Introduction

Conservation-tillage systems are primarily used to address concerns about soil erosion, soil quality, and water availability (Blevins et al., 1971; Reeves, 1994; Reeves, 1997; Kaspar et al., 2001). Peanut (Arachis hypogaea L.) hectarage in conservation-tillage systems is estimated to be 45, 30, 25, 20, 10% respectively, in Alabama, Florida, Georgia, North Carolina, and Virginia (personal communications, Dallas Hartsog, Barry Brecke, Eric Prostco, David Jordan, and Joel Faircloth). Approximately 98% of peanut grown in Alabama, Florida, Georgia, North Carolina and Texas in 2004 received herbicides (Anonymous 2005). Practical alternatives to the intensive use of herbicides for peanut weed control offers economical as well as environmental benefits.

The use of cover crops in conservation tillage has advantages, one of which is weed suppression through physical and chemical allelopathic effects (Nagabhushana et al. 2001; Phatak 1998). Cereal rye and soft red winter wheat are common winter cover crops recommended for peanut production in the U.S. (Wright et al., 2002). These cover crops contain allelopathic compounds that inhibit weed growth (Akemo et al., 2000; Chase et al., 1991; Perez and Ormeno-Nunez, 1991; Yenish et al., 1996).

In southern Brazil, black oat is the predominate cover crop on millions of hectares of conservation-tilled soybean [Glycine max (L.) Merr.] due in part to its weed suppressive capabilities (Derpsch et al., 1991). Black oat has recently been introduced in the southeastern U.S. through a joint release between Auburn University and The Institute of Agronomy of Paraná, Brazil, and is currently marketed as “SoilSaver black oat” (Bauer and Reeves, 1999). In a greenhouse study, allelopathic compounds released from black oat inhibited cotton root elongation 16% compared to rye when residue was mixed with soil (Bauer and Reeves, 1999). However, in a field study where residue remained on the soil surface, cotton stand establishment was not affected by black oat, rye, or wheat winter cover crops and cotton lint yield was higher in plots containing black oat residue compared to rye (Bauer and Reeves, 1999). No other published research has been conducted evaluating black oat as a winter cover crop preceding row crop establishment in the U.S.

Typically, cooperative extension service recommendations in the southeastern U.S. encourage growers to terminate cereal winter cover crops relatively early prior to row crop production, citing concerns for excessive residue interfering with planting operations or excessive moisture depletion (Jost et al., 2006). Cooperative extension service recommend waiting approximately 2 wk after desiccating cereal winter cover crops before cash crop planting to avoid allelopathic effects on the following crop (Reeves, 1994).

The Brazilian conservation-tillage system is based on terminating cover crops during early reproductive growth, by treating with glyphosate and mechanically rolling the cover crop to form a dense mat of residue on the soil surface into which crop seeds are planted (Derpsch et al., 1991; Reeves, 2003). Technological advancements in cover crop residue management are allowing producers to maximize cover crop biomass in conservation agriculture systems located where water utilization by winter cover crops is not usually a concern (Kornecki et al., 2005; Raper et al., 2004). In the southeastern U.S., winter cereal cover crops reach anthesis and can be terminated in a timely fashion prior to the recommended planting window for peanut. Ashford and Reeves (2003) evaluated a mechanical roller-crimper as an alternative method for termination of a black oat, rye, and wheat cover crop. Their results indicated that use of a roller-crimper in combination with glyphosate at 0.84 kg ae/ha at anthesis was as effective as using glyphosate at 1.68 kg ae/ha alone when treatments were applied at the same crop growth stage, for all cover crops evaluated. Few growers are currently utilizing roller-crimpers to manage cover crops; however, grower interest in this management technique exists due to the potential to reduce soil erosion and increase water infiltration and soil moisture storage (Truman et al., 2002).

While research has evaluated weed-suppressive qualities of winter cover crops, few experiments have evaluated peanut response. Therefore, our objective was to evaluate weed control provided by black oat, rye, and wheat as winter cover crops within three herbicide input systems, compared to winter fallow, for conservation-tilled peanut using the Brazilian system of managing cover crops. Peanut yield was also evaluated for each cover crop and herbicide input system.

Materials and Methods

Field experiments were conducted from autumn 1994 through autumn 1997 at the Alabama Agricultural Experiment Station's Wiregrass Research and Extension Center (31°24′N, 85°15′W), located near Headland, AL. The soil was a Dothan fine sandy loam (fine-loamy, siliceous, thermic Plinthic Paleudults). The experimental area had been in conservation tillage (strip-tillage) for the previous 8 y and had a high population of Palmer amaranth (Amaranthus palmeri S. Watts.).

The experimental was set up as a strip-plot design with a factorial arrangement of treatments with four replications. Horizontal strips consisted of black oat, rye, wheat, or fallow. The seeding rate for all cereal cover crops was 120 kg/ha and were established utilizing a Great Plains^{H 5} no-till drill⁶ in early November of 1994, 1995, and 1996. Ammonium nitrate was applied at 56 kg/ha to the cover crops in fall of 1994 and 1995 after establishment. Ammonium nitrate was not applied in 1997 due to an oversight. Cover crops were terminated 3 wks prior to planting peanut in early May each year with an application of glyphosate at 1.12 kg ae/ha utilizing a compressed CO₂ backpack sprayer delivering 140 L/ha at 147 kPa. Biomass from black oat, rye, wheat, and fallow plots was measured immediately before glyphosate application in all years. The above-ground portion of each cover crop and weeds in the winter fallow plots was clipped from three randomly-selected 0.25-m² sections in each plot, dried at 60 C for 72 h, and weighed.

Within 3 d following glyphosate application, the cover crops were rolled with a mechanical roller-crimper as described by Ashford and Reeves (2003) to flatten all residues on the soil surface. The peanut variety GK 7 was planted each year at a rate of 28 seed per meter of row with a four-row John Deere MaxiMerge^H planter⁷ equipped with Martin® row cleaners⁸ and ACRA-PLANT^H retrofit seeding double-disk openers⁹. Plots were four 92 cm rows wide and 9.1 m long.

Vertical plots were herbicide input systems consisting of no herbicide, a low input system consisting of preemergence (PRE) followed by (fb) postemergence (POST) herbicides, or a high input system consisting of PRE fb sequential POST herbicides. The PRE fb POST herbicide low input system consisted of pendimethalin at 1.12 kg ai/ha fb an additional early POST application of paraquat at 0.14 kg ai/ha plus bentazon at 0.56 kg ai/ha. The PRE fb sequential POST herbicide high input system contained the aforementioned herbicides fb 2,4-DB at 0.22 kg ai/ha plus chlorimuron at 0.14 kg ai/ha applied late POST.

In 1995, because the site had a well-developed hardpan, the experimental area was in-row sub-soiled prior to planting with a KMC^H narrow-shanked parabolic subsoiler¹⁰, equipped with pneumatic tires to close the subsoil channel (Raper, 2005). In 1996 and 1997, the area was in-row subsoiled prior to planting with a bent-leg subsoiler (Paratill¹¹) 2 wks prior to planting. In all years, residue disturbance was minimal and residue formed a thick (approximately 10 cm) mat over the soil surface with exception of the winter-fallow plot treatment. Weed control was determined by visual ratings a 0%  =  no control, 100%  =  complete control) early in the season [30 d after planting (DAP)] and late in the season (51–98 DAP). Only late season ratings are reported. All weed species present at both ratings were evaluated for control, as a reduction in total above ground biomass resulting from both reduced emergence and growth, and the combined average for each rating and treatment was calculated. Late season weed control ratings were averaged over all dominant weed species.

Alabama Cooperative Extension System recommendations were used for insect control and nutrient management. Peanut yield was determined by machine-digging the middle two rows of each plot and harvesting with a combine.

All data were subjected to analysis of variance (ANOVA) using the general linear models procedure in SAS (SAS, 1998) to evaluate the effect of a three (herbicide input level) by four (cover crops) factorial treatment arrangement. Herbicide input levels and winter cover crops were considered fixed effects while year effects were considered random variables. Non-transformed data for visual evaluations were presented because arcsine square root transformation did not affect data interpretation. Means for appropriate main effects and interactions were separated using Fisher's protected LSD test at P ≤0.10 a priori. Where interactions occurred, data were presented separately and where interactions did not occur, data were combined.

Results and Discussion

Cover Crop Biomass

Analysis revealed a year by treatment interaction; therefore, results are presented by year. In 1995, residue production was similar for all winter cereal cover crops, averaging 5,230 kg dry matter/ha (Table 1). Winter weeds produced 1,410 kg dry matter/ha in fallow plots. Dominant winter weeds in the fallow system in all three years were cutleaf evening primrose (Oenothera laciniata Hill) and chickweed [Stellaria media (L.) Vill.]. The severe winter of 1995–1996 resulted in differences in residue production between all cover crops. Dry matter averaged 6,550, 4,370, 1,320, and 870 kg/ha for rye, wheat, black oat, and winter fallow, respectively, in 1996. The minimum nighttime temperature from November 1^st through March 31^st was below 0 C for 56 nights in 1995–1996 (−13 C lowest temperature) compared to 33 nights in 1994–1995 (−8 C lowest temperature) and 26 nights in 1996–1997 (−10 C lowest temperature) (Alabama Mesonet weather data). In 1997, residue production was similar for rye (2,840 kg/ha) and black oat (2,770 kg/ha); however, wheat (1,600 kg/ha) and winter fallow (770 kg/ha) produced less biomass. Because nitrogen fertilizer was not applied to winter cover crops in 1997, dry matter production was less than prior years. Yenish et al. (1996) reported rye planted into a sandy loam soil resulted in biomass ranging from 4,540 to 5,140 kg/ha in an experiment conducted in North Carolina. Bauer and Reeves (1999) reported an average biomass of 5,300, 2,980, and 3,010 kg/ha for rye, black oat, and wheat, respectively, planted into a loamy sand soil in an experiment conducted in South Carolina. Ashford and Reeves (2003) reported higher biomass for rye, black oat, and wheat in experiments conducted in east-central Alabama when evaluating effectiveness of a roller-crimper for cover crop desiccation. Averaged over two years, dry biomass was 10,100, 9,700 and 9,100 kg/ha for rye, black oat, and wheat; respectively (Ashford and Reeves, 2003).

Table 1

Cover crop biomass for three years at the Alabama Agricultural Experiment Station's Wiregrass Research and Extension Center in Headland, AL prior to termination.

Weed Control

Analysis revealed a year by treatment interaction; therefore, results are presented by year. Cover crop and herbicide input system main effects were significant for weed control. There were no interactions in 1995, 1996, or 1997 between cover crops and herbicide input systems for weed control. Grasses {primarily large crabgrass [Digitaria sanguinalis (L.) Scop.] and Texas panicum (Panicum texanum Buckl.)}, sedges [(Cyperus esculentus L.) and (C. rotundus L.)], sicklepod (Cassia obtusifolia L.), and Palmer amaranth were the dominant weed species present during peanut production all three years. No cover crop was adequate in controlling weeds without herbicide season long in all three years. In 1995, black oat and rye cover crops controlled the weed complex 70% and 61% respectively without herbicides, compared to wheat (43%) and winter fallow (24%) (Table 1). PRE fb POST herbicides increased control to 93% and greater when a winter cover crop was utilized. Additional sequential POST applications in the high input system did not result in increased weed control compared to the low input system. However, in 1996, sequential POST applications in the high input system result in increased weed control ranging from 5 (rye winter cover crop) to 18 percentage points (black oat winter cover crop) compared to the low input system. Without herbicide, rye gave similar visual control to black oat and wheat (25%); all cover crops provided superior weed control compared to winter fallow (15%). In 1997, the combination of low herbicide input system and rye provided 84% weed control compared to 69, 60, and 59% for black oat, winter fallow, or wheat systems, respectively. Similar to 1995, additional sequential POST applications in the high input system did not result in increased weed control compared to the low input system in 1997 regardless of winter cover crop.

In three years, rye cover crop; in one year, black oat; and in two years, wheat in combination with PRE fb POST herbicides provided similar weed control compared to the PRE fb sequential POST herbicides herbicide systems (Table 2). Yenish et al. (1996) reported increased short term weed control utilizing a non-rolled rye cover crop in no-till corn (Zea mays L.), but not season-long control. Reddy (2003) reported that rye reduced total weed density 27% in a no-till soybean [Glycine max (L.) Merr.] system 6 wks after planting. Black oat's popularity as a cover crop in Brazil is largely due to its ability to control both annual grasses and small-seeded broadleaf weeds (Derpsch, 1985; Derpsch, 1990). Weed control following black oat was similar to rye in 1995 and 1997 when biomass production between these two covers was similar (Table 1).

Table 2

Weed control^a affected by cover crop and herbicide system for three years at the Alabama Agricultural Experiment Station's Wiregrass Research and Extension Center in Headland, AL 51 to 98 DAP.

Table 3

Peanut yields affected by cover crop and herbicide system for three years at the Alabama Agricultural Experiment Station's Wiregrass Research and Extension Center in Headland, AL.

The authors would like to thank Jeffrey A. Walker (Agricultural Research Technician, USDA-ARS) and Eric B. Schwab (Support Agronomist, USDA-ARS) for their assistance in conducting this experiment.

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