K.W. Freeman, W.E. Thomason, D.A. Keahey, K.J. Wynn,
R.W. Mullen, G.V. Johnson, W.R. Raun, and M.T. Humphreys.

Abstract

Many alternative management systems have been evaluated for corn (Zea mays L.), soybeans (Glycine max L.), and wheat (Triticum aestivum L.) production, however, most have involved rotations from one year to the next.  Legume interseeding systems which employ canopy reduction techniques in corn have not been thoroughly evaluated.  One study was initiated in 1994 at the Panhandle Research Station near Goodwell, OK, on a Richfield clay loam soil, to evaluate five legume species: yellow sweet clover (Melilotus officinalis L.), subterranean clover (Trifolium subterraneum L.), alfalfa (Medicago sativa L.), arrowleaf clover (T. vesiculosum L.) and crimson clover (T. incarnatum L.) interseeded into established corn.  In addition, the effect of removing the corn canopy above the ear (canopy reduction) at physiological maturity was evaluated.  Canopy reduction increased light interception beneath the corn thus enhancing legume growth in late summer, early fall, and early spring the following year prior to planting.  Legumes incorporated prior to planting were expected to lower the amount of inorganic nitrogen fertilizer needed for corn production.  Crimson clover appeared to be more shade tolerant than the other species, and intereseeding this species resulted in the highest corn grain yields when no N was applied.  In the last two years, interseeding crimson clover at physiological maturity, followed by canopy reduction resulted in a 21 bu/ac increase in yield compared to conventionally grown corn with no N applied. 

   Introduction

Canopy reduction has been used in third world countries as a means of increasing light interception for a relay crop.  Canopy reduction is imposed when the corn reaches physiological maturity when nutrient and water uptake has ceased).  Over the past 20 years, various researchers have evaluated intercropped legumes for increased N supply in corn (Zea mays L.) production.  As sources of inorganic nitrogen fertilizer become less dependable and prices increase, organic forms, particularly legumes, are being considered as alternative sources for non-legume crops.  Searle et al. (1981) stated that corn grain yield was not affected by legume intercrop, indicating neither competitive depression nor nitrogen transfer from the legume.  Nair et al. (1979) showed that intercropping corn with soybeans increased yield 19.5% when compared to monoculture corn.  Scott et al. (1987) noted yields following medium red clover (T. pratense L.) were equivalent to the addition of 17 kg ha^-1 fertilizer-N. 

Even though intercropping usually includes a legume, applied nitrogen may still confer some benefits to the system as the cereal component depends heavily on nitrogen for maximum yield (Ofori and Stern, 1986).  Chowdhury and Rosario, (1993) found that intercropping corn with mungbeans (Vigna radiata L.) increased yields 71% when the N application rate was increased from 0 to 90 kg/ha.  Ebelhar et al. (1984) reported with no fertilizer N applied, there was an increase in corn grain yield from 2.5 to 6.2 Mg ha^-1 with hairy vetch (Vicia villosa L.) treatment compared with corn residue.  Corn yields increased 62% with applied N (0 versus 120 kg N ha^-1). 

Canopy reduction is defined as the removal of the corn canopy just above the ear at physiological maturity, where the cut portion is allowed to drop to the soil surface.  Some of the basis of canopy reduction come from regions where a relay crop like common beans is produced following corn.  In order to increase light interception beneath the corn canopy for the bean plant, the tops of the corn can be removed once physiological maturity is reached.  This in turn does not sacrifice the corn yield while increasing the chances of producing a bean crop that would not have been possible if planting took place following corn harvest.

            The objective of this work was to evaluate the effect of interseeded legume species and nitrogen rates combined with canopy reduction on corn grain yield and grain protein.

   Materials and Methods

One experiment was established in the spring of 1994 at the Oklahoma Panhandle Research and Extension Center near Goodwell, OK on a Richfield clay loam (fine, montmorillonitic, mesic Aridic Argiustoll).  Initial soil test characteristics and soil classification are reported in Table 1.  A randomized complete block experimental design with three replications was used.  Plot size consisted of four rows (30 inch) x 25 ft.  All treatments received 90 lb N/ac of urea (45-0-0) in the fall of 1995.  In 1996 and for the remaining years of this experiment, treatments 1-5, 7 and 12 received no N to assess legume N fixation compared to identical treatments with 45 lb N/ac.  Each year, corn was planted at a seeding rate of 30,000 seeds ac between late April and early May.

Canopy reduction was imposed by removing the tops of the corn plants just above the ear using a machete.  This allowed sunlight to reach the legume seedbed.  In August, when the corn had reached physiological maturity, five legume species were interseeded by hand at the following seeding rates: yellow sweet clover (Melilotis officinalis L.) 40 lb/ac, subterranean clover (Trifolium subterraneum L.) 40 lb/ac, alfalfa (Medicago sativa L.) 30 lb/ac, arrowleaf clover (T. vesiculosum L.) 20 lb/ac and crimson clover (T. incarnatum L.) 40 lb/ac.  Physiological maturity was determined by periodic monitoring grain black layer formation.  Following interseeding and canopy reduction, 5 cm of irrigation water was applied for legume establishment and to prevent reduction in growth caused by moisture stress.  The legume seeds were inoculated prior to planting with a mixture of Rhizobium meliloti and R. trifolii bacteria.  Harvest area consisted of two rows (30 inches) x 25 ft.  Harvesting and shelling were performed by hand.  Plot weights were recorded and sub-sampled for moisture and chemical analysis.  Subsamples were dried in a forced-air oven at 150°F and ground to pass a 140 mesh screen.  Total nitrogen concentration was determined on all grain samples using dry combustion (Schepers et al. 1989).  Protein N in corn grain can be determined by multiplying %N by 6.25.

Interseeded legumes remained in the field until the following spring when they were incorporated prior to corn planting using a shallow (4 inches) disk.  Legumes were only used for ground cover and potential nitrogen fixation and as such were not harvested for seed or forage.

   Results and Discussion

By imposing the alternative management practice of canopy reduction, we visually observed an increase in light interception beneath the corn canopy, thus enhancing legume growth in late summer, early fall before corn harvest, and early spring the following year prior to planting.  Crimson clover had superior spring growth compared to the other species evaluated as visual biomass production was greater when incorporated in early April prior to planting.  No grain yield response to applied N was observed in 1996, or 1997, but by 1998, yields increased 21 bushels as a result of applying N (12 vs 13, Table 2).  The lack of fertilizer N response at this site restricted the early evaluation of legume N contribution and species comparison. 

There was no significant difference between grain yields when tops were cut at physiological maturity compared to the normal practice (5 vs 7, crimson clover with and without canopy reduction, with no N applied) in 1996, 1997 or 1998.  However, by 1999, interseeding crimson clover and using canopy reduction resulted in increased yields when compared to that observed where no canopy reduction was employed.  It was important to find no differences between canopy reduction and conventional management early on, because it demonstrated the applicability of interseeding in late summer. 

            In the last two years, interseeding crimson clover at physiological maturity, followed by canopy reduction resulted in a 21 bu/ac increase in yield when compared to conventionally grown corn with no N applied (5 versus 12).  This N fertilizer savings of approximately 24 lb N/ac would have an economic value of $4.80.  Legume interseeding and canopy reduction costs would likely be greater than $4.80, thus restricting what can be promoted at this point in time.

Although not evaluated in this study, mechanized canopy reduction could decrease the time required for grain to lose moisture since more sunlight would directly come in contact with the corn ears when the tops were removed.  When grain moisture is high it can delay harvest and/or significantly increase drying costs.  Legume seeding rates, alternative species, method of interseeding and interseeding date will all need to be thoroughly evaluated prior to the mechanization and implementation of this practice. 

Since nitrate leaching and soil erosion are becoming major concerns in production agriculture today, this experiment may lead to practices that can decrease both, via lowering the amount of inorganic fertilizer N needed for corn production and reducing the amount of bare soil susceptible to wind and water erosion.

References

Chowdhury, M.K. and E.L. Rosario.  1994.  Comparison of nitrogen, phosphorus and potassium utilization efficiency in maize/mungbean intercropping.  J. of Agric. Sci., Cambridge.  122:193-199.

Ebelhar, S.A., W.W. Frye and R.L. Blevins.  1984.  Nitrogen from legume cover crops for no-tillage corn.  Agron. J.  76:51-55.

Nair, K.P., U.K. Patel, R.P. Singh and M.K. Kaushik.  1979.  Evaluation of legume intercropping in conservation of fertilizer nitrogen in maize culture.  J. Agric. Sci. Camb.  93:189-194.

Ofori, Francis and W.R. Stern.  1986.  Maize/cowpea intercrop system: effect of nitrogen fertilizer on productivity and efficiency.  Field Crops Research  14:247-261.

Schepers, J.S., D.D. Francis and M.T. Thompson.  1989.  Simultaneous determination of total C, total N and 15N on soil and plant material.  Commun. Soil Sci. Plant Anal.  20:949-959.

Scott, T.W., J. Mt. Pleasant, R.F. Burt and D.J. Otis.  1987.  Contributions of ground cover, dry matter, and nitrogen from intercrops and cover crops in a corn polyculture system.  Agron. J. 79:792-798

Searle, P.G.E., Yuthapong Comudom, D.C. Shedden and R.A. Nance.  1981.  Effect of maize + legume intercropping systems and fertilizer nitrogen on crop yields and residual nitrogen.  Field Crops Res.  4:133-145.