WEED MANAGEMENT IN NONIRRIGATED AND IRRIGATED ESP'S PLANTINGS MADE FOLLOWING MINIMUM AND SUBSOIL FALL TILLAGE

OBJECTIVES & SIGNIFICANT ACCOMPLISHMENTS:  

Objective 1) Determine economically best set of production and management practices that include planting date, fall tillage, weed control, and irrigation for stale seedbed production of ESPS management systems.

Objective 2) Determine weed control practices using conventional and Roundup Ready soybean varieties that sustain or increase profits from stale seedbed ESPS soybean grown with and without irrigation.

Objective 3) Determine economic effect of using conventional and RR MG IV varieties in an ESPS environment following rice.

Objective 4) Determine economic feasibility of fall subsoiling and weed management for controlling redvine in nonirrigated soybean grown on a clay soil.

Experiments were conducted at Stoneville, MS on Sharkey clay soil. Row spacing was 20 in. and seeding rate was 5 seed/ft. of row in all experiments. All experiments were planted in a stale seedbed following a preplant application of Roundup to kill weeds. Estimates of costs and returns were developed for each annual cycle of each experimental unit in each experiment.

• Total specified expenses (direct and fixed costs, excluding costs for land, management, and general farm overhead) were calculated using actual inputs for each treatment in each year of the experiment.

• Direct expenses were costs for herbicides, seed, labor, fuel, repair and maintenance, hauling, and interest on operating capital. All weed management costs included costs for ground application.

• Fixed expenses were ownership costs for tractors, self-propelled equipment, implements, and the irrigation system. Costs of variable inputs and machinery were based on prices paid by Mississippi farmers each year.

• Irrigation costs included the cost of an engine, well, pump, gearhead, generator, fuel tank and lines, and polypipe.

• Total fixed costs of the irrigation system consisted of annual depreciation, interest on investment, and insurance. Annual depreciation was calculated using 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 each year using the Mississippi November price of $6.58/bu in 1995, $6.80 in 1996, $6.79 in 1997, $5.69 in 1998, and $4.65 in 1999.

The November soybean price was highly correlated (r = 0.983) with the seasonal average price for the 1984-1993 period. In 1997, beans delivered to the elevator before Sept. 15 received a price bonus that averaged $0.65/bu, and this was added to their income in the 1997 experiments. From these calculations, net returns above total specified expenses were determined for each experimental unit in each experiment each year.

WEED MANAGEMENT USING ROUNDUP READY AND CONVENTIONAL TECHNOLOGY IN ESPS SOYBEAN--1997, 1998, and 1999

Experiments were initiated in 1997 to compare agronomic and economic ramifications of using Roundup Ready and Conventional weed control technology (PRE = preemergent and POST = postemergent) in ESPS soybean plantings grown with and without irrigation.

The treatments for 1997, 1998, and 1999 are shown in Table 1.

The experiment was planted in a stale seedbed on 14 Apr. 1997, 2 Apr. 1998, and 17 May 1999. The 1998 nonirrigated planting site was subsoiled in the fall of 1997, and both nonirrigated and irrigated sites were subsoiled in the fall of 1998. All weed management was with broadcast applications of herbicides.

Canopy was applied at 10 oz. product/acre to treatment 1 of all varieties except H 5545 (Scepter at 2.8 oz. product/acre) in 1997 and to treatment 1 of all varieties in 1998. In 1999, Canopy was applied at 8 oz. product/A to treatments 1 and 2 of all varieties. In 1997 and 1998, the PRE + POST treatments received one POST application of Roundup Ultra (RR), or POST applications of Poast Plus, Storm, and/or 2,4-DB + Lorox (CONVENTIONAL). The POST-only treatments received two applications of Roundup Ultra (RR), or applications of Storm, Poast Plus, and 2,4-DB + Lorox (CONVENTIONAL). In 1999, treatment 1 received PRE Canopy, treatment 2 received PRE Canopy and POST 2,4-DB + Lorox (CONVENTIONAL) or POST Roundup Ultra (RR), treatment 3 received POST Conclude Xtra (CONVENTIONAL) or POST one application of Roundup Ultra (RR), and treatment 4 received POST Conclude Xtra and 2,4-DB + Lorox (CONVENTIONAL) or POST two applications of Roundup Ultra (RR). All second applications of Roundup and the 2,4-DB + Lorox were applied as a directed spray underneath the soybean canopy.

Net returns (excluding costs for land, management, and general farm overhead) were determined. Prices received in 1997 were $7.44/bu. for early-harvested D 478 and $6.79 for the other varieties. In 1998, $5.69/bu. was received for all varieties, while in 1999, a price of $4.65/bu. was the price used to calculate income. The weed expense figures for RR varieties in 1999 include a $9.50/A surcharge for seed cost.

In the nonirrigated 1997 planting, H 5545 RR produced the highest yield and net returns of any variety. The earliest-maturing variety, D 478, produced the lowest NI yield. The other RR varieties, DK 5961 and H 4998, yielded similarly to DP 3588 and Hutcheson; however, because of the slightly lower cost associated with the RR program, net returns from these two varieties were greater than those from DP 3588 and Hutcheson. Use of PRE herbicides in conjunction with POST Roundup on RR varieties vs. POST-only Roundup resulted in lower net returns. Thus, in nonirrigated plantings, use of only POST Roundup on RR varieties resulted in greater net returns.

In the irrigated 1997 planting, both D 478 and Hutcheson outyielded RR varieties, and net returns from them were greater. Use of PRE herbicides in conjunction with POST Roundup on RR varieties vs. POST-only Roundup resulted in lower net returns from irrigated plantings.

In the 1998 nonirrigated planting, the earlier-maturing varieties H 4994 RR, A 4601 RR, and D 478 produced the highest and similar yields, and all yielded above 30 bu/acre. The POST-only treatment of the RR varieties resulted in much higher net returns because of the lower costs and higher yields associated with this weed management. The PRE + POST and POST-only treatments of D 478 produced similar yields and similar net returns. The MG V DP 5806 RR produced the lowest yields and net returns from both weed management treatments.

In the 1998 irrigated planting, use of PRE Canopy resulted in extremely large yield reductions compared to the POST-only treatments of all varieties. The POST-only treatment of Pioneer 9511 produced the highest yield and net returns, while the POST-only treatment of A 4601 RR and D 478 produced similar yields and net returns.

In the 1999 nonirrigated planting, P 9492RR had the highest average yield of 18.4 bu/A, which was at least 5 bu/A greater than yield from any of the other varieties. Yield was not significantly affected by weed management. Net returns to all variety/weed management systems were negative because of the low yields and low price. Net return was affected by both variety and weed management treatment. In general, use of only PRE Canopy or minimum POST herbicides in both RR and Conventional varieties resulted in greater net returns, or the least negative returns.

In the 1999 irrigated planting, variety and weed management treatment interacted to affect both yield and net return. Among RR varieties, H 4994RR yielded the most and produced the highest net returns when PRE Canopy was used, but it and P 9492RR yielded similarly and produced similar net returns when only POST Roundup Ultra was used. DP 5644RR yielded lowest and produced the least return in all weed management treatments. Among conventional varieties, AP 4880 and P 9594 yielded similarly and produced similar net returns in all weed management treatments, and both outyielded P 9511.

These results from three years of experiments indicate that: 1) use of PRE herbicides on RR varieties will result in lower net returns compared to use of POST Roundup only; 2) Use of PRE Canopy at the rate used in these experiments can be detrimental to both RR and conventional varieties in ESPS plantings in an irrigated environment; 3) Use of a POST-only weed management program in both RR and conventional varieties will result in lower costs; 4) high yields and net returns can be obtained from ESPS plantings of early-maturing RR varieties that are irrigated; and 5) the lower weed management costs associated with a RR system, combined with the relatively high yields from these varieties when grown without irrigation (H 5545 in 1997, H 4994 in 1998, and P 9492 in 1999), indicate that the RR system of weed management has greater utility in NI ESPS plantings.

The early-planted soybean production system (ESPS) produces maximum yields in both I and NI environments. Subsoiling has been shown to enhance ESPS dryland yields on some clay soils. Its effect on ESPS I yields has been assumed to be nil, but recent observations indicate this may not be true.

The use of NI and I environments will dictate a different weed control strategy for conventional and RR systems since the water environment in these two situations is different, especially when fall tillage operations may differ. Thus, determination of economically feasible weed control using PRE and POST broadleaf and grass herbicides in I and NI ESPS plantings following minimum and subsoil fall tillage is necessary.

Certain assumptions have been made to arrive at reasonable treatments. Previous research indicates that PRE or POST broadleaf herbicides must be applied in both I and NI environments. However, NI environments rarely require grass weed control measures. Total POST weed control can be used effectively during the first 4-5 weeks after emergence. This experiment will ensure that all early-season weed control treatments are "good" or effective; i.e., an attempt will be made to control broadleaf and grass weeds with no treatments designed to be bad.

The current setup is divided into irrigated and nonirrigated experiments with subsoiled and non-subsoiled environments using MG IV conventional and RR varieties.

Fall subsoiling was conducted on designated areas on 22 Sept. 1998. Subsoiling was followed by surface smoothing of both the subsoiled and non-subsoiled areas with a disk harrow and spring-tooth field cultivator. AP 4880 and SG 468 (MG IV) soybean varieties were planted on 23 Apr. (irrigated) and 3 May (nonirrigated) 1999. Separate nonirrigated and irrigated studies were conducted. Intended weed management treatments are shown in Table 3, while inputs actually used are shown in Table 4. Duplicates or apparent duplicates of some treatments occurred in reality because some intended POST applications were not necessary, or times of application of POST herbicides were different.

Results--Table 5

In the nonirrigated study, yields were extremely low and all net returns were negative. Fall subsoiling increased average yield of AP 4880 by 3.2 bu/A, but average net returns to the two fall tillage treatments were near equal (-63 and -62 $/A). Fall tillage did not significantly affect yield of SG 468RR, and this resulted in a $21/A lower net return from the subsoiled treatment of SG 468. Yields resulting from the weed management treatments of AP 4880 were similar, but net returns were highest from the PRE Canopy- only treatment that had the lowest weed management cost. For SG 468RR, yields from the POST Roundup Ultra-only treatments were slightly higher, and this slightly higher yield coupled with the lower cost of this treatment resulted in the highest net return.

In the irrigated study, fall subsoiling increased average yield by about 4 bu/A, but this yield increase was not enough to significantly increase net return to subsoiling. Average yield of AP 4880 was 61.5 bu/A, and this was significantly greater than the 52.9 bu/A average yield of SG 468RR. Weed management did not significantly affect yield of either variety. However, weed management treatment did affect net returns from each variety. For SG 468RR, highest average net returns of $84 and $96/A resulted from weed management treatments that received only POST Roundup Ultra. For AP 4880, the highest average net returns resulted from weed management that included PRE Canopy plus POST Poast Plus ($111/A) or POST Storm plus Select ($123/A).

MG IV and V soybean varieties were planted on 18 Apr. and 9 May 1995, 30 Apr. and 15 May 1996, 9 Apr. and 12 May 1997, 9 Apr. and 10 June 1998, and 4 May and 24 May following subsoiling of a dry Sharkey clay site. Subsoiling was 15-18 in. deep with a curved-shank implement on 28 Sept. 1994, 2 Oct. 1995, none in 1996, 4 Oct. 1997, and 23 Sept/5 Oct. 1998. Wet soil in the fall of 1996 prevented effective subsoiling of the study area that had been used for the 1996 study. Rainfall preceding subsoiling was: 0.43 in. in Aug. and 1.14 in. in Sept. 1994; 1.41 in. in Aug. and 1.63 in. in Sept. 1995; 4.33 in. in Aug. and 4.39 in. in Sept. 1996 (no subsoiling); 2.81 in. in Aug. (all prior to Aug. 15) and 2.19 in. in Sept. 1997; and 0.71 in Aug. and 2.9 in. in Sept. (all prior to Sept. 17) 1998. All plots each year were maintained in the same location to preserve the integrity of the site.

Results and Discussion--Tables 6 & 7

Weather--Table 6. In 1995, average maximum air temperatures were 4 or more above normal in April, May, and Aug. Moisture deficits (rainfall - pan evaporation) in 1995 were above normal in May and Aug., but below normal in July. The 15 July to 31 Aug. period received only 1 in. of rain. In 1996, May average maximum air temperature was 6 above normal, and May moisture deficit was 5.5 in. above normal. All other months were near normal in temperature, and at or below normal in moisture deficit. In 1997, average maximum temperature and moisture deficit were near or below normal for most months. In 1998, the May through Aug. period generally was hotter than normal, and moisture deficits were below-normal in all months except July. The 15 July to 31 Aug. period received only 0.7 in. of rain. In 1999, the months of June, July, and August were hotter and greatly drier than normal, with only 4.0 in. of rain during the 3-month period.

Yield and Net returns--Table 7. In the ESPS (April) plantings, fall subsoiling resulted in an average yield increase of 3.1 bu/acre in 1995 and 4.7 bu/acre in 1996. Increased costs of $8-9/acre for the subsoiling operation and $3/acre for the additional subsequent seedbed preparation vs. the costs of normal stale seedbed operations resulted in net returns to subsoiling being similar to those from the non-subsoiled treatment in 1995. Net returns in 1996 averaged $21/acre higher from subsoiling. Differences in both yield and net returns among varieties occurred in both years. Subsoiling for subsequent May plantings did not appreciably affect yields or net returns in 1995, and only slightly increased both in 1996.

As stated above, subsoiling could not be effectively performed in the fall of 1996 because of wet soil. Thus, 1997 average yields and net returns from the subsoiled and non-subsoiled treatments both resulted from the same fall seedbed tillage. In 1998, subsoiling significantly increased average yields by 8.7 bu/acre and average net returns by $33/acre in the 9 April planting. The increase was 11.6 bu/acre for MG IV varieties and 5.9 bu/acre for MG V varieties. Net returns from MG IV varieties following subsoiling were similarly higher than those from MG V varieties following subsoiling. The 10 June 1998 planting produced low yields and net returns that were similar in the subsoiled and nonsubsoiled treatments. In 1999, there was no April planting because of wet soil during April. All 1999 yields were low, and all net returns were negative. Subsoiling resulted in small yield increases that only covered additional costs associated with the deep tillage in both May plantings. Yields and net returns from MG IV and V varieties were similar in the 4 May planting, while yields and net returns from MG V varieties were slightly higher in the 24 May planting.

Average net returns to subsoiling Sharkey clay were similar to those from the non-subsoiled treatment in most cases, and were greater in only isolated cases. These results indicate that fall subsoiling of dry clay soil will be a breakeven proposition in the worst-case scenario, and will increase net returns in April but not May plantings in the best-case scenario. In years when the yield response was small, it was still large enough to cover the $11-12/acre additional cost associated with subsoiling and subsequent additional tillage for seedbed preparation. Fall subsoiling was especially beneficial for the early-April planting of 1998 when extremely droughty conditions occurred during the growing season. The extreme drought of 1999 in conjunction with the 4 May early planting date precluded any appreciable effect from subsoiling.

It is possible that Roundup applied to RR soybean varieties will affect Rhizobium spp. that infect and nodulate soybean roots, and fix N. This affect may be manifested by a failure of bacteria to infect roots with Roundup in them, or by the Roundup applied after nodulation killing or reducing the activity of nodulating bacteria.

This experiment was designed to determine if the possible effect of Roundup on RR soybean varieties (yellowing, reduced yield) through its possible effect on N-fixing nodulating bacteria can be overcome or compensated for by the addition of N at planting or soon thereafter. This was done by measuring seed yield from RR and conventional varieties that received PRE and/or POST weed management in conjunction with 0 or 30 lb./acre of added N.

Three MG IV varieties, AP 4880 conventional, SG 468RR, and DP 4750RR, were used.

Four weed management/N combinations were used. They were:

1) PRE and POST weed management without 30# N per acre.

2) PRE and POST weed management with 30# N per acre added shortly after planting.

3) POST weed management without 30# N per acre.

4) POST weed management with 30# N per acre added shortly after planting.

The entire study area was subsoiled in early Oct. 1998, and the experiment was planted on 17 May. Nitrogen (ammonium nitrate source) was applied 3 weeks after planting, with a $7.88 cost of material plus application. Herbicides used for each treatment are shown in Table 8. The weed management expense for Roundup Ready varieties includes a surcharge of $9.50/A for seed. A November 1999 soybean price of $4.65/bu was used to calculate net return to each treatment.

Results are shown in Table 8. Application of early-season N resulted in more expense, no increase in yield, and lower net returns in both RR and conventional varieties grown in both nonirrigated and irrigated environments. In the nonirrigated environment, RR varieties produced slightly higher yields, but the opposite occurred in the irrigated environment. In the nonirrigated environment, all net returns were highly negative because of the extremely low yields from this 17 May planting. In the irrigated environment, highest net returns were obtained with AP 4880 that received PRE + POST weed management. Net returns from RR varieties were higher when POST Roundup weed management vs. PRE + POST weed management was used.

These results indicate that early-season N application to soybean does not benefit yield of either conventional or RR varieties, and will result in lower net returns. These results also indicate that using PRE weed management with RR varieties will result in lower net returns because of the increased cost incurred from using PRE herbicides.

An experiment was initiated to determine the economic feasibility of using fall subsoiling, soybean canopy structure, and in-season roundup for redvine control, and to determine the effect of redvine infestation on ESPS yields in a nonirrigated environment on a clay soil.

Soybean is grown on millions of Delta clay acres that are not irrigated. This dryland or NI environment often results in poor canopy development of soybean, even when grown in narrow rows. These areas are often infested with redvine that manifests itself in the absence of a healthy soybean canopy. Redvine is a deep-rooted perennial species. This allows it to proliferate in this environment, and extract water from soil depths that are not affected by summer drought and that are below the depths used for soybean water extraction. Since drought is the overriding yield limiter of soybeans in a monocrop system, early maturing soybean varieties that avoid drought are the only solution. Use of these early-maturing varieties will allow for early harvest (late September--early October) and fall subsoiling of dry soil that may be used as a management tool to disrupt the root system of redvine, and render it ineffective on a year-to-year basis.

Two basic fall tillage treatments were used. They are:

Subsoiling annually, followed by surface smoothing with a disk and/or spring-tooth harrow, followed by bedding when allowed by soil conditions.

Shallow surface tillage with a disk- and/or spring-tooth harrow followed by bedding when allowed by soil conditions.

Fall tillage treatments were applied 22 Sept. 1998. The entire area was subsoiled and smoothed so that the experimental site would be uniformly treated at the outset. Planting was on 23 April 1999 in a stale seedbed preceded by a Roundup burndown application.

Eight variety/weed management systems were used, and are shown in the following Table 9.

Since the entire area was subsoiled in the fall of 1998, results shown in Table 10 are from the subsoiled treatment only. The weed expense for RR varieties includes a $9.50/A surcharge. SG 468RR was the only variety that provided a positive net return, and the amount was only significant from the POST-only treatment where weed management costs were the lowest. These results indicate that POST-only weed management in ESPS plantings of both conventional and RR varieties will result in the highest net returns. The extremely high weed management costs in the PRE + POST treatment of all varieties indicates that the PRE Canopy application was not effective in controlling weeds in this study.

An experiment was initiated and conducted to determine the economic return from mid-April planted, irrigated, conventional and Roundup Ready MG IV soybean varieties in rotation with rice using PRE + POST and POST-only weed control.

Soybeans following rice will be irrigated in producer situations. Flood and furrow irrigation of soybean are comparable. Roundup Ready soybean varieties have a special utility in soybean/rice rotations. The merits and economic comparisons of PRE + POST vs. POST-only weed control in conventional and Roundup Ready soybean varieties planted in a rotation with rice have not been investigated for early-April plantings on clay soils. This experiment will elucidate the feasibility of these systems in this environment.

Variety/weed management treatments are as follows:

AP 4880, DP 3478, and D 478 conventional varieties with PRE + POST and POST-only weed management.

DP 4750RR, DK 4762RR, and SG 468RR varieties with PRE + POST and POST-only weed management.

The study site is in a 1:1 rotation with rice. The experiment was planted on 23 April. The PRE + POST treatments of both conventional and RR varieties received Canopy + Dual, and follow-up POST weed management was not needed in these treatments. The POST-only treatment for the conventional varieties consisted of applications of Poast Plus, Storm, and Select. The POST-only treatment for RR varieties consisted of 2 applications of Roundup Ultra. All weed management treatments provided excellent weed control. The weed management expense for Roundup Ready varieties includes a surcharge of $9.50/A for seed. A November 1999 soybean price of $4.65/bu was used to calculate net return to each treatment.

Results are shown in Table 11. Total expenses assigned to conventional varieties were greater in the PRE + POST than in the POST-only treatment because of the greater weed management costs in this treatment. The opposite was true for RR varieties. Yields of both conventional and RR varieties were greater from the POST-only than from the PRE + POST treatment. Differences in yield between these two treatments varied with variety, and ranged from a low of 5.5 bu/A difference (AP 4880) to a high of 19 bu/A difference (D 478). Difference between the two treatments ranged from 7.3 to 16.4 bu/A in RR varieties. This difference in yield between the two weed management systems translated to greater net return from the POST-only treatment. The average difference in net return between the PRE + POST and POST-only treatments was greater for the RR varieties. Average yield of SG 468RR and DP 4750RR was similar to the average yield from DP 3478 and D 478.

These results indicate that a total POST weed management program will result in higher yields and greater net returns when ESPS soybeans are grown following rice.

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