Introduction
In the fall of 1989 a five-year study of the impact of various alternative crop rotations on crop yield and soil fertility was initiated at the North Willamette Research and Extension Center. The rotations involved ranged from five years of grass seed production on one extreme, to a vegetable-only rotation, unbroken by grain, grass seed, or legume crop, or by winter cover crops. The primary objective of the vegetable portion of this study is to investigate the effects of several rotations on the yield and quality of vegetable crops in the rotation and to determine the contribution of winter cover crops to the nutrient needs of the vegetable crop. Spring, 1990 saw the first introduction of vegetables into the rotation plots. Future years will involve different vegetable crops in the rotations.
Methods
The plot area is a Willamette silt loam, pH 5.5, which was planted to wheat in the fall of 1988. Four tons per acre of ground agricultural limestone was applied in September, 1989, to bring the soil pH to 6.0. Fescue, clover, and wheat plots of 30 x 60 feet were established in late September and early October of 1989, while other plots were plowed and left fallow. Another set of plots was seeded to cereal rye at 60 pounds per acre or to cereal rye at 40 pounds per acre plus Austrian pea at 80 pounds per acre.
On 2 March, 1990, all fallow, rye, and rye-pea plots were disked to turn under the cover crop and start preparations for planting. The plots were again disked on 30 March and were tilled with a Roterra on 2 April. Following cultimulching, the plots were seeded with 'Earliking' sweet corn on 26 April, with 12 rows per plot on 30-inch centers. No fertilizer was applied at planting. On 3 May, atrazine (1.2 pounds per acre) or atrazine (1.2 pounds) plus alachlor (2.0 pounds per acre) were applied to the appropriate plots. At the same time 50 pounds N per acre was applied as urea to eight rows of each 12-row plot, establishing a split between fertilized and unfertilized subplots.
Due to insufficient stands, the plots were disked, cultimulched, and replanted on 15 May, with no additional herbicide. On 18 June, additional urea was applied at 150 pounds N per acre to half (20 x 30 feet) of the previously fertilized area, resulting in a total N rate of 200 pounds per acre on these subplots. The experimental design was a factorial combination of cover crop-herbicide treatments applied to main plots, with N fertilizer rate as subplot. The plots were irrigated weekly at about one inch per week with a drip system set between every other row. Ears and stover were harvested from 70 square foot sections of each subplot on 16 August. Subsamples of ears and stover were transported to the OSU Department of Crop and Soil Science for determination of dry weight yields. The plots were disked, plowed, and prepared for cover crop seeding in September.
Results
Seedling stands were not sufficient for the first planting (Table 1). This was due to a combination of poor emergence and stand loss to birds. Emergence was significantly affected by the presence of a winter cover crop, with nearly double the stand on fallowed plots compared to the mean for plots with either the rye or rye plus pea cover crops. Type of cover crop and herbicide applied did not affect the stand. Soil condition at time of planting seemed ideal, with no surface cover crop residues to interfere with the seeding operation. Soil moisture was nearly ideal for planting. The poor emergence may have been due to an allelopathic effect of the cereal rye residues. An adequate stand was obtained on all plots after replanting and the replanted stand did not vary with treatment.
Fresh weight ear yield increased on cover crop plots as opposed to winter-fallowed plots, and yields were slightly higher on plots wintered with a rye-legume cover rather than rye alone (Table 2). Stover fresh weight and mean ear weight responded similarly. Number of ears harvested per plot and ear length did not vary with cover crop treatment. Herbicide had no effect on yield. Tipfill was rated on a three point scale, with higher means for ears from plots which had a winter cover crop. Yield of ears and stover, mean ear weight, and ear length all increased with increasing rate of nitrogen. The number of ears harvested per subplot also increased with increasing rate of N as there were more mature ears at time of harvest on highly fertilized plots. The tipfill rating did not vary with N rate. Stover moisture content increased with increasing rate of applied N and on cover crop plots as compared to fallowed plots. Kernel moisture content did not vary significantly with treatment but there was a tendency toward higher moisture content with increasing rate of applied N.
Crop response to added N was normal in this season, allowing a rough estimate of the fertilizer value of the cover crop. The first increment of applied urea resulted in increased fresh yield of 0.8 tons per acre. A rye cover crop resulted in an average yield increase of 0.4 tons per acre. If we assume that the response is due solely to cover crop nitrogen content, then the rye cover appeared to contribute about 25 pounds N per acre to the corn crop. Similarly, the rye-pea mix contributed about 50 pounds N per acre.
Table 1. Effects of winter cover crop and herbicide on the stand of sweet corn seeded on April 26, 1990. Stands recorded on May 14, NWREC, Oregon Winter cover crop Herbicide Seedlings per meter Fallow atrazine and alachlor 3.5 Rye atrazine 1.4 Rye atrazine and alachlor 1.8 Rye and Austrian pea atrazine 1.6 Rye and Austrian pea atrazine and alachlor 2.1 LSD (0.05) 1.2 Single degree of freedom comparisons Cover vs fallow: ** Rye vs. rye-pea NS atrazine vs. combination NS Cover x herbicide NS Table 2. Main effects of cover crop and nitrogen fertilizer rate on yield and quality of 'Earliking' sweet corn, crop rotation experiment, NWREC, Oregon, 1990 Fresh wt. (T/A) No. ears Mean ear Ear length Tipfill % moisture Treatment Ears Stover per plot fr. wt. (g) (inches) rating Ear Stover Cover crop Fallow 4.1 5.7 34.8 169 8.6 0.8z 80 76.7 Rye-low inputy 4.4 6.6 34.8 183 8.7 1.1 78 80.5 Legume-low input 5.0 7.4 35.7 205 8.7 1.3 77 77.5 Rye 4.3 6.9 33.3 184 8.5 1.1 75 79.0 Legume 4.6 7.0 35.0 190 8.8 1.2 80 79.5 LSD (0.05) NS 0.9 NS 19 NS NS 2.5 N rate 0 lb/A 3.6 6.4 29.8 177 8.4 1.1 77 77.7 50 4.4 6.4 35.5 181 8.7 1.1 78 78.3 200 5.4 7.3 38.9 202 8.9 1.2 79 80.0 **lin **quad **lin **lin *lin NS *lin Single degree of freedom comparisons Cover crop:none * ** NS ** NS NS * Rye:legume-rye * * NS * NS NS NS Atraz:atraz-alach NS NS NS NS NS NS NS zThree point scale with 0=more than 1 inch of tip unfilled, 1=0.25-1 inch unfilled, 2=less than 0.25 inch unfilled. yLow input: atrazine only at planting rather than the standard combination of atrazine plus alachlor.