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© 2007 Plant Management Network.
Accepted for publication 20 July 2007. Published 23 October 2007.

Sequence and Rotation Effects on Pest Incidence and Grain Yield of Double-cropped Soybean and Pearl Millet After Winter Wheat and Canola

G. D. Buntin, Department of Entomology, B. M. Cunfer and D. V. Phillips, Department of Plant Pathology, University of Georgia, Griffin 30223; and J. P. Wilson, USDA-ARS, Crop Genetics and Breeding Research Unit, University of Georgia, Tifton 31793

Corresponding author: G. D. Buntin. gbuntin@griffin.uga.edu

Buntin, G. D., Cunfer, B. M., Phillips, D. V. and Wilson, J. P. 2007. Sequence and rotation effects on pest incidence and grain yield of double-cropped soybean and pearl millet after wheat and canola. Online. Crop Management doi:10.1094/CM-2007-1023-01-RS.

Abstract

The crop-sequence and rotational effect of incorporating alternative crops of canola (Brassica napus) as a winter crop and pearl millet (Pennisetum glaucum) as a summer crop into a double-crop system of winter wheat (Triticum aestivum) and soybean (Glycine max) on crop pest incidence, stand, and grain yield was studied at Plains, GA. Twelve double-cropping sequences that substituted winter canola and pearl millet at various intervals in a wheat/soybean double-crop system were studied for five seasons. Previous crop sequence had little effect on stand and grain yield of winter canola but the incidence of Sclerotinia stem rot was greater when canola was grown continuously. Winter infestations of the Hessian fly, Mayetiola destructor (Say), in wheat were reduced following canola than wheat but tended to be greater following pearl millet than soybean. Stand of both pearl millet and soybean were reduced following canola as compared to wheat. Infestations of false chinch bugs (Nysius raphanus Howard) were greater on seedling stands of both summer crops following canola. Stand losses in pearl millet were associated with false chinch bug infestations but stand losses in soybean most likely were due to several factors. Both crops compensated for stand differences so that grain yield was not affected by previous winter crop. Infection rate of soybean by soybean stem canker was not affected by previous crop sequence. In general pearl millet had little effect on pest incidence and agronomic performance of other crops studied. Stand reductions of summer crops probably can be mitigated so that the beneficial effects of canola in reducing insect and diseases in winter wheat can be realized in a wheat/soybean double-crop system.

Introduction

Double-cropping is an important cropping practice in areas of the southern United States where length of growing season and adequate rainfall or irrigation permit timely stand emergence, development, and maturity of a summer crop. The predominant double-crop sequence is winter wheat (Triticum aestivum L.) and soybean [Glycine max (L.) Merr.], although grain sorghum [Sorghum bicolor (L.) Moench] and cotton (Gossypium hirsutum L.) are sometimes grown as a double-crop with wheat. Double-cropping has advantages of increased cash flow for producers, as well as reduced soil erosion and water losses by having ground cover most of the year. Double-cropping also provides cost savings from more intensive use of the land and better utilization of crop inputs, labor, and capital investments (16). However, over time double-cropping can result in continuous production of the same crops in the same field each year which can cause a build up of damaging levels of disease, insect, and weed populations. Indeed, in the 1970s and 1980s continuous double-crop production of winter wheat resulted in take-all root and crown rot, caused by the fungus Gaeumannomyces graminis var. tritici (Ggt), becoming a serious limiting factor in many fields (12,13). Devastating outbreaks of the Hessian fly, Mayetiola destructor (Say), also occurred in wheat throughout the Southeast in the mid to late 1980s in part because of the lack of rotation in double-crop systems (3,4).

Incorporating alternative crops that are culturally and biologically compatible with a soybean/wheat double-crop system could help reduce pest incidence and severity and also provide farmers with commodity marketing alternatives. Winter canola (Brassica napus L.) is an alternative winter grain crop that provides high quality edible oil for various uses and defatted meal for livestock and poultry (11,15). Improved, adapted cultivars and improved agronomic practices and pest control information have made canola a profitable alternative to winter wheat in the Southeast (19). Pearl millet [Pennisetum glaucum (L.) R. Br.] is a new alternative high-quality feed grain for poultry (1,7,8,14). Although other crops such as cotton or grain sorghum can be substituted for soybean in a double-crop system, pearl millet for grain production is attractive in non-irrigated systems because of its short growing season and inherent tolerance to heat and drought conditions (1,17).

Limited information is available on the biological and agronomic compatibility of incorporating winter canola and pearl millet into wheat-soybean double-crop systems. We established a five-year study in the Coastal Plain region of GA to examine the effects of incorporating canola and pearl millet in multiple year rotational sequences on the agronomic performance and pest incidence and severity in a wheat-soybean double-crop system. Wilson et al. (17) presented results from this study of previous crop and crop rotation sequences on pearl millet diseases and agronomic characteristics. They found that pearl millet stand was lower following canola than wheat in two of three years. They also found that the incidence of stalk and neck rot (caused by Fusarium graminearum) infection on pearl millet was greater following canola than wheat, and the severity of smut (caused by Moesziomyces penicillariae) on pearl millet was enhanced after three continuous years of cultivation. Nevertheless, despite these effects, previous winter or summer crop or number of sequential years of pearl millet cultivation had no detrimental, limiting impacts on pearl millet disease severity or grain yield (17). In contrast, Cunfer at al. (5) showed from this five-year study that a single year of canola production greatly reduced the incidence and severity of infection by take-all root and crown rot in winter wheat as compared to continuous wheat production thereby preventing yield loss in wheat the following season due to this disease. However, pearl millet did not affect the incidence or severity of take all in the following wheat crop.

Presented here are the results from this five-year study not previously reported (5,17) on the effects of cropping patterns and rotational effects on Hessian fly in wheat and on seedling insect pests of all crops in the study and agronomic performance of canola and soybean. Rotational effects on two diseases known to be enhanced by continuous cropping, Sclerotinia stem rot caused by Sclerotinia sclerotiorum in canola and soybean stem canker infection caused by Diaporthe phaseolorum var. caulivora also were examined.

Cropping Sequence and Rotation Field Study

The experiment was conducted on a Greenville sandy loam (pH 6.3 and < 1% organic matter) at the Southwest Branch Experiment Station near Plains, GA. The plot area was in a winter wheat soybean double crop system before plots were established with the winter crop planting in 1994. Twelve rotation treatments listed in Table 1 were established in a balanced randomized complete block design with four replications. Plots measured 35 by 35 ft (0.028 acre). Rotations included winter wheat (cv. Savannah), winter canola (cv. Oscar), winter rye (cv. Wrens Abruzzi), or fallow. Summer crops were pearl millet (hybrid HGM 100) for grain production or soybean, cv. Brim soybean in 1995 and cv. Deltapine 105 in the other years. Deltapine 105 is a short season, maturity group V variety soybean. Cotton (cv. Deltapine 51) was included in some rotations in 1998 and 1999 and pearl millet was not planted in 1999.

Table 1. Crops and double-cropping rotation sequences in twelve treatments over five seasons.

 * Winter crop-Summer crop: C = canola, Co = cotton, F = fallow, M = pearl millet, R = rye, S = soybean, W = wheat.

Tillage in the fall was disk harrowing twice and in the spring was strip tillage which conventionally prepared an 8-inch strip for planting summer crops in 36-inch rows. Winter wheat and rye were planted on 9 to 12 November each year in 7-inch rows with a small-plot grain drill at the rate of 23 seeds per ft of row. Canola was planted on 22 October to 8 November in 7-inch rows at 5 lb of seed/acre. Severe cold temperatures killed canola stands in December 1995. Canola was re-planted in 14 March 1996 and killed with glyphosate on 23 May before summer crop planting. Summer crops were planted with a Monosem air planter. Soybeans were planted on 3 to 19 June at the rate of 8 seeds per ft of row and pearl millet was planted on 8 to 24 June at 2.5 to 3.0 lb of seed per acre. Yield was measured from a 5-ft wide strip for winter crops or 2 rows for summer crops using a Hege small-plot combine. Wheat and canola were harvested on 18 May to 4 June. Pearl millet was harvested in mid September and soybeans were harvested late October in each year.

Fertility and herbicide regimes are previous reported for pearl millet (17) and the other crops (5). No insecticides were used on any crop and no fungicides were used on soybean and pearl millet in any year. Canola seed was treated with benomyl (Benlate, DuPont, Wilmington, DE) and wheat seed was treated with carboxin (Vitavax 200, Bayer CropSciences, Research Triangle Park, NC). The experiment was not irrigated in the first three years, but summer crops were irrigated with 5 and 7 inches of water in 1998 and 1999, respectively.

Winter wheat plots were sampled for Hessian fly infestation on 1 February and 9 May 1995, 26 January and 18 April 1996, 5 February and 24 April 1997, and 29 January and 5 May 1998 by collecting plants in two samples of 1 ft of row per plot. Tillers were dissected and the number of larvae plus puparia counted and the percentage of infested tillers (stems) also determined.

Winter canola stand was measured 25 to 30 days after planting by counting plants in two adjacent 3.2-ft sections of row. Aphids on seedling plants also were counted. Insects infesting canola during pod/seed fill stage in the spring were sampled twice in 1997 and 1999 by taking 10 pendulum sweeps per plot. The incidence of stem infection by Sclerotinia stem rot also was determined in spring of 1997, 1998, and 1999 by counting all infected stems in one row per plot.

Soybean and pearl millet plant numbers were counted at 20 to 26 days after planting in each year by counting plants in two 10-ft sections of row in each plot. The incidence of soybean stem canker on soybean also was measured in each plot by counting the number of infected symptomatic plants in one row. Numbers of false chinch bug (Nysius raphanus Howard) were measured in each plot from three 1-ft² areas of ground centered over a row of plants at 12 to 16 days after planting in each year. The occurrence of other insects attacking pearl millet and soybean was noted.

Results of rotation treatments were analyzed by sample date with analysis of variance for a randomized complete block design. Main effects of winter and summer crops and cropping sequence and number of continuous years of production were compared for most variables using F tests of selected rotation treatments. Means were separated using Fisher’s protected LSD (α = 0.05).

Effect of Crop Sequence and Rotation on Pests and Diseases of Winter Crops

Wheat. Rotational effects were not established in the 1994/1995 crop season. In the 1995/1996 and 1996/1997 seasons, winter infestations of Hessian fly were significantly greater following the wheat-pearl millet rotation than the wheat-soybean, canola-millet and canola-soybean rotations (Table 2). Spring infestations also were greater following wheat than canola and pearl millet than soybean with this result being highly significant in the 1996/1997 season. Hessian fly infestations were not significantly different between rotations in 1997/1998. Rotational effects on wheat diseases and yield are reported previously (5). Despite differences in Hessian fly infestations, wheat yields were not significantly different between treatments of continuous wheat and wheat rotated with canola in control plots where take all infection was low (5).

Table 2. Effect of previous crop rotation on Hessian fly infestations in winter wheat over three seasons.

 ‡ Analysis of square-root arsine transformed percentage values.

Means within previous crop treatments and year followed by the same letter are not significantly different (P < 0.05; LSD); *, ** indicate significant F value at P < 0.05 and P < 0.01, respectively.

Canola. Canola stand averaged 4.2 ± 1.1, 3.0 ± 1.0, 3.2 ± 0.6, and 6.0 ± 1.5 plants per ft of row in 1995, 1997, 1998, and 1999, respectively, and were not affected be previous winter or summer crop (F = 0.70-2.56; df = 1, 6-12; P > 0.05). Freezing temperatures in December 1995 completely killed canola stands before any results could be collected. Incidence of Sclerotinia sclerotiorum was very low in 1996/1997 with only two infected plants being recorded from all plots. In 1998 and 1999, Sclerotinia incidence was greater after previous winter canola than wheat but was not affected by the number of years of canola cultivation (Table 3). Sclerotinia infection also was greater after soybean than millet in 1998 but previous summer crop did not affect incidence in 1999.

Table 3. Effect of double-cropping and crop rotation treatment on infection by Sclerotinia sclerotiorum in winter canola.

Means within previous crop treatments and year followed by the same letter are not significantly different (P < 0.05; LSD); * significant F value at P < 0.05.

The number of aphids, mainly turnip aphid [Lipaphis erysimi (Kaltehbach)], on seedling canola were low in all years and were not significantly different among rotational treatments (data not shown, F = 0.10-1.65; df = 2-4, 6-12; P > 0.05). Large numbers of false chinch bugs were present in canola plots canola immediately before harvest in 1995, 1997, and 1999. Sweep sampling of canola during pod and seed development in 1997 and 1999 revealed that one generation of tarnished plant bug [Lygus lineolaris (Palisot de Beauvois)] also developed on plants between flowering and harvest.

Canola grain yield averaged (± SE) 1450 ± 44 lb/acre in 1995, 1350 ± 88 lb/acre in 1997, 1440 ± 62 lb/acre in 1998, and 2860 ± 88 lb/acre in 1999. No grain was produced in spring 1996. Grain yield was not significantly different between rotations in any year (F = 0.60-1.47; df = 2-4, 6-12; P = 0.67-0.27). Furthermore, no significant rotational effects were found between the sequence and number of years of canola cultivation and canola grain yield (F = 0.52-1.16; df = 1-2, 6-12; P > 0.05).

Effect of Crop Sequence and Rotation on Pests and Diseases of Summer Crops

Pearl millet. Treatment effects on pearl millet stand, yield and disease incidence during the first three years have been presented previously (17). They showed that millet plant stands were significantly lower following canola than wheat in 1995 and 1997, but were not different between winter crops in 1996. Stands in 1998 also were not affected by winter crop (Table 4). False chinch bugs infested millet seedlings in all years but numbers were greatest in 1995 when they were more abundant following canola than wheat or rye (Table 4). Feeding injury by false chinch bugs caused seedlings in several plots to be stunted and killed resulting in large gaps in the stand. False chinch bugs also were more numerous following canola than wheat in the other years but differences were statistically significant only in 1997.

Table 4. Effect of previous winter crop on pearl millet stand and number of false chinch bugs on seedling plants^x during four years.

Means within rows and years followed by different letters are significantly different (P < 0.05; LSD); * significant F value at P < 0.05.

 ^x Plants and chinch bugs were sampled 20 to 26 days after planting in each year.

 ^y Plant stand data for 1995, 1996, and 1997 from Wilson et al. (17).

Grain yields averaged (±SE) 2100 ± 150, 2010 ± 360, and 1620 ± 150 lb/acre in 1995, 1996, and 1997, respectively, and were not affected by previous winter crop or rotational sequence of pearl millet cultivation (17). Yield in 1998 averaged 2330 ± 190 lb/acre and also was not affected by previous winter crop or rotational sequence (F = 0.81; df = 1, 15; P = 0.45).

Soybean. Plant stands were significantly lower following canola than wheat or rye in four of five years (Table 5). Previous summer crop and years of soybean cultivation did not affect soybean stand in any year (data not shown: F = 0.01-0.23; df = 1, 6 to 18; P > 0.05), except 1999 when stand was lower after millet than soybean in the previous year (F = 12.14; df = 1, 15; P < 0.01). False chinch bugs were more abundant on soybean seedlings following canola than wheat in all years, although differences were significant only in 1997 and 1999 (Table 5). However, little visible feeding injury to seedlings was observed in any year. Incidence of stem canker infection was not significantly affected by previous winter, except in 1997 when stem canker incidence was lower after canola than wheat (Table 5). Previous summer crop or sequence of soybean cultivation did not affect stem canker infection in any year (data not shown, F = 0.07-3.35; df = 1, 6-18; P > 0.05).

Table 5. Effect of previous winter crop on stand, number of false chinch bugs, stem canker infection and grain yield of soybean in five years.

Means within rows and years followed by different letters are significantly different (P < 0.05; LSD); * and ** significant F value at P < 0.05 and P < 0.05, respectively.

Severe drought in 1997 limited soybean yields in all rotations. Furthermore, defoliation during grain fill by soybean looper [Pseudoplusia includens (Walker)] and velvetbean caterpillar (Anticarsia gemmatalis Hübner) exceeded 50% in all soybean plots by late September in 1995 and 1997 which also may have reduced yield. Soybean grain yield was not significantly affected by previous winter crop in any year except 1996 when soybean yielded less after canola than winter wheat or rye (Table 5). Reduced grain yield in 1996 may have been associated with the replanting of canola in March 1996 and planting soybeans into early-flowering stage canola. Previous summer crop and previous years of soybean cultivation did not affect soybean yield except in 1996 when continuous soybean production yielded less than 1-year rotated soybeans (data not shown, F = 18.10; df = 1, 12; P < 0.01).

Implications for Double-Crop Management

Previous summer and winter crop and cropping sequence had little detrimental effect on canola grain yield in any year. However, planting canola after canola did enhance infection levels of Sclerotinia sclerotiorum in both years where the disease was present, although increased infection rate did not reduce grain yield. Current canola production guidelines recommend planting canola only one in four years to help avoid infection by blackleg disease, caused by the fungus Leptospharia maculans (4). Blackleg infection was not found in any year until the last year when very low levels were observed. Our results suggest that more frequent rotations of every one or two years may be feasible if blackleg is absent or if canola varieties with high levels of blackleg resistance are grown.

Cunfer et al. (5) demonstrated that wheat rotation with canola for one year was effective in suppressing take-all stem and root rot in wheat the following season. We also showed that canola as the previous winter crop reduced winter infestations and, to some extent, spring infestations of Hessian fly. Furthermore, the wheat-soybean rotation had lower winter infestations levels of Hessian fly than a wheat-pearl millet rotation. Reduced Hessian fly infestations in rotations with canola is understandable because of the lack of host plants, but the reason for increased infestation levels following pearl millet compared with soybean is not clear. Possibly the herbicide used in millet did not control volunteer wheat in late summer as well as in soybean, thereby providing a bridging host for the first fall generation of Hessian fly which develops in volunteer wheat before planting of the winter wheat crop (3,4).

Large numbers of false chinch bugs infested canola as the crop matured and remained in plots following canola where they infested seedling stands of pearl millet and soybean. Infestations following wheat and rye usually were much smaller. Millet seedling injury from false chinch bug feeding was evident 1995 and 1997 when millet stands were reduced following canola. Finding false chinch bug as a seedling pest of pearl millet has not been previously documented. Not observed during this study was the chinch bug [Blissus leucopterus leucopterus (Say)], which is recognized as a pest of summer grass crops such as pearl millet especially after double-cropping with winter small grains (9).

Soybean stands were 18 to 25% lower following canola than following small grains in all years except in 1998. False chinch bugs also were more abundant on soybean seedlings following canola but seedling injury was not apparent. Although false chinch bug injury may be a contributing factor, soybean stand losses most likely were caused by physical interference of the canola stubble with planter performance or possibly by undetermined chemical or biological parameters associated with canola stubble.

Stands of soybean and pearl millet usually were reduced when planted into canola stubble as compared to winter wheat, rye, or fallow. However, previous cropping sequence did not reduce grain yield of pearl millet (17) or soybean indicating that both crops compensated for stand losses. Porter (10) also found no detrimental effect of canola on yield of double cropped soybeans under several tillage systems. Both soybean and pearl millet can tolerate a considerable range of plant populations without affecting grain yield (18). Therefore, rotating canola with wheat to disrupt take-all disease (5) and Hessian fly in wheat can be done without detrimental, limiting effects on subsequent soybean or millet crops as long as seeding rates of these crops are not below the minimum for a full stand. Nevertheless, scouting of seedling stands of summer crops following canola for false chinch bugs should be done to prevent seedling injury and stand loss.

Acknowledgments

We thank W. R. Slaughter, Jr., J. Youmans, D. Spradlin, and H. Fowler for technical assistance, and S. Jones and R. Pines for assistance with management of field plots. This research was funded in part by a grant (L94-57) from the Southern Region USDA Sustainable Agriculture Research and Education program.

Literature Cited

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