Screening of cowpea and soybean varieties for weed suppression

Dan Brainard, 2005

The primary objective of this research was to identify promising cowpea and soybean varieties that could be used as cover crops to suppress weeds while improving soil health under Northeast growing conditions. Five varieties of cowpeas and five varieties of soybeans were grown at two sites: Freeville Organic Research Farm, in Freeville and Starflower Farm in Candor. All varieties were sown in early June at approximately 350,000 seeds/A (70 – 220 lbs/A depending on seed size) and harvested in mid-July, approximately 40 days after planting. Overall, weed suppression of these legumes was inadequate at both sites, and the dominant weed species ( Amaranthus spp. at Freeville, and Portulaca oleracea at Candor) would have produced large amounts of seeds had the trial continued beyond 40 days. Soybean varieties generally performed better than cowpeas, producing more biomass and comparable weed suppression at a considerably lower cost per acre (Table 1). The most promising soybean varieties in both trials were Tara and Tyrone, forage varieties developed by Tom Devine at USDA in Beltsville , MD. These varieties produced the highest biomass at both sites, and were able to suppress weeds better than the other three varieties. Among cowpea varieties, Red Ripper and Blackeye were the best performers at both sites. However, the small seed size and lower seed cost of Red Ripper makes it a much more affordable cover crop than Blackeye. Unusually warm and dry conditions during these trials may have reduced the ability of these legumes to suppress heat-loving Amaranthus spp. and Portulaca oleracea . Future research will examine whether higher seeding rates, or mixtures of these legumes with more weed-suppressive non-legumes (see cover crop mixture trial results) could overcome the weed problems observed in this trial.

Mixtures of legume and non-legume summer cover crops for integrated weed and soil management

Objectives: The primary objectives of this research were to (i) compare biomass production and weed suppressive ability of two legume (soybean and cowpea) and two non-legume (buckwheat and sorghum-sudangrass) cover crops grown in late summer, (ii) test whether mixtures of legumes with non-legumes could improve weed suppression and legume nodulation relative to legumes grown alone, and (iii) evaluate the effect of fertilization on growth, nodulation, and weed suppressive ability of cover crops both alone and in mixture.

Methods: C owpeas (Red Ripper; 168 kg/ha), soybeans (Tyrone; 168 kg/ha), buckwheat (112 kg/ha), and sorghum-sudangrass (56 kg/ha) were sown either alone or in all 4 legume-non-legume combinations (at 50% seeding rate for each component) on July 8, by broadcasting and shallowly incorporating seeds with a harrow. A no-cover crop, unweeded treatment was included as a control. Subplots within each cover crop main treatment were either unfertilized, or fertilized with composted chicken manure (5 5 3) at a rate of 96 kg N/ha. Cover crop and weed samples were taken at 39 Days After Seeding (DAS)—around the time that buckwheat would need to be killed to avoid buckwheat seed production—and at 63 DAS (non-buckwheat treatments only).

Results: At 39 DAS, buckwheat and sorghum-sudangrass both produced about 5 T/ha biomass, more than twice that of cowpea or soybean (Table 1). Buckwheat was by far the best weed suppressor at 39 DAS, reducing weed biomass by 98% compared to the non-cover crop control. However, since no weed seeds were produced at the time of buckwheat incorporation, buckwheat did not provide any meaningful weed suppressive benefit compared to other cover crops. By 63 DAS, sorghum-sudangrass produced about 8 T/ha biomass compared to only 3 T/ha for both cowpea and soybean. By 63 DAS, both legumes were over-run by pigweed ( Amaranthus spp), which was able to produce over 150,000 seeds/m 2 . Even in sorghum-sudangrass treatments, where pigweed biomass was reduced by about 90% compared to weedy control plots, pigweed was able to produce over 20,000 seeds/m 2 . Mixtures of legumes with non-legumes tended to be dominated by the non-legume, resulting in low legume biomass and poor nodulation. For example, at 63 DAS, cowpea and soybean accounted for only 4% and 10% of the final biomass when mixed with sorghum-sudangrass, and only 3% of plants had healthy nodules. Fertilization had no significant effect on the growth of non-legumes (data not shown). However, fertilization of legume cover crops resulted in a 45% increase in pigweed biomass, a 23% reduction in legume biomass and a 55% reduction in nodulation (Table 2). Unusually hot and dry conditions this season may have contributed to the dominance of pigweed and sorghum-sudangrass (both C4 species) relative to soybean and cowpea.

Effects of buckwheat cover crop on selected weeds and winter wheat

The objectives of this experiment were to determine (i) in-season weed suppressive ability of buckwheat and (ii) the effect of buckwheat residues on the establishment of selected weed species and winter wheat. Buckwheat was drilled at a rate of 50 lbs/A on either 15 July (Buckwheat 1) or 3 August (Buckwheat 2). At the second planting date, two buckwheat treatments were established to allow comparison of tilled versus no-till wheat planting. A weed-free control, and 2 weedy control treatments (one for each planting date) were also included. Plots were 10 x 60 ft, arranged in a randomized complete block design with 4 replications. Seeds of barnyardgrass, ECHCG, and hairy galinsoga, GASCI (about 100 seeds of each) were sown at the time of buckwheat planting in four permanent 0.25m 2 quadrats in all buckwheat and weedy treatments. Immediately before mowing buckwheat, shoots of both weeds (by species) and buckwheat were harvested from four permanent quadrats per plot, oven dried for 7 days at 65 C and weighed. Buckwheat was mowed approximately 10 days after anthesis (25 August or 13 September for Buckwheat 1 and Buckwheat 2, respectively) and either disked (Buckwheat 1; Buckwheat 2) or left on the soil surface (Buckwheat 2, no-till). Winter wheat was sown with a no-till drill in all treatments on 19 September. Weed and crop emergence counts, and wheat height were evaluated in two 0.25 m 2 quadrats in each plot at 13 and 28 DAS. In addition, 8 randomly selected plants of winter wheat in each plot were clipped from the soil surface 38 DAS, dried and weighed. To determine the effects of buckwheat residues on the establishment of selected weeds, soil samples were collected from the buckwheat 1 and weed free plots 0, and 15 days after buckwheat incorporation, placed in pots in a growth chamber (25/20 C), and sown with 50 or 100 seeds of four summer annual species (pigweed AMAPO, hairy galinsoga GASCI, barnyardgrass ECHCG, and common purslane POROL) and four winter annual species (yellow rocket BARVU, corn chamomile ANTAR, common chickweed STEME, and shepherd's-purse CAPBP). Emergence of these 8 weed species was monitored daily for 20 days. In addition, five plants per pot were left for biomass sampling. In-season weed suppression by buckwheat in the field ranged from 97 to 99% (Table 1). Buckwheat residue had no significant effect on wheat emergence, but reduced wheat height and biomass, especially when wheat was planted into fresh buckwheat residue (Table 1). In the field, no significant differences in weed emergence were detected in buckwheat and non-buckwheat treatments. However, in growth chamber testing, fresh buckwheat residue significantly reduced emergence and biomass of all weed species except barnyardgrass (Figures 1 and 2). However, buckwheat residue allowed to decompose for 15 days had no effect on emergence (with the exception of AMAPO) or dry weight of any of the weed species (Figures 1 and 3).

Figure 1. Percent reduction in emergence of 8 weed species in 0 or 15 day old buckwheat residue, compared to bare soil.

Figure 2. Dry weight of 8 weed species sown into fresh buckwheat residue and bare soil.

Figure 3. Dry weight of 8 weed species sown into 15 day old buckwheat residue and bare soil.

Table 1. In season weed suppression by buckwheat and effect of buckwheat residues on winter wheat and winter annual weeds.

Treatment	In-season weed biomass (g/m 2 )				Residue effect on winter wheat establishment					Residue effect on winter annual weeds in wheat
Emergence (#) 0.5 m 2		Height (cm)		DW (g)
AMAPO	ECHCG	GASCI	POROL	13DAS	28DAS	13DAS	28DAS	38DAS	CAPBP	STEME
Weed free	0.0	0.0	0.0	0.0	142	172	8.3	19.2	1.2	8.3	0.5
Weedy 1 a	73.4	97.3	7.6	69.9	152	164	7.6	18.8	1.1	4.8	0.8
Buckwheat 1 a	0.4	1.7	0.2	0.4	129	149	6.8	16.2	1.1	0.8	1.3
LSD (0.05)	32.0	55.6	6.0	52.4	-	-	-	-	-	-	-
Weedy 2 a	-	55.5	27.0	5.8	152	150	7.5	18.7	1.1	3.3	2.8
Buckwheat 2 b	-	2.3	2.1	0.4	133	161	6.2	15.2	0.6	7.5	1.8
Buckwheat 2 (No-till)	-	-	-	-	128	156	5.6	16.0	0.6	2.0	0.5
LSD (0.05)	-	45.1	23.1	4.2	28	29	1.0	1.8	0.2	9.0	2.8