Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2004
Publication Date: 1/1/2005
Citation: Clay, S., Kleinjan, J., Clay, D.E., Forcella, F., Batchelor, W. 2005. Growth and fecundity of several weed species in corn and soybean. Agronomy Journal. 97:294-302. Interpretive Summary: Late emerging weeds that grow and develop after the last scheduled herbicide application may reduce crop yields or add seeds to the seedbank, which may cause serious problems in future years. Consequently, information is needed on growth and seed production of important weeds as influenced by emergence date and crop type. Weed growth and seed production were measured for several common weeds in corn and soybean. The weeds were allowed to emerge at four different times of crop development: before crop emergence, at crop emergence, at the first true-leaf stage and at the second true leaf stage. With delayed emergence, barnyardgrass, pigweed and velvetleaf produced more seeds than other species. Furthermore, more weed seeds were produced in corn than in soybean crops. In general, weeds that emerge when the crop reaches the first or second true-leaf stages may not require control because they do not inhibit crop yield and they produce few seeds. However, control still may be desirable if the infestation of the species is very high or if the species is not common in the field or general area. These results will be of interest to primarily to crop advisors, agrichemical industry personnel and agronomic researchers.
Technical Abstract: Late emerging weeds that grow and develop after the last scheduled herbicide application may reduce crop yields or add seeds to the seedbank. Growth and fecundity were measured for eight weed species sown in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] 1) prior to crop emergence, 2) at crop emergence, 3) at V-1 and 4) at V-2 stages of crop growth in 2002 and 2003. Weed seed was sown close to the crop row and thinned to one plant m**-1 row. Weed growth and fecundity were influenced by species, time of planting and year. Barnyardgrass (Echinochloa crus-galli [L.] Beauv.) in corn and redroot pigweed (Amaranthus retroflexus L.) and velvetleaf (Abutilon theophrasti Medik.) in both crops survived to produce seed. Barnyardgrass in corn averaged 380 seeds/plant in 2002, regardless of planting date, whereas in 2003 seed production ranged from 3860 seeds/plant when sown preemergence to 260 seeds/plant when sown at V-2. Redroot pigweed sown preemergence to each crop averaged 5400 seed/plant in corn and 7100 seeds/plant in soybean. Redroot pigweed sown at V-2, in contrast, averaged 70 seeds/plant in corn and produced no seed in soybean. Weeds that grew from seed sown at V-1 and V-2 crop growth stages did not reduce crop yield or biomass of adjacent crop plants, had low fecundity and may not warrant treatment. Control may be necessary, however, to prevent yield losses if weeds are present at high densities or to stymie establishment of uncommon species.