Yield Components as Indirect Selection Criteria for Late-Planted Soybean Cultivars J. E. Board^'r' M. S. Kang. and M. L. Bodrero ABSTRACT Cultivar selection for late-planted soy'bean lGlycine max (L.\ Merr.l in the wheat-soybean doublecropping system is an important production problem. Top'yielding cultivars for late plantings could tre selected more efficiently by identification of rield components that indicate top 1ield, compared Nith the traditional combine-haryested plot yield method. Our objective rvas to identify yield components that could be used as indirect selection criteria to identify top-yielding cultivars for late planting dates. A 2-yr studl'(1998 and 1999) with 26 cultivars was conducted at a mid-July planting at Baton Rouge, LA (30' N, 90" W). A similar study involving 27 difl'erent cultivars was conducted for I yr at Los C)liveros, Santa Fe Province, Argentina (32",18' S, 62" W), planted in early January 1999. Experirnental designs were randomized complete blocks with four replications and one factor (cultivar). Data were obtained on combine-harvested plot yield, seed m 2, seed size, seed per pod, pods m 2, pods per reproductive node, and reproductive node m 2. Across years, yields at Baton Rouge ranged from ll.83 to 2992 kg ha--r, while yields in Argentina ranged from 1688 to 2809 kg ha 1. Yield at Baton Rouge increased x'ith maturity group, whereas in Argentina there *as no relationship be- hveen yield and maturity group. For both phenotl.pic and genot.rpic levels, selection for either seed m : or pods m : identilied top-lielding cultivars, although seed m. 2 was more accurate. Tr" wHEAr-soyBEAN doublecropping svstem has I gained wide acceptance in the southeastern USA, as well as other soybean-growing regions such as Argen- tina, because of greater profitability compared with mo- nocropped soybean (Larreche and Brenta, 1999; Wesley et aI., 1994, 1995). Besides greater profitability, dou- blecropping also provides for better erosion and pollu- tion control (Elmore et aL.,1992; Kessavalou and Wal- ters, L997), avoidance of some diseases (Whitam, 1996), and better seed quality and viability (Purcell and Vories, personal communication, 2001). Flowever, acceptance of doublecropping has been limited by low yields for late-planted soybean (planted at mid-June or later) (Boerma and Ashley, 1982). Reduced yields at late planting dates mainly result from shorter daylengths at late vs. normal planting dates during vegetative and early reproductive periods (Board and Settimi, 1986). This decreases the period emergence to R5 (stages ac- cording to Fehr and Caviness, 1977) resulting in too J.E. Board and M.S. Kang, Dep. of Agronoml,, Louisiana Agric. Exp. Stn., LSU Agric. Ctr., Baton Rouge, LA 70803; and M.L. Bodrero. EEA Oliveros INTA. 2206 Oliveros, Santa Fe, Argentina. Research support provided in part by the Louisiana Soybean Promotion Board. Approved for publication by the Director of the Louisiana Agric. Exp. Stn. as manuscript no. 02-09-0113. Received 12 Mar.2002. 'tCor- responding author (iboard@agctr.Isu.edu). Published in Agron. 1.95:420429 (2003). SOYBEAN iittle vegetative grou,th for optimum yield (Egli et al.. 1987). Drought stress during mid-July to late August is another problem for late-planted soybean in the south- eastern USA (Morrison and Rabb, 1996). Associated with rcduced crop growth rate and dry matter accumulation during the pcriod emergence to R5, yieid losses at late planting dates are linked to lowcr pod number per area (pods m-r). mediated by reproduc- tive node m 2 and/or pods per reproductive node (Board et al.. 1999). Another problem with late-planted soy- bcan is reduced plant height and lowcr height to the lowest pod (Ouattara and Weaver, 199-5). This problem is more severe in determinatc vs. indetermiualc cultivars and can result in combine vield losses. Cultural pracLices that increase light interception and crop grorvth ratc during the emergence to R5 period resulting in greater yield are narrow-row culture (Board and Harville, 1994). increased plant population (Ball et al.. 2000a). and avoidance of stresses such as waterlogging (Lin- kemer et a1., 1998) and drought (Ball et al., 2000b). Although cultural practices to increase late-planted yield have been identified, determination of genetic methods is less clear. Previous studies demonstrated large yield increases through proper cultivar selection for late plantings, with potential yield improvement ranging from29 to 2J6"/", depending on range of culti- vars and lateness of planting (Board, 2002). Cultivar recommendations for late-planted soybcan are ham- pered by significant cultivar x planting date interactions (Carter and Boerma,1979; Boquet et a1., 1982). There- fore, recommendations based on state-wide trials at nor- mal planting dates are nol. generally applicable to late planting dates. Incorporation of the long-juvenile trait (Tomkins and Shipe, 1996) and development of indeter- minate late-maturing [Maturity Group VII (MG VII)] cultivars have not been promising avenues for improv- ing late-planted soybean yields. Late-planted studies in Louisiana among MG V, VI, and VII cultivars showed yield to increase with maturity group (Board, 2002). Currently, cultivar recommendations for late planting in the southeastern USA are based on state-wide culti- var trials similar to those conducted for normal planting dates (Bowman. 1993; Thurlow et a1.. 1991). Cultivar recommendations for late-planted culture, as well as cultivar-genotype development, would be enhanced by the identificalion of yield components that indicate high genetic vield potential. Identification of such markers would be helpcd by a grcater understanding of what explains cultivar yield differences at late plantings. AI- 420 Abbreviations: MG. rnaturitv group.