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a USDA-ARS, Crop Genetics and Prod. Res. Unit, P.O. Box 343, Stoneville, MS 38776
b USDA-ARS Application and Production Technology Research Unit, P.O. Box 36, Stoneville, MS 38776
c Economist, Dep. of Agric. Econ., P. O. Box 9755, Mississippi State, MS 39762
* Corresponding author (lheatherly{at}ars.usda.gov)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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$62 ha-1.
Abbreviations: CSPS, conventional soybean production system • DAP, days after planting • ESPS, early soybean production system • MG, maturity group • NR, narrow-row system • NETRET, net return • PFA, preplant foliar-applied • POST, postemergence • PRE, preemergence • WR, wide-row system • WTRT, weed management treatment
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The ESPS is suited for the use of narrow-row (0.5 m) culture to accomodate the narrow growth habit of indeterminate MG IV cultivars (Heatherly and Bowers, 1998). However, wide-row soybean production systems are still used because they match the row spacing requirements of other row crops used in a producer's rotation. Narrow-row systems preclude POST cultivation that normally has been used in wide rows (
0.75 m) of the CSPS (Buhler et al., 1997; Hooker et al., 1997; Newsom and Shaw, 1996; Swanton et al., 1998). Thus, determination of economically feasible weed management systems using broadcast-applied PRE and POST herbicides without POST cultivation in narrow rows and band-applied PRE and POST herbicides with POST cultivation in wide rows in the ESPS is necessary. These management systems must provide options for economical management of both summer broadleafs and grasses to attain maximum economic yield.
Soybean, especially that not irrigated, provides relatively low gross return with a small profit margin in the midsouthern USA (Heatherly et al., 1994; Heatherly and Spurlock, 1999; Williams, 1999). The small profit margin from soybean grown without irrigation dictates that all costs associated with production, including pest management costs, must be minimized. In previous research at Stoneville where drought was common during the reproductive period of soybean, level of weed management in a nonirrigated CSPS was of little consequence in regard to soybean yield when the weeds present were not highly competitive species (Heatherly et al., 1994). However, it is impractical to plan a weed management program for soybean based on the assumption that drought stress will result in low and unprofitable yield since weed management expenditures are made early in the growing season.
Inputs used for weed management in soybean represent a significant cost (Buhler et al., 1997; Heatherly et al., 1994; Johnson et al., 1997), and must be managed early (PRE) or on an as-needed basis (POST). In narrow-row soybean plantings made in a stale seedbed, effective weed management systems will almost exclusively involve herbicides (Johnson et al., 1997, 1998; Oliver et al., 1993). Use of combinations of PRE and POST herbicides with POST cultivation is commonplace in wide-row production systems in the midsouthern USA (Askew et al., 1998; Heatherly et al., 1993, 1994; Hydrick and Shaw, 1995; Oliver et al., 1993; Poston et al., 1992).
Many weed management systems will provide a similar level of weed control, but cost differences can be large (Buhler et al., 1997; Heatherly et al., 1993, 1994). These cost differences, coupled with differences in yield among weed management systems, can mean significant differences in net return among systems of weed management (Buhler et al., 1997; Heatherly et al., 1993, 1994; Johnson et al., 1997). The best weed management systems for ESPS plantings must be determined to maximize profits from this higher-yield-potential production system.
Plantings using the ESPS are being made in fields previously cropped by the CSPS where weed control was excellent. Row spacing–cultivar–weed management systems for continuous ESPS plantings in these situations have not been evaluated. The objective of this study was to determine the effect of PRE and POST weed management on weed cover, seed yield, and net return for continuous nonirrigated plantings of a MG IV cultivar grown in 0.5-m-wide rows and a MG V cultivar grown in 1-m-wide rows.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Several assumptions were made in the selection of the two systems used in this study. An indeterminate MG IV cultivar with its non-branching canopy structure was assumed to be suited to NR, while a determinate MG V cultivar with its branching, bushy canopy structure was assumed to be suited to WR. A wide-row system will necessarily have a wider open space to fill, and MG V cultivars that branch profusely and form a bushy canopy should fill the between-row area. On the other hand, MG IV cultivars do not branch significantly above the lower nodes and have a narrow, upright profile. Thus, they are not as likely to close the open area in wide rows. Therefore, the selection of the two systems was based on the expectation of each cultivar's particular canopy structure fitting its assigned row width.
Plots were 4 m wide (eight rows in NR and four rows in WR) and 30.5 m long. Seeding rate was 16 seed m-1 of 0.5-m-wide rows and 32 seed m-1 of 1.0-m-wide rows. Treatments were randomly assigned to plots in 1994, and remained in the same location thereafter. ‘RA 452’ (1994 and 1995) and ‘DK 4875’ (1996) (MG IV indeterminates) were planted in NR and ‘DP 3589’ (MG V determinate) was planted in WR on 20 April 1994, 17 April 1995, and 10 April 1996. All plantings were made in a stale seedbed where glyphosate at 840 g ai ha-1 in 94 L water ha-1 was applied preplant to kill emerged weeds at planting.
Weed management treatments each year were: WTRT 1, PRE broadleaf management; WTRT 2, POST broadleaf management; WTRT 3, PRE broadleaf plus POST broadleaf management; WTRT 4, PRE broadleaf plus PRE grass management; WTRT 5, PRE broadleaf plus POST grass management; WTRT 6, PRE grass plus POST broadleaf management; WTRT 7, POST broadleaf plus POST grass management; WTRT 8, PRE broadleaf plus POST broadleaf plus PRE grass management; WTRT 9, PRE broadleaf plus POST broadleaf plus POST grass management; and WTRT 10, PRE broadleaf plus POST broadleaf plus PRE grass plus POST grass management. The study rationale and selection of the above WTRTs assumed the following.
Selection of PRE herbicides was based on expected weed populations, whereas selection of POST herbicides was based on expert opinion resulting from assessment of the presence and size of particular weed species in plots of each WTRT within each system. Herbicides applied to each WTRT within each system are shown in Table 1. Herbicides were broadcast-applied to NR and applied to a 0.5-m-wide band centered over each row in WR. Between-row areas in WR were cultivated three times in 1994 and 1995 and twice in 1996. Rainfall of 20 mm or greater was received 9, 2, and 3 d following PRE herbicide applications in 1994, 1995, and 1996, respectively.
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Total weed cover by species was determined prior to harvest each year, but after soybean leaf senescence had begun (Elmore and Heatherly, 1988) to measure the season-long effect of the weed management treatments. Visual estimates of weed cover in 10% increments from 0 to 100% were made in five randomly chosen 0.5-m2 sample areas in each plot to estimate cover for each weed species. If a species was merely present in any of the samples of an individual plot, then its relative abundance was assigned the lowest possible score (0–10%), with an average of 5% cover in that sample. This is similar to the process used by Yelverton and Coble (1991) to measure weed resurgence at the end of the growing season following early-season application of weed management treatments intended to give 100% control.
Estimates of costs and returns were developed for each annual cycle of each experimental unit by the Mississippi State Budget Generator (referred to as MSBG—Spurlock and Laughlin, 1992). Total specified expenses were calculated from retail cost for each treatment input in each year of the study, and included all direct and fixed costs, but excluded costs for land, management, and general farm overhead which were assumed to be the same for all treatment combinations. Direct expenses included costs for herbicides, seed, labor, fuel, machinery repair and maintenance, hauling harvested seed, and interest on operating capital. Fixed expenses were ownership costs for tractors, self-propelled harvesters, implements, and sprayers. Costs of variable inputs and machinery were based on prices paid by Mississippi farmers each year; i.e., machinery costs varied with year. Annual depreciation of all machinery was calculated by the straight-line method with zero salvage value. Annual interest charges were based on one-half of the original investment times an appropriate interest rate for each year of the study. Insurance was estimated at 1% of the original investment.
Income from each experimental unit was calculated from the market-year average price of $0.21/kg in 1994, $0.24/kg in 1995, and $0.26/kg in 1996 for Mississippi. Yearly prices vs. an average long-term price were used to reflect the effect of market forces on income for each individual year. Net return above total specified expenses was determined for each experimental unit each year.
Soybean plant height at maturity was recorded for each sub-plot just prior to harvest. A field combine modified for small plots was used to harvest the four center rows of NR plots and the two center rows of WR plots. Soybean seed were harvested on 14 Sept. (RA 452) and 22 Sept. (DP 3589) 1994, 7 Sept. (RA 452) and 20 Sept. (DP 3589) 1995, and 9 Sept. (DK 4875) and 24 Sept. (DP 3589) 1996. Harvested seed were weighed and adjusted to 130 g moisture kg-1 of seed. Analysis of variance [PROC MIXED (SAS Institute, 1996)] was used to evaluate the significance of effects on weed cover, seed yield, and net returns. Analyses across years treated year as a fixed effect to determine interactions involving year. Analyses for individual years treated system (NR and WR) and WTRT as fixed effects. Mean separation was achieved with an LSD at P 0.05.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Average minimum and maximum air temperatures, and rainfall and pan evaporation for May, June, July, and August of each year are shown in Table 2. The major difference in the weather of the 3 yr occurred in July and August. July rainfall was greatest in 1994 (295 mm) when it was spread throughout the month (last rain of 66 mm on 26 July), and exceeded pan evaporation by 130 mm. August rainfall in 1994 was negligible. The high July rainfall evidently provided moisture for most of the seedfill period of both cultivars (21 July–19 Aug. for MG IV cultivar and 29 July–2 Sept. for the MG V cultivar). In 1995, all but 30 mm of the 148 mm of July rain occurred before 6 July. Only 66 mm of rain was received during the remainder of July and all of August, and this low amount provided inadequate moisture during the seedfill period of both cultivars. In 1996, over one-half of the low July rainfall of 84 mm occurred on 31 July, and August rainfall totaled 110 mm. These rainfall amounts and times favored the seedfill period of the MG V cultivar (19 July–27 Aug.) vs. that of the MG IV cultivar (24 June–2 Aug.).
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Weed Cover and Weed Management Costs
In 1994, all POST herbicides were applied between 6 May (16 DAP) and 7 June (48 DAP). In 1995, all POST herbicides were applied between 31 May (44 DAP) and 19 June (64 DAP). In 1996, all POST herbicides were applied between 20 May (40 DAP) and 5 June (56 DAP). POST grass control was not needed in 1995 and 1996 (no grasses present), and this component was not applied to treatments designated to get a POST grass herbicide. All weed management treatments controlled target species at time of application. Thus, weed cover at harvest represented those weeds that appeared after weed management measures had been completed each year.
All weed cover values in 1994 and 1995 were below 10% in both NR and WR (Table 3). In 1994, average weed cover in the NR system was 6%, and this significantly exceeded the 2% average cover in the WR system (Table 3). Pitted morningglory was the dominant species. System x WTRT interacted in 1995 and 1996 to significantly affect weed cover. In both years, weed cover in the PRE broadleaf management treatment (WTRT 1) of the NR system (8% in 1995 and 21% in 1996) exceeded that in all other WTRTs of that system, as well as that in all WTRTs of WR. This sometimes greater weed cover in the NR system of this study is different from the relationship between NR and WR weed populations in the northern USA (Mickelson and Renner, 1997) and Ontario, Canada, (Swanton et al., 1998). This is attributed to the early maturity of the MG IV cultivar in the NR system of this study, which resulted in late-season weed infestations during soybean maturity and canopy opening in August. In the WR system, all weed cover values were 2% in 1995, and ranged from 5 to 11% in 1996, with no significant differences among WTRTs. The dominant weed species in 1995 again was pitted morningglory, while in 1996, both browntop millet and pitted morningglory were dominant in both NR and WR. Barnyardgrass [Echinochloa crus-galli (L.) Beauv.] was the only other annual grass that appeared prominently in any treatment. The occurrence of annual grass at soybean maturity in 1996 was likely due to 110 mm of rain that occurred in August when soybean was maturing and the soybean canopy was opening.
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Yield and Economics
Analysis across years indicated that yield and net return were significantly affected by year x system, year x WTRT, and/or year x system x WTRT interactions. Therefore, yield and net return data are presented by years.
In 1994, system, WTRT, and the system x WTRT interaction significantly affected yield and net return. Yield from the NR system was greater in all WTRTs except the PRE broadleaf management (WTRT 1), PRE grass plus POST broadleaf management (WTRT 6), and PRE plus POST broadleaf plus PRE grass management (WTRT 8) treatments (Table 4). The lack of a difference between NR and WR in the PRE broadleaf management treatment (WTRT 1) resulted from the relatively high yield in WR, while the lack of a difference between NR and WR in the POST broadleaf plus PRE grass management (WTRT 6) and PRE plus POST broadleaf plus PRE grass management (WTRT 8) treatments resulted from relatively low yield in NR. Within WR, yields from the PRE broadleaf management (WTRT 1) and PRE grass plus POST broadleaf management (WTRT 6) treatments exceeded that from the POST broadleaf plus POST grass management (WTRT 7). In NR, yields from the PRE broadleaf plus PRE grass management (WTRT 4), PRE broadleaf plus POST grass management (WTRT 5), PRE plus POST broadleaf plus POST grass management (WTRT 9), and PRE plus POST broadleaf and PRE plus POST grass management (WTRT 10) treatments exceeded those from the POST broadleaf plus PRE grass (WTRT 6) and PRE plus POST broadleaf plus PRE grass management (WTRT 8) treatments. In the NR system, highest net returns were attained from PRE broadleaf management (WTRT 1), POST broadleaf management (WTRT 2), PRE broadleaf plus PRE grass management (WTRT 4), and PRE broadleaf plus POST grass management (WTRT 5) treatments (Table 5). These treatments consisted of only PRE or POST broadleaf management, or PRE broadleaf and either PRE or POST grass management, and were the cheapest. Within the WR system, PRE broadleaf management plus POST cultivation (WTRT 1) resulted in greater profit.
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In 1996, the 2360 kg ha-1 average yield from WR exceeded the 1515 kg ha-1 average yield from NR (Table 4). This difference was attributed to the aforementioned 1996 rainfall pattern that favored the MG V cultivar in WR. The large yield difference between WR and NR resulted in higher average net return from WR ($366 ha-1 vs. $118 ha-1) (Table 5). Average yield was not significantly affected by WTRT, but average net return was. Highest net returns were $297 ha-1 (WTRT 1) and $292 ha-1 (WTRT 5), and these WTRTs also had the lowest weed management costs.
In both NR and WR, use of only PRE broadleaf management or only POST broadleaf management resulted in similar yields all 3 yr (Table 4). However, greater cost was associated with using only POST broadleaf management in NR, and this resulted in greater net return from the PRE broadleaf management in 1 yr, and a trend toward greater net return from this treatment in the other 2 yr in NR (Table 5). In WR, there was no clearcut trend in net returns that favored either treatment. As mentioned earlier, band application of all herbicides in WR vs. broadcast application in NR resulted in lower weed management costs for WR in all treatments all years. This contributed to the different pattern in net return differences between the two systems. Swanton et al. (1998) determined that glyphosate followed by a PRE application of a residual herbicide was the most "risk-efficient" weed management system in both NR and WR systems.
In both NR and WR, the addition of either PRE or POST grass herbicides to either PRE of POST broadleaf herbicides (WTRT 4 and WTRT 5 vs. WTRT 1, or WTRT 6 and WTRT 7 vs. WTRT 2) did not improve yield (Table 4) or net return (Table 5). In fact, the additional cost of grass herbicides with no concommitant increase in yield sometimes resulted in lower net returns (Table 5). Thus, use of grass herbicides in either NR or WR systems was not necessary in these nonirrigated ESPS plantings. This is counter to the results of Johnson et al. (1997), who found that both grass and broadleaf weed management was necessary to attain full yield potential in May and June plantings in Missouri.
In NR, soybean with PRE and POST broadleaf plus PRE grass management (WTRT 8) had yield similar to that from the PRE and POST broadleaf management only treatment (WTRT 3), but net return was usually lower. Soybean yield using PRE grass plus POST broadleaf management (WTRT 6) was similar to that from using only POST broadleaf management (WTRT 2), but net return was lower. The large amount of money spent for weed management in WTRT 6 was spent on unnecessary herbicides, since yield from this treatment was low.
In NR, several combinations of PRE and POST broadleaf and grass herbicides produced similar soybean yields (Table 4), but a large difference in the costs of these combinations occurred (Table 5). For example, the PRE broadleaf plus POST grass management treatment (WTRT 5) averaged 2065 kg ha-1 across the 3 yr with weed management costs of $96, $59, and $59 ha-1, respectively. On the other hand, WTRT 10, the most expensive weed management treatment at $175, $175, and $138 ha-1 for 1994, 1995, and 1996, respectively, averaged only 2155 kg ha-1 across the 3 yr. Evidently, the money spent for weed management in WTRT 10, as well as others, was unnecessarily high, since yields produced from these treatments were not significantly higher than yields from other treatments with lower cost and resulted in net returns that usually were lower.
In WR, yield differences among WTRTs were small and usually not significant (Table 4). However, significant differences in net returns among WTRTs within WR existed (Table 5). In all 3 yr, the five WTRTs with the least weed management costs were also the five highest ranking treatments in yield. Generally, the less money spent on weed management in WR, the greater the net returns. Cultivation used in conjunction with band application of herbicides in WR resulted in reduced production costs and increased net returns. This agrees with results from earlier studies conducted on early-May plantings at this location (Heatherly et al., 1993) and on June plantings in South Carolina (Poston et al. 1992). We did not experience any of the problems with interrow cultivation of clay soil that are mentioned by Swanton et al. (1998).
The use of net return to assess the weed management combinations resulted in a different conclusion than if yield alone was used. Use of any one of the 10 weed management combinations in this short-term study delivered similar weed management (Table 3) and yields (Table 4), but net returns (Table 5) were often different among WTRTs. In essence, using only PRE broadleaf management with either NR or WR in early-planted soybean in this dryland environment was sufficient to obtain the highest net return all 3 yr. This was true even though late-season weed infestations occurred in WTRT 1 in NR in 1996 (Table 3). Conversely, the use of only POST broadleaf management either alone (WTRT 2) or in combination with PRE or POST grass herbicides (WTRTs 6 and 7) resulted in lowered net returns because of higher weed control costs in NR, while the use of only POST broadleaf management (WTRT 2) in WR resulted in net returns that were always similar to those from WTRT 1. In this dryland environment where natural weed populations and yield potential were low, weed control expenditures of $62 ha-1 in both NR and WR were sufficient to achieve maximum net returns. The results from this short-term study indicate that neither weed management costs nor soybean yield can be used separately to determine the most economical system of weed management in nonirrigated ESPS plantings.
These results offer no clearcut reason to choose NR over WR or vice versa in a nonirrigated ESPS, which is counter to results obtained in more northern latitudes (Mickelson and Renner, 1997; Swanton et al., 1998). As stated earlier, weed management in NR was more expensive than in WR with no commensurate increase in weed control. Yields from most WTRTs used in NR were greater in 1994, while average yield from WR was greater in 1996. The difference in yield between the two was not significant in 1995. In 1994, 4 of the 10 WTRTs in NR resulted in greater net returns than the same treatments in WR, while in 1995, 3 of the 10 WTRTs in WR resulted in greater net returns than the same treatments in NR. In 1996, average net return across WTRTs in WR was greater than that from NR, and this was probably related more to weather pattern than to the system. The choice of using NR or WR in the ESPS depends on whether a MG IV cultivar (non-branching growth type, short stature, and early maturity suited to narrow rows) or a MG V cultivar (branching growth type, taller stature, and later maturity suited to wide rows) is desired. These results show that the cheapest weed management used for both NR and WR systems in nonirrigated ESPS plantings resulted in the greatest net returns, and only management of broadleaf weeds was required at this site which is representative of millions of dryland soybean hectares in the region. We suggest that annual grasses either emerge in these early plantings too late to be competitive, or they are not competitive in the dryland environments where late-season soil moisture is limited as typified by the conditions of this study. These conclusions should pertain to the large clay hectarage in the midsouthern USA where johnsongrass has been controlled.
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ACKNOWLEDGMENTS |
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Received for publication April 21, 2000.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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