CROP ROTATION AND RESIDUE DURABILITY EFFECTS OF BRASSICAS AND OTHER CROPS ON WINTER WHEAT
Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow, Idaho 83844-2339, USA. E-mail: firstname.lastname@example.org
Spring crops of mustard (Sinapis Alba L.), canola or rapeseed (Brassica napus L.) and grain legumes such as dry pea (Pisum sativum L.) and lentil (Lens culinaris Medik.), are critical rotation crops with cereals in northern Idaho and eastern Washington (the Palouse). However, steep-sloping cultivated areas and predominant winter precipitation in the cool Mediterranean climate of the Palouse create erosion and nutrient loss. Two large-scale field trials evaluated mustard, dry pea, and lentil for subsequent effect on winter wheat. Spring crop yields and initial residue levels were high, but residues were greatly reduced after planting winter wheat. Residues from the spring crops showed little overwinter decomposition. Surface cover stayed above 50% for mustard, 30% for pea and 25% for lentil. Winter wheat yields were 7360 kg ha-1 following both pea and lentil, and 6840 kg ha-1 after mustard the first year, but were not different the second year. There were no significant differences in residual soil N among spring crops because mustard was top-dressed. Sub-plot N application to winter wheat on a portion of the large strips showed no significant differences among N response regression equations for the three previous crops. Linear Plateau regression analysis showed winter wheat N response up to 135 kg ha-1 following pea and lentil and 180 kg ha-1 after mustard. In small-plot rotation trials, winter wheat yields were highest following dry pea when moisture was limiting. Compared to wheat following wheat, wheat yields averaged 8% more following spring barley, 17% more following spring canola or mustard, and 21% more following dry pea. Grain legumes and Brassicas are important in sustainable cropping systems that optimize crop production and Brassica crop residue is an important resource to minimize soil erosion.
KEYWORDS: Sinapis Alba, Brassica napus, the Palouse, N response, Sustainable cropping systems, Grain legumes
Winter wheat in northern Idaho has shown significantly higher yields following dry pea than following spring canola, mustard, or barley (H. vulgare L.) during years of limited moisture (Guy and Gareau, 1998; Guy et al., 1995). Mustard and canola crops provide greater production potential and reduced disease levels in subsequent winter wheat crops when compared to wheat or barley preceding winter wheat (Angus et al., 1991; Guy et al., 1995; Hammel, 1995). Nitrogen (N) response in winter wheat can vary due to previous crop because of differences in N fixation, residual inorganic N, soil moisture, soil physical changes, pest dynamics and crop residue interactions (Angus et al., 1991; Guy et al., 1995; López-Bellido et al., 1996).
The objective of these experiments is to evaluate rotational effects of crops grown previous to winter wheat for previous crop performance, residue production and persistence, and relative winter wheat performance from the previous crop effects.
Condiment mustard, dry pea, and lentil were planted in April of 1994 and 1995 in a randomized complete block design with four replications in a Palouse silt loam soil near Moscow, Idaho, USA. Individual plots were 7.6 m by 152 m in 1994 and 4.9 m by 183 m in 1995. The harvest combine was equipped with a chaff spreader to distribute residue onto each plot area. Crop residues were collected from three areas in each plot inside a template area of 0.75 m2 placed across the crop rows. Residual inorganic N was determined to 0.9 m soil depth before winter wheat planting. Seedbed preparation for the subsequent winter wheat crops (Crop Year 2 of each 2-year cycle) was done using conventional tillage and seeding operations. A line-point method was applied to determine percent residue surface cover along a 15 m tape-line. Monthly residue evaluations were taken when conditions allowed from October through April. A 15 m end section in each plot area was divided into sub-plots to establish N fertilizer trials at rates of 0, 45, 90, 135, 180, and 225 kg N ha-1 using the previous spring crop as the main plots, and N rates as sub-plots. The remaining large plot areas were top-dressed with 80 kg ha-1 of N fertilizer. All areas received 49 kg N ha-2, 27 kg P2O5 ha-2, and 18 kg SO4 ha-2 incorporated prior to winter wheat seeding.
In small plot rotation trials (9.1 m by 7.6 m), winter wheat, spring wheat, spring barley, spring canola, mustard, crambe (Crambe abssinica Hochst.), winter rape (B. napus L.), and dry pea were grown from 1991 to 1995 prior to a winter wheat crop. Winter wheat was uniformly cropped across the previous crop areas before dividing into sub-plots for variable N fertilizer application evaluation. The previous crop influences were assessed for the following winter wheat crop agronomic performance and N fertilizer response. Yield responses across years for previous crop effects on winter wheat were compared as a percent of the yield of wheat yield following pea.
Seed yields were different among the three spring crops in the large plot trials (Table 1). The pea and lentil seed yields were above average for the region. Mustard seed yields were slightly lower than expected due to lack of moisture from May through July in 1994 and due to establishment and weed problems in 1995. Soil N nitrate and ammonia assessed to 0.9 m soil depth after harvest was not different among the spring crops.
Table 1. Seed yield, residue production and after-seeding residue surface cover persistence of three spring crops grown during 1994 and 1995 and winter wheat yield following the spring crops near Moscow, Idaho, USA
-- Crop Yield -- Residual ----- Overwinter Residue ----- Winter Wheat
Spring Crop Seed Residue Soil N Oct Dec Feb Mar Yield
----------- kg ha-1 ----------- ------- % Surface Cover ------ kg ha-1
Mustard 1230 6300 48 58 59 62 61 7100
Pea 3460 3470 49 33 31 30 28 7500
Lentil 2420 4170 39 28 28 26 26 7600
Average 2370 4650 46 40 38 36 35 7400
LSD (0.05) 480 920 ns 10 6 7 3 ns
Despite the lower seed yield, the residue produced by mustard was significantly higher than pea or lentil (Table 1). Total biomass (seed + residue) averaged 7530 kg ha-1 for mustard, 6930 kg ha-1 for pea and 6590 kg ha-1 for lentil. This benefit of high residue production and potential erosion reduction contributes to the value of mustard as a crop and is an advantage over dry pea or lentil. Harvest index (seed/(seed + residue) x 100), was significantly different among the three spring crops, 50% for pea, 37% for lentil, and 16% for mustard.
Residue surface cover was 100% for mustard, 88% for lentil, and 84% for pea after harvest. Residue surface cover was reduced during seedbed preparation and planting of winter wheat to 58% for mustard, 33% for pea, and 28% for lentil (Table 1). Residues from all three crops were more durable over the winter than expected, showing little change in surface cover until March. The seedbed preparations and planting of the winter wheat in 1994 were done in dry conditions and little rain fell until late October, slowing the residue decomposition rate. When residue surface cover started to decrease, winter wheat and weed growth after March more than compensated for the decline in crop residue. Residue cover measured on the mustard plots was consistently above the 30 to 50% target level set by USDA conservation plans for erosion control. Pea residue overwinter surface cover was just below this target level after December, while lentil residue was below the target level at all times after seedbed preparation and planting of the winter wheat. This shows the difficulty in providing adequate surface groundcover following legume crops. Also, lentil residue is incorporated or lost from the surface to a greater extent than pea residue from tillage used in these experiments. This probably is due to the smaller size and greater fragility of the lentil residue.
In the 1994-95 trial, winter wheat yields were significantly higher following pea and lentil than following mustard. In the 1995-96 trial, however, wheat yield was not significantly different among previous crop treatments. Averaged over both trials, wheat yields were not different among previous crop treatments (Table 1). Winter wheat grain test weight differences were small, but were highest when wheat followed lentil. There were no significant differences due to previous crop for winter wheat grain protein content or seed weight.
Winter wheat seed yield increased 60% as spring applied N rate increased from 0 to 180 kg ha-1. As shown by regression analysis, yield response to fertilizer N was not different among previous crops in regression line slope or position (R2 = 0.98 to 0.99). However, Linear plateau regression analysis of winter wheat yield response to fertilizer N (R2 = 0.97 to 0.99) revealed optimum fertilizer rates of 135 kg ha-1 following pea or lentil, and 180 kg ha-1 following mustard. This plateau was at a wheat yield of 9400 kg ha-1 following pea and mustard, and 9200 kg ha-1 following lentil.
Results pooled from five crop sequence trials show that winter wheat yield following pea and spring Brassica crops were highest (Figure 1). These trials did not include results from 1994 trials when a dry growing season allowed wheat yield response to residual soil moisture after pea that gave a 20 to 30% yield advantage for winter wheat following pea. Results from other years do not show this advantage to pea over mustard, canola, and crambe as previous crops to wheat. Winter rape did not appear to be as beneficial of a previous crop as spring canola, mustard, or crambe. Wheat yields were at least 25% less following wheat than after canola, mustard, crambe, or pea. These crop sequence trials show that the more different a crop is previous to winter wheat, e.g. spring broadleaf, the greater the rotation advantage. Rotation of broadleaf crops with grass crops appears to be more important than rotating spring and fall crops when weed control is not a factor. In some experiments it appears there was also a rotational advantage to previous crops that do not use all the soil moisture when the following crop season available moisture is limiting to wheat production.
Mustard, canola, crambe, pea and lentil are excellent rotation crops prior to winter wheat. Additionally, some years pea and lentil can provide some carryover soil moisture for dry growing conditions and fix nitrogen. Brassica crops such as mustard, canola, and crambe are excellent rotation crops and provide large amounts of crop residue that is important for erosion control. Grain legumes must be better managed for residue production and retention for soil erosion control. These broadleaf crops are important for sustainable crop production systems on the Palouse and will continue to be grown in rotation with winter wheat.
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