Soybean improvement via plant breeding has been critical for the success of the crop. The objective of this study was to quantify genetic change in yield and other traits that occurred during the past 80 yr of North American soybean breeding in Maturity Groups (MGs) II, III, and IV. Historic sets of 60 MG II, 59 MG III, and 49 MG IV soybean cultivars, released from 1923 to 2008, were evaluated in field trials conducted in 17 U.S. states and one Canadian province during 2010 to 2011. Averaged over 27 MG II and MG IV and 26 MG III site-years of data, the estimated rates of yield improvement during the 80 yr were 23 kg ha -1 yr -1 for MGs II and III, and 20 kg ha -1 yr -1 for MG IV cultivars. However, a two-segment linear regression model provided a better fit to the data and indicated that the average current rate of genetic yield gain across MGs is 29 kg ha -1 yr -1 . Modern cultivars yielded more than old cultivars in all environments, but particularly in high-yielding environments. New cultivars in the historic sets used in this study are shorter in height, mature later, lodge less, and have seeds with less protein and greater oil concentration. Given that on-farm soybean yields in the United States are also increasing at a rate of 29 kg ha -1 yr -1 , it can be inferred that continual release of greater-yielding cultivars has been a substantive driver of the U.S. onfarm realized yield increases.

Experiments were conducted to investigate the interactions of tank-mix combinations of sethoxydim plus the sodium salt of bentazon, the sodium salt of acifluorfen, fomesafen, imazaquin, or the ethyl ester of chlorimuron. Antagonistic interactions were observed with tank-mixes of sethoxydim plus bentazon, imazaquin, or chlorimuron applied for fall panicum, large crabgrass, and goosegrass control in field experiments. Antagonism was observed in greenhouse experiments with tank-mixes of sethoxydim plus bentazon or imazaquin applied to goosegrass. Bentazon, acifluorfen, and fomesafen reduced14C-sethoxydim uptake by large crabgrass. However, imazaquin and chlorimuron did not affect14C-sethoxydim uptake. In field experiments, no interactions occurred with tank-mixes of sethoxydim plus any of the broadleaf weed control herbicides applied to entireleaf or tall morningglory.

A B S T R A C TThe effects of supplemental nitrogen (N) on soybean [Glycine max (L.) Merr.] seed yield have been the focus of much research over the past four decades. However, most experiments were region-specific and focused on the effect of a single N-related management choice, thus resulting in a limited inference space. Here, we composited data from individual experiments conducted across the US that examined the effect of N fertilization on soybean yield. The combined database included 207 environments (experiment × year combinations) for a total of 5991 N-treated soybean yields. We used hierarchical modeling and conditional inference tree analysis on the combined dataset to establish the relationship and contribution of several N management choices on soybean yield. The N treatment variables were: N-application (single or split), N-method (soil incorporated, foliar, etc.), Ntiming (pre-plant, at a reproductive stage, etc.), and N-rate (from a 0 N control to as much as 560 kg ha). Of the total yield variability, 68% was associated with the effect of environment, whereas only a small fraction of that variability (< 1%) was attributable to each N variable. Averaged over all experiments, a single N application and the split N application were 60 and 110 kg ha −1 greater yielding than the zero N control treatment, respectively. A split N application with more than one method (e.g., soil incorporated and foliar) resulted in 120 kg ha −1 greater yield than zero N plots. Split N application between planting and reproductive stages (Rn) resulted in greater yield than zero N and single application during a Rn; however, the effect was not significantly different than N application at other growth stages. Increasing the N rate increased the environment average soybean yield; however, 93% of the environment-specific N-rate responses were not significant which suggested a minimal effect of N across the examined region. A large yield variability was observed among environments E-mail address: mourtzinis@wisc.edu (S. Mourtzinis).Abbreviations: BNF, biological nitrogen fixation; C, check (no nitrogen was applied); MM, major management practices; N, nitrogen; N-rate, nitrogen rate; N-application, number of nitrogen applications; N-method, method of nitrogen application; N-timing, timing of nitrogen application (growth stage/s); P, all nitrogen was applied at planting only; PR, split nitrogen application at planting and reproductive growth stages; pP, all nitrogen was applied at pre-planting only; Rn, reproductive growth stage; R, all nitrogen was applied at a reproductive growth stage only; RR, split nitrogen application at two reproductive growth stages; V, all nitrogen was applied at a vegetative growth stage only; Vn, vegetative growth stage MARKwithin the same N rates, which was attributed to growing environment differences (e.g., in-season weather conditions, soil type etc.) and non-N related management (e.g., irrigation). Conditional inference tree analysis identified N-timing and N-rate to be conditional to irriga...

Greenhouse and field experiments were initiated to evaluate rapeseed and mustard species as green manure crops for weed suppression. Under greenhouse conditions incorporating 20 g fresh wt leaf and stem tissue of rapeseed, two white mustards, and brown mustard into 450 g Sharpsburg, silty clay loam soil resulted in significant reductions in weed emergence, biomass, and height. Kochia, shepherd's-purse, and green foxtail emergences were reduced by all green manure crops. Redroot pigweed emergence was reduced by all green manure crops except brown mustard, and velvetleaf emergence was reduced by white mustards only. Kochia and shepherd's-purse fresh weights were reduced by all green manure crops, while redroot pigweed and velvetleaf fresh weights were reduced by brown mustard and white mustard var. Salvo. Green foxtail fresh weight was reduced by all green manure crops except rapeseed. With the exception of shepherd's-purse, no relationship between glucosinolate content of the incorporated green manure and suppression of weed growth was found. Under field conditions, early spring-planted green manure crops reduced early season weed biomass in soybean at one of the two locations. Mustard species as green manure crops reduced total weed biomass in soybean by 40% 4 weeks after emergence (WAE) and 49% 6 WAE. Soybean biomass and yield were sometimes reduced by the incorporation of green manure crops in treatments containing weeds; however, hand-weeded plots with green manure treatments yielded similar to hand-weeded plots without green manure.

This stability allows for reduced risk of crop failure and encourages wider acceptance of ESPS (Boquet, 1998).

from several sources, including mineralized soil organic matter, symbiotically fixed N, and N incorporated into Research on the effects of N and B applications on soybean [Glyplant tissue. Demand for seed N is highest from the R5 cine max (L.) Merr.] yield is limited. Experiments were conducted kg ha Ϫ1 applied from seedling emergence to R1 in-Holland Rd., Suffolk, VA 23437; and M.M. Alley, Dep. of Crop and creased yields by 720 kg ha Ϫ1 , averaged across all treat-

Experiments were conducted from 1989 to 1991 to determine the critical period of johnsongrass control in field corn. Maximum weed-infested and weed-free periods of 0 to 20 wk after corn emergence were maintained by either hand weeding or nicosulfuron application. Interference duration effects on corn grain yield were not affected by johnsongrass control methods. The critical period for johnsongrass control was determined to be between 3 and 6.5 wk after corn emergence to avoid losses above 5% of yield produced by full-season weed-free corn.

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