e., droughty soils, seed placement at greater depth may be necessary to ensure placement into moisture. Regardless of the formulation chosen, the goal is to place as many Rhizobium bacteria in the area of the emerging root as possible. All of the formulations currently available can do the job, given the appropriate environmental conditions. Research evidence from western Canada suggests that the granular formulations typically are as good as and often better than either peat or liquid formulations. However, convenience, expense, equipment requirements, and farmer experience and preference are all important considerations when deciding which formulation is best for any commercial operation. Literature Cited 1. Brockwell, J., Gault, R. R., Chase, D. L., Hely, F. W., Zorin, F. W., and Corbin, J. E. 1980. An appraisal of practical alternatives to legume seed inoculation: Field experiments on seedbed inoculation with solid and liquid inoculants. Aust. J. Agric. Res. 31:47-60. 2. Brockwell, J., Bottomley, P. J., and Thies, J. E. 1995. Manipulation of rhizobia microflora for improving legume productivity and soil fertility: A critical assessment. Plant Soil 174:143-180. 3. Burton, J. C. 1982. Modern concept in legume production. Pages 105-114 in: Biological Nitrogen Fixation Technology for Tropical Agriculture. P. H. Graham and S. C. Harris, ed. CIAT, Cali, Columbia. 4. Clayton, G., Rice, W. A., Lupwayi, N. Z., Johnston, A. M., Lafond, G. P., Grant, C. A., and Walley, F. 2003. Inoculant formulation and fertilizer nitrogen effects on field pea: Crop yield and seed quality. Can. J. Plant Sci. 84:89-96. 5. Hansen, A. P. 1994. Symbiotic N2 fixation of crop legumes: Achievements and perspectives. Margraf Verlag, Weikersheim, Germany. 6. Hansen, R. 1922. Inoculation for legumes. Agric. Ext., Bull. No. 1. Coll. Agric.e, Univ. Saskatchewan. Saskatoon, SK. 7. Hynes, R. K., Craig, K. A., Covert, D., Smith, R. S., and Rennie, R. J. 1995. Liquid rhizobial inoculants for lentil and field pea. J. Prod. Agric. 8:547-552. 8. Hynes, R. K., Jans, D. C., Bremer, E., Lupwayi, N. Z., Rice, W. A., Clayton, G. W., and Collins, M. M. 2001. Rhizobium population dynamics in the pea rhizosphere of rhizobial inoculant strain applied in different formulations. Can. J. Microbiol. 47:595-600. 9. Kyei-Boahen, S., Slinkard, A. E., and Walley, F. L. 2002. Evaluation of rhizobial inoculation methods for chickpea. Agron. J. 94:851-859. 11. Rice, W. A., Clayton, G. W., Olsen, P. E., and Lupwayi, N. Z. 2000. Rhizobial inoculant formulations and soil pH influence field pea nodulation and nitrogen fixation. Can. J. Soil Sci. 80:395-400. |
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