Soybean Yield Reductions Caused by Defoliation during Mid to Late Seed Filling

Abstract

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.

Resposta da cultura da soja a diferentes intensidades de injúria nos estágios iniciais de desenvolvimento

“…The same results were observed in the second trial for treatment 5; however, did not differ from treatment 10 (total defoliation), 4 (removal of both cotyledons + one unifoliate leaf), 6 (removal of one unifoliate leaf), and 7 (removal of both unifoliate leaves) ( Table 1). The reduction of plant height was linearly correlated with defoliation, as previously reported for soybeans by OSTLIE & PEDIGO (1985), although with a small effect on yield, mainly due to the recovery capacity of soybean plants, which produce many new leaves during vegetative growth (BOARD et al, 1994).…”

Efeitos De Níveis E Épocas De Desfolhamento Sobre O Rendimento E Os Componentes Do Rendimento De Grãos Da Soja

“…A redução do peso do grão ocorre principalmente quando o desfolhamento se dá no período reprodutivo tardio, ou seja, durante o período efetivo de enchimento de grãos [17]; [3,5]). …”

Soybean Yield Formation: What Controls It and How It Can Be Improved

“…Greatest yield loss (75-88%) occurred at R5 , a finding substantiated by later studies (Fehr et al, 1981;Gazzoni and Moscardi, 1998;Goli and Weaver, 1986). Delay of total defoliation to R6.6 resulted in only a 20% yield loss (Board et al, 1994), supporting the view of greater tolerance to defoliation as seed filling progresses. Partial defoliation treatments initiated at R1 and terminated at R3, R4, R5, and R6.5 resulted in significant yield loss (approximately 15%) when average LI during the R1 to R5 period was reduced at least by 17-20% (Board and Harville, 1993;Board and Tan, 1995).…”