Little is known about soybean [Glycine max (L.) Merr.] yield response to defoliation during the last half of the seed‐filling period (R6.3 to R7) and how it is affected by source/sink ratio. Because defoliating insect pests in the southeastern USA frequently attack soybean during this period, greater knowledge of yield responses to defoliation at this time would aid in making management decisions. Therefore, the objectives of this field study were to (i) determine yield losses to total defoliation near the temporal midpoint (R6.3) and three‐quarter point (R6.6) of the seed‐filling period; (ii) determine if alterations in source/sink ratio affect this response; and (iii) determine yield component mechanisms responsible for the yield reduction. ‘Centennial’ soybean was planted in 1991 and 1992 near St. Gabriel, LA. Treatments were no defoliation (control), 100% defoliation at R6.3, and 100% defoliation at R6.6 arranged as split plots within high, normal, and low source/sink ratios during seed filling (main plots). Defoliation at R6.3 resulted in 40% yield reduction, whereas defoliation at R6.6 caused a 20% yield loss. Lower yield resulted from reduced seed size. Source/sink ratio did not affect this response. Smaller seed size in both defoliation treatments resulted partly from reduced seed‐filling rate (28% less than control). Shorter effective filling period also contributed to smaller seed size, but this effect was influenced by treatments and years. In conclusion, 100% defoliation during R6.3 to R6.6 must be avoided to maintain optimum yield.

Relationships between defoliation, light interception, and grain yield for soybean [Glycine max (L.) Merr.] during the last half of the seed filling period are not fully understood. Because defoliating insect pests in the southeastern USA frequently attack soybean during this period, knowledge of such relationships would aid development of criteria for economic insecticide application. Our first objective was to determine the efficacy of using light interception and leaf area index as criteria for economic insecticide application to manage defoliating insect pests. The second objective was to determine the effects of partial and total defoliations during seed filling on grain yield and grain yield formation. ‘Centennial’ soybean (Maturity Group VI) was planted in 1993 and 1994 near St. Gabriel, LA. Experimental design was a randomized complete block in a split‐plot arrangement. Main plots were the temporal midpoint and 3/4‐point seed‐filling stages. Split plots were four defoliation treatments: 0% leaf removal (control), 37 to 41% leaf removal, 56 to 61% leaf removal, and 100% leaf removal. At the temporal midpoint, grain yield declined linearly as light interception fell from 93 to 50%. Thus, for a crop achieving near‐maximal light interception, insecticide application would be recommended whenever light interception at the midpoint seed‐filling stage fell below 93%. Yield and light interception were not closely related at the 3/4‐point seed‐filling stage. Maintenance of maximal grain yield (yield at 0% defoliation) required retention of at least 39% of the leaf area at this time. In conclusion, light interception potentially could be used as an insecticide application criterion at midpoint seed‐filling stage, but not the 3/4‐point. Partial defoliation at the midpoint will affect grain yield if it reduces light interception below 93%, whereas at the 3/4‐point total defoliation is required to affect grain yield.

Delayed maturity in soybean, Glycine max (L.) Merr., occurred in response to infestation by southern green stink bug, Nezara viridula (L.), in 4 yr of field studies. Maturity delays followed stink bug infestation that occurred only during the pod set and filling stages (R3-R5.5), and infestations at R3-4 and R5 resulted in delayed maturity more consistently than did infestation at R5.5. Infestation levels of six stink bugs per 0.3 m of row for 7-14 d generally were required to delay soybean maturity. The greatest impact on seed yield and quality parameters followed stink bug infestations that occurred during R3-R5.5, which corresponded closely with the periods of infestation that resulted in delayed maturity. If both delayed maturity and yield reduction are considered, the pod elongation through late pod filling stages were most critical for protecting soybeans from southern green stink bugs.

Soybean [Giycine max (L.) Merr.] yield is affected more by assimilatory capacity (source strength) during reproductive (R1-RT) than vegetative (emergence to RI) growth periods, but relative sensitivities of yield components to source strength have not been clearly demanstrated. This study was conducted to verify that source strength (R1-R7) affects yield primarily through regulation of pod number. Source strength effects (R1-R6) on pod number and effects of altered sourcesink strength during R5 to R7 on seed per pod and seed size were determined. Soybean cultivar Centennial was planted at an optimal date in 1991 and 1992 near St. Gabriel, LA. Source strength during RI to R6 and source-sink strength during R~ to R7 were altered with three treatments: control, 30% shade (R1-R7), and partial pod removal 10 d after RS. Shade vs. control reduced source strength and yield 30% through a 20% reduction in pod number. Number of branch nodes and main stem pods per reproductive node, yield components shown previously to respond to source strength at late plantings, responded similarly to altered source strength at optimal planting dates. In contrast, a 28% decrease in source-sink strength during R5 to R7 reduced seed per pod and seed size by only 7.7 and 4.5%, respectively. The results substantiate that source strength during RI to R7 influences soybean yield primarily by affecting pod number and that RI to R6 is the most responsive part of the reproductive period to altered source strength. K NOWLEDGE of soybean yield component responses to assimilatory capacity (source sensitivity), coupled with identification of the growth periods in which yield components are formed, can aid in designing genetic and cultural strategies for optimizing yield. Previous studies (Brun, 1978; Christy and Porter, 1982) have determined that yield was affected by altered source strength during the reproductive (R1-R7, stages according to Fehr and Caviness, 1977) rather than the vegetative period (emergence to R1). However, reports of source sensitivity for yield components formed during early (R1-R5) and late (RS-R7) reproductive periods have not been consistent. During R1 to R5, yield components affecting pod number (node number, reproductive node number, and pods per reproductive node) are formed. Final determination of pods per reproductive node occurs near R6, 10 to 12 d after R5 (Board and Tan, 1995). Drastic reductions in source strength (i.e., total defoliation) applied shortly after R6 have little effect on pod number (Board et al., 1994).