Minimizing Risk and Increasing Yield Stability in Field Pea Production

Clayton, G.¹, W. Rice², S. Blade³, C. Grant⁴, N. Harker¹,

A. Johnston⁵, G. Lafond⁶ and N. Lupwayi²

AAFC Lacombe¹, Beaverlodge², Brandon⁴, Melfort⁵,

Indian Head⁶and AAFRD Edmonton³.

Yield Instability

Field pea is a relatively new crop for western Canadian farmers that is grown on 2,000,000 acres across the three prairie provinces. Field pea has been used to extend the rotation as a summer fallow replacement, to prevent disease in other crops and is known to have rotational benefits that increase yields in subsequent crops. Field pea yield has been variable across western Canada with reported yields ranging from 10 to 100 bushels per acre with the average yield approximately 35 bushels per acre. This wide range of yield has been attributable to two general principles: lack of nitrogen and insufficient weed control. The key to increasing yield stability then is to develop management practices that minimize the risk of growing field pea.

Pre-Harvest Weed Control

There are relatively few herbicides that control perennial weeds effectively in pulse crops compared to cereal crops in the rotation. Field selection for a pulse crop like peas or beans should be considered the year before seeding the pulse crop. Pre-harvest Roundup in the previous crop provides an opportunity to control hard to kill perennial weeds for the following field pea crop.

The use of pre-harvest Roundup as a means for perennial weed control has increased from about 500,000 acres in 1991 to over 7 million acres in 1997. This increase is associated with the limited opportunities that exist for perennial weed control with pre-seeding and post-harvest applications of Roundup. Other problems that exist include: dry spring or fall gives insufficient perennial weed regrowth, wet spring or fall makes application difficult, late harvest leaves little time for application and early, heavy, killing frost makes fall application ineffective. The advantages of pre-harvest Roundup include: consistent timing for perennial weed control, perennial weeds are more susceptible and harvest management benefits that include crop and weed dry down. In the year following treatment, pre-harvest Roundup provides excellent control of quackgrass (91%), Canada Thistle (85%), perennial sowthistle (84%), toadflax (91%) and dandelion (87%) and suppresses field horsetail.

Fields with perennial weeds should be avoided for field pea crop production or pre-harvest Roundup should be the first step in the management of pulse crops in rotation.

Soil Inoculation vs. Seed Inoculation

Seed inoculation is still the most important and common method of applying Rhizobia spp. to legumes, even though an alternative to conventional application has been available for 30 years. In western Canada, direct application of peat powder inoculants to seed was the most common until an easier method of seed application was introduced with liquid inoculants. There are several situations in which seed application of rhizobia may be inefficient and include: 1) inoculation of legumes with large seed size and high seeding rates is a major task, restricting the speed of the seeding operation; 2) sometimes seeds are too fragile to be inoculated and over-handling can cause reduced germination and emergence; 3) the seed surface places a limit on the number of rhizobia which may be applied, a common problem when seed size is small or it is necessary to apply more of an introduced strain when there is naturally occurring rhizobia present; 4) the seed coats of some species may contain materials toxic to rhizobia.; 5) environmental stresses may contribute to increased rhizobia die-off on the seed and 6) seeding is delayed for 1-2 days because of inclement weather or equipment breakdowns and re-inoculation is necessary.

Seeding rates of field pea are commonly 150-180 kg ha^-1 and the logistics of properly inoculating enough seed on the farm to seed a quarter section is a difficult task. Solid forms of inoculant introduced separately into the seedbed represents a satisfactory alternative to direct application of liquid or peat inoculant to the seed.

Starter nitrogen has been recommended for field pea production when soils are low in nitrogen. Also, farmers have increasingly utilized nitrogen when disappointing pea yields have prevailed on their farms to improve pea yield stability. Therefore, it is important to understand the role nitrogen plays in field pea production.

The comparative effects of soil and seed inoculants is summarized for a number of studies in Table 1. Twelve of the 14 studies cited showed that soil applied inoculant resulted in an increase in one or more of nodule number, nodule weight, nitrogen fixation, plant biomass or grain yield. In the other two studies there was no response to inoculation. The significance of these findings is enhanced by the fact that the results are based mainly on field studies.

Only two studies reported on the use of liquid formulations for seed inoculation (Table 1). In one report peat seed-applied inoculant gave a significant response over liquid seed-applied inoculant. In the other report there was no difference between liquid and peat, and in general no significant response to inoculation.

The conclusion reached by Brockwell et al. (4) is certainly justifiable. The practical implications of the superior performance of soil inoculation is dependent on the comparative cost/production ratio of soil and seed applied inoculants. Over the past three years we have observed a range in grain yield increase of 0 to 140% for field pea with granular soil inoculant as compared to seed-applied inoculants or the check. The sites at Beaverlodge and Fort Vermilion in 1995 and 1996 showed the biggest variation in pea yield from the inoculant formulation, where soil applied inoculant produced pea yields that were higher than uninoculated and liquid treatments.

Where no N was added, field pea yielded 76, 62, and 44% higher than the uninoculated check for field pea inoculated with granular, peat powder and liquid formulations, respectively (Table 2). Averaged over all inoculant formulations and N rates, the relative pea yield was 183, 162, 132 and 95% for granular, peat powder, liquid and uninoculated treated peas, respectively, compared to the yield of field pea with no inoculant and no added N at Beaverlodge (Table 2). The site at Indian Head (Table 3) also had pea yields that were higher from soil inoculation than from where pea seed was treated with liquid inoculant or were not inoculated at all. Relative pea yield was higher with the granular and peat powder formulations than with the liquid formulation or the uninoculated check in 1996. However, in 1997 when growing season precipitation was low, the

relative pea yield was higher when field pea was inoculated with the granular formulation than the other formulations or the uninoculated check (Table 3). It appears that soil inoculation with the granular formulation may overcome environmental stresses that the pea plant may encounter such as dry soil conditions, cool soils and/or soil pH below 6.0. There were no significant differences in yield from the other sites (i.e. Melfort, Table 4) between inoculant formulation or between inoculated and uninoculated treatments. Generally at these sites there was sufficient nodulation from naturally occurring rhizobia to support the plants need for nitrogen and consequently inoculation did not make a difference in productivity.

At Minnedosa, Melfort, Lacombe and Vegreville it appeared the treatments nodulated in a way to express differences in yield between formulation treatments, however this expression wasn't apparent in growth.

At all sites, adding starter N did not increase pea yield. Starter N decreased nodulation at most sites and pea yield declined at the higher N rate at some sites (Table 2). Increasing the N rate at seeding decreased the nodulation on pea roots of the uninoculated treatment which suggests that applying nitrogen at seeding can inhibit the naturally occurring rhizobia.

Granular Inoculant Rate x Placement

Seed-placed granular inoculant resulted in higher pea biomass and biomass N content at flatpod than when the granular inoculant was placed in a band, to the side and below the seed, or surrounding the seed when the seed and inoculant were distributed below the soil surface with a sweep at Fort Vermilion in 1996 (Table 5). The placement effect at the flatpod stage did not carry over to seed yield, straw yield, seed protein, seed N or straw N at harvest. This resulted because the accumulation of N in pea during the filling period (flatpod to maturity) was greater when the granular inoculant was banded or applied with a sweep. Pea yield was 48, 44 and 54% higher than the uninoculated check when granular inoculant was applied in the band, in the seed row or with a sweep, respectively. Pea yielded the same when 2.5, 5, 7.5 or 10 lbs per acre were applied (Table 5). Pea yield was 49, 47, 49 and 49% higher than the check with 2.5, 5, 7.5 and 10 lbs per acre of granular inoculant at Fort Vermilion in 1996 (Table 5). At Beaverlodge in 1996, placement or granular inoculant rate had no effect on any of the variables collected (Table 6).

There were no significant differences between inoculant rate or placement of the granular inoculant in the soil on pea biomass and nodulation at flatpod, grain yield or straw yield at Fort Vermilion in 1997 (Table 7). Pea yielded 98, 105 and 107% higher than the uninoculated check when granular inoculant was applied in the band, with the seed or with the sweep, respectively. Pea yielded 101 - 105 % higher than the inoculated check with granular inoculant rates of 2.5-10 lbs per acre. At Beaverlodge in 1997, there were no significant differences between granular inoculant rate or placement for pea biomass and nodulation at flatpod, grain yield or straw yield straw yield (Table 8). Pea yielded 20% higher than the inoculated check averaged over rates and placement of granular inoculant.

This study indicates that a granular inoculant rate of 2.5 lbs per acre is adequate for high pea yield and that placement of the inoculant can be in the band, with the seed or applied in a sweep type opener. In farm scale trials conducted in the Peace River region, there is an indication that yield increases can be achieved by applying 10 lbs per acre of granular inoculant as compared to 5 lbs per acre under some conditions. It appears that lower rates of granular inoculant may be appropriate in the absence of environmental stresses, however there is a possibility that pea yields could benefit from higher applications of granular inoculant when environmental stresses occur i.e. dry soil conditions or low pH.

In Crop Weed Control:

Time of Weed Removal

Producers often ask "Should I control weeds early or wait until they all emerge, and will this influence pea yield stability"?. Remove weeds early. Peas compete very poorly with weeds and will often loose 5-10 bu/ac for every week that weed control is delayed. Most people worry too much about late emerging weeds. While late emerging weeds may look a little worse at the end of the year, they have relatively small impacts on crop yield. We often wait for late weed flushes which come as a result of rain. When the rain comes it also prevents us from getting into the field for a few days longer than we or the herbicide manufacturer would like. By that time some the weeds will be pretty big , and the peas will have already incurred a yield loss. In peas, it doesn't pay to wait and spray.

Does early weed removal have other advantages? Early weed removal has two other distinct advantages. First, pea tolerance to most broadleaved herbicides is greatest at early growth stages. Herbicides such as MCPA will often severely injure peas when applied after 5 leaf pairs. Even relatively safe herbicides such as Pursuit can injure peas when applied too late. Second, when weeds are small, they are much more susceptible to herbicides. Environmental conditions that stress weed growth will also reduce herbicide performance. Reduced herbicide performance is minimized when weeds are treated at early growth stages.

Do grassy weeds affect field pea yield stability more strongly than broadleaved weeds? It would not be accurate to say that grassy weeds are more competitive than broadleaved weeds or vice versa. The weeds that emerge the quickest often have the greatest impact on crop yields. In a study at Lacombe and Lethbridge, wild oat tended to emerge before Tartary buckwheat and did cause greater yield losses in peas. However, there may be early emerging weeds that grow slowly and have only limited impacts on yield. A particular weed may dominate a field in one year and have only limited effects in the next year. Usually, individual farmers have a good grasp of which weeds are most detrimental to pea yields on a particular field.

Weed Management Issues:

Herbicide Residues

Herbicide residues can be both good and bad. A pulse crop like peas are weak competitors and herbicides such as Pursuit or Treflan can provide season-long weed control. However, peas are very sensitive to residues from sulfonylurea herbicides such as Ally or Amber or to pyridine herbicides such as Lontrel or Tordon and reduced or no emergence or weak seedlings may occur.

Tank Mixtures

Mixing grass and broadleaf herbicides together is often necessary to obtain broad spectrum weed control but risky because of potential crop injury (i.e. Poast and MCPA) or because of antagonism; i.e. Assure and Pursuit tank mixtures give less wild oat control than with either herbicide alone.

Surface-applied Herbicides

If fields have been in a low disturbance direct seeding system for a minimum of three years, then herbicides such as Edge or Treflan need not be incorporated for effective weed control.

References

• 1. Bezdicek, D.F.; Evans, D.W., Abede, B. and Witters, R.T. 1978. Evaluation of peat and granular inoculum for soybean yield and N fixation under irrigation. Agronomy Journal 70: 865-868.

  2. Boonkerd, N.; Weber, D.F. and Bezdicek, D.F. 1978. Influence of Rhizobium japonicum and inoculation methods on soybeans grown in rhizobia-populated soil. Agronomy Journal 70: 547-549.

  3. Brockwell, J.; Gault, R.R.; Chase, D.L.; Hely, F.W.; Zorin, M. and Corbin, E.J. 1980. An appraisal of practical alternatives to legume seed inoculation: field experiments on seedbed inoculation with solid and liquid inoculants. Australian Journal of Agricultural Research 31: 47-60.

  4. 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 and Soil 174:143-180.

  5. Burton, J.C. and Curley, R.L. 1965. Comparative efficiency of liquid and peat-base inoculants on field-grown soybeans. Agronomy Journal 57: 379-381.

  6. Chambers, M.A. 1983. Influence of several methods of rhizobial inoculation on nodulation and yield of soybeans. Plant and Soil 74: 203-209.

  7. Danso, S.K.A.; Kapuya, J. and Hardarson, G. 1990. Nitrogen fixation and growth of soybean as influenced by varying methods of inoculation with Bradyrhizobium japonicum. Plant and Soil 125: 81-86.

  8. Dean, J.R. and Clark, K.W. 1977. Nodulation, acetylene reduction and yield of faba beans as affected by inoculum concentration and soil nitrate level. Canadian Journal of Plant Science 57: 1055-1061.

  9. Dubetz, S.; Major, D.J. and Rennie, R.A. 1983. Production practices for early maturing soybeans in southern Alberta. Canadian Journal of Plant Science 63: 641-646.

  10. Hardarson, G.; Golbs, M. and Danso, S.K.A. 1989. Effect of nodulation patterns on nitrogen fixation by soybean (Glycine max (L.) Merrill). Soil Biology and Biochemistry 21: 783-787.

  11. Hynes, R.K.; Kraig, K.A.; Covert, D.; Smith, R.S. and Rennie, R.J. 1995. Liquid rhizobial inoculants for lentil and field pea. Journal of Production Agriculture 8: 547-552.

  12. Muldoon, J.F.; Hume, D.J. and Beversdorf, W.D. 1980. Effects of seed- and soil-applied Rhizobium japonicum inoculants on soybeans in Ontario. Canadian Journal of Plant Science 60: 399-410.

  13. Nelson, D.W.; Swearingin, M.L. and Beckham, L.S. 1978. Response of soybeans to commercial soil-applied inoculants. Agronomy Journal 70: 517-518.

  14. Rice, W.A. and Olsen, P.E. 1992. Effects of inoculation method and size of Rhizobium meliloti population in the soil on nodulation of alfalfa. Canadian Journal of Soil Science 72: 57-68