Common waterhemp is native to the Midwest, although it has only developed into a serious problem in corn and soybeans in the last 15 years due to changes in production practices. Decreases in tillage allow common waterhemp seed to remain at or near the soil surface, therefore providing optimum conditions for germination. The delayed emergence pattern of common waterhemp allows it to avoid many control tactics that are targeted to control weeds already competing with the crop. Repeated use of triazine, ALS, and PPO herbicides have selected for resistant varieties of common waterhemp. Speculation is already occurring on when glyphosate will be added to that list.
The growth and seed production of waterhemp as affected by delayed emergence has been examined in soybeans, however little is know about these parameters in corn and if narrow row spacing will be effective in controlling the late emerging plants. Corn yield loss associated with late emerging Amaranthus spp. (V6- V8) has ranged between 5 - 20% depending on weed density (Knezevic et al. 1994 and Massinga et al. 2001). Decreasing the amount of light transmitted through the crop canopy (narrowing row spacing) has decreased growth and seed production of common lambsquarter, velvetleaf, redroot pigweed, palmer amaranth, and the foxtails. Narrow-row spacing in soybean has been effective in increasing weed control and yield.
Four experiments were conducted in 2001 and 2002 near Ames, Iowa to evaluate the response of common waterhemp to corn row spacing and date of waterhemp emergence. Corn was planted at a population of 32,000 plants/acre in 15 and 30 inch rows. Four waterhemp emergence cohorts were established in each experiment: 1) plants emerging with corn (VE), 2) V3 corn stage; 3) V5 corn stage, and 4) V8 corn stage. A total of 30 plants per cohort were selected in each experiment and their fate monitored throughout the growing season. Other weeds were controlled with directed applications of glyphosate and hand weeding. At maturity, waterhemp plants were harvested at the soil line. Plants were dried at 30o C for 4 days and weighed. Seed production was based on plant dry weight using a linear regression as determined by previous research.
Results: Delays in waterhemp emergence relative to corn emergence reduced the survival of common waterhemp plants (Figure 1). Corn row spacing did not affect survival rate at any emergence time. Common waterhemp emerging at the VE stage of corn averaged 80% survival, whereas plants emerging at the V3 and V5 stages averaged 44% and 3.5% survival, respectively. Less than 1% of plants emerging at the V8 time survived to maturity. The mortality of these plants occurred throughout the growing season, rather than single events resulting in a majority of the deaths. The low survival rates of plants emerging after the V5 corn stage should be considered when evaluating the need for implementing control tactics for late-emerging waterhemp.
The height of common waterhemp that emerged at the VE corn stage averaged 140 cm and decreased approximately 40, 80 and 95% with each successive delay in emergence (Figure 2). Corn row spacing did not eaffect common waterhemp height. Heights of first cohort common waterhemp ranged from 60 to 240 cm. Many plants that emerged at the VE corn stage became etiolated and the plants either fell down or relied on neighboring corn plants for support. Very few plants reached above the corn canopy (2.5 m). Later-emerging plants rarely grew above ear height on the corn plant.
Biomass accumulation of common waterhemp declined more rapidly in response to emergence delays than plant height (Figures 2 and 3). The majority of plants emerging at VE corn stage weighed less than 40 g and plants emerging at V8 weighed less than 1 g. Common waterhemp plants that emerged at the VE corn stage were affected by corn row spacing, whereas later emerging plants were not. Narrow row corn decreased waterhemp shoot dry weight 20% compared to waterhemp in 30" corn rows. Plants that emerged after VE corn stage were not affected by row spacing; however, delays in emergence accounted for 80, 95 and 99% reductions in biomass compared to plants that emerged with corn (Figure 3).
Seed production by female common waterhemp plants emerging at the VE corn stage was greater in wide rows than narrow rows (Figure 4). Plants in wide rows averaged over 48,000 seeds per plant, whereas plants in narrow rows only produced around 39,000 seeds per plant. Row spacing did not affect later emergence cohorts, but delays in emergence did decrease seed production to approximately 9000 and 950 seeds when emerging at the V3 and V5 corn stages. Plants that emerged at the V8 stage did not produce seed.
Common waterhemp seed production in this study with corn is much less than reported by Hartzler et al.(2001. . Average seed production of plants emerging with soybeans was 309,000 seeds per plant, six times greater than that of plants emerging with corn. Knezevic et al. )reported redroot pigweed grown in competition with corn produced 32,000 seeds per plant emerging before the V4 corn stage and 1,500 seeds per plant when emerging between V4-V7. The fecundity of common waterhemp declines rapidly with delayed emergence; even so, the later emerging plants are still capable of producing sufficient seed to recharge the seedbank. Results from this study show that emergence time was more influential than row spacing on common waterhemp growth and survival in corn. Common waterhemp plants produced an average of 1,000 seeds when emerging at the V5 corn stage; however, when combined with only a 3% chance of survival they would be of little economic significance under most scenarios. Narrow row spacing was only effective in reducing the biomass and seed production of the first cohort, not the later emerging weeds. Decreasing row spacing will increase the amount of light intercepted by crop through faster canopy closure, leaving less light available for weed germination and growth earlier in the season. The use of narrow row spacing could also decrease the chance of leaving gaps that occur between planter passes in wide rows where weeds may appear.
Prepared by Bob Hartzler, extension weed management specialist and Dawn Nordby, graduate research assistant at Iowa State University.