Patrick M. Manning

IS IT FAIR TO COMPARE? ISSUES REGARDING THE TESTING OF GLYPHOSATE-TOLERANT SOYBEANS IN UNIVERSITY VARIETY PERFORMANCE TRIALS

ABSTRACT With the introduction of herbicide-tolerant crop varieties, there is concern that university variety performance trials conducted under conventional herbicide programs may be biased in terms of yield potential. Experiments on dryland and irrigated glyphosate-tolerant soybean (Glycine max) varieties conducted at two Arkansas locations in 1998 and 2000 provide evidence there are no differences in plant height or yield potential across herbicide regimes if effective weed control is established. The research compared conventional herbicide programs to various glyphosate products. While the evidence suggests that university trials are an appropriate means of determining the yield potential of glyphosate-tolerant varieties, producers are still advised to evaluate whether effective weed control will be better attained with a glyphosate or conventional herbicide program.

Preplant and post-plant tillage for full-season soybean on clayey and silt loam soils

Definitive responses to pre- and post-plant tillage are hard to locate in the literature. These tillage responses are usually confounded with other factors. A 2×2 factorial treatment design for pre- and post-plant tillage was implemented in a modified split plot experimental design. Main plots were preplant tillage and sub-plots were post-plant tillage. The experiment was conducted from 1992 through 1994 on a clayey soil at Keiser, AR; 1992 through 1993 on a silt loam at Marianna, AR; and in 1994 on a silt loam at Pine Tree, AR. Grain yield was determined. Soil samples were taken for soil moisture determinations at depths of 0–5 cm, 5–15 cm, 15–30 cm, and 30–45 cm. Crop budgets were generated using production practices and equipment commensurate with commercial farming operations in the area. Results showed that preplant tillage and no-till resulted in equivalent grain yields on clay soil. However, net returns on no-till were equal to or superior to preplant tillage on clay soils across all years. On silt loam soils, both preplant and post-plant tillage main effects were significant for both yield and net returns. Results varied with year on both clayey and silt loam soils. Breakeven analysis indicated that breakeven yield differences were relatively minor across tillage systems. Further, breakeven prices on the clayey soils were generally lower than on the silt loam soils reflecting changes in relative profitability across soil type. Soil water measurements clearly demonstrated the soils function of water storage for ultimate release to the plant during droughty periods. This function was not affected by tillage system in this study.

Comparison of Yield Potential and Herbicide Program Cost Between Herbicide‐Resistant and Conventional Soybean

Abstract Using the results of seventy‐one soybean (Glycine max) variety performance trials, the existence of a yield drag (YD) of herbicide‐resistant varieties when compared to conventional varieties is verified. The trials were conducted from 1996 to 2000 at five locations, on 569 varieties covering three maturity groups in irrigated and dryland conditions using conventional tillage. Trend analysis of YD suggests that it (i) is magnified by irrigation; (ii) has declined to negligible levels; (iii) is not significantly different across maturity group; and (iv) exhibits some locational differences. An analysis of six herbicide programs revealed that glyphosate programs are generally more expensive than reduced‐rate conventional programs but are comparable to or cheaper than full‐rate conventional herbicide regimes. It is further conjectured that seed/technology and herbicide cost differences across seed/herbicide systems will adjust to competitive conditions in the long run.

Analysis of a Novel Bedded Planting System for Dry Clay Soil Management of Full‐Season and Double‐Crop Soybean

Clay soils are difficult to manage to obtain a soybean [Glycine max (L.) Merr.] stand, especially when dry. A novel production system, recently observed on several farms in Arkansas, consists of bedding the dry clayey soil with disk‐bedders, broadcasting the soybean seed over the surface, re‐bedding the seedbed to cover the seed, rolling the beds to flatten the tops and finally furrow irrigating immediately thereafter. Typically this planting system provides a stand of soybean within 4 to 6 days after irrigation and is designated as “hipped” after the common reference of a disk‐bedder as a hipper. Studies were conducted on Sharkey soil at Keiser, AR from 1998 to 2000 for comparing this “hipped” system to more widely used planting methods. On dry clayey soils, a randomized complete block design with three replications was used to compare full‐season soybean under a conventional 96‐cm row system, drilled‐planting into a stale seedbed, and the “hipped” system. A similar study for double‐cropped wheat‐soybean involved straw management (burn or leave) coupled with no‐till drill, tilled drill, and “hipped” systems. Other small studies on the “hipped” system were conducted to investigate the sensitivity to planting depth, soybean plant population, and suitability for obtaining a stand on other crops such as cotton [Gossypium hirsutum (L.)] and grain sorghum [Sorghum bicolor (L.) Moench]. The “hipped” system worked well for obtaining stands of soybean, cotton, and grain sorghum. In essence the “hipped” system 1) provides insurance against poor planting conditions; 2) allows for a reduction in the uncertainty of planting and stand establishment time required; 3) can be used to achieve high plant populations; 4) requires a relatively higher seeding rate in double‐crop than full‐season systems; 5) shows promise for situations when wheat stubble burning becomes curtailed especially if planting can occur early to lead to improved canopy coverage and 6) leads to yield reductions in fields where depth of seed placement cannot be controlled easily.

Evaluation of row-spacing, seedbed preparation, and weed control options for dryland soybean

Production systems for dryland full season soybean consist of seedbed preparation, planting operations, weed control activities and harvesting operations. Often production system choices are interdependent across seedbed preparation, planting equipment and weed control options. For example, the selection of one system option such as no-till seedbed preparation restricts planting equipment choices. Evaluating production systems requires that a holistic approach be employed to account not only for agronomic factors but economic ones as well. Experiments were conducted in 1992, 1993, and 1994 at Keiser, AR and in 1992 and 1993 at Marianna, AR on clayey and silt loam soils, respectively. Treatments consisted of various combinations of preplant tillage (yes or no), row-spacing (narrow or wide), and post-plant cultivation (yes or no) for weed control. Grain yield data was collected and analyzed using SAS. In addition, economic analysis consisted of generating crop production budgets that were used to estimate returns to land, labor and management as well as to conduct breakeven and sensitivity analysis. A no-till system lead to higher grain yield and net returns on clayey soils at Keiser, AR, while the tilled system lead to more stable returns on silt loam soils at Marianna, AR. Results showed that narrow row spacing was superior regardless of soil type and location. Finally, post-plant cultivation was of little consequence given the above recommendations.