- Fall ammonia and dry fertilizer applications were delayed in some areas due to adverse fall conditions impacting the timing of preemergence residual applications.
- Delayed or even lack of preemergence residual applications will affect season long weed control and adversely impact corn and soybean yields.
- A preemergence residual herbicide program is key to season long weed control as there are very limited options for post emergence residual herbicides.
With less than ideal conditions for field work last fall, very little fall ammonia, fall dry fertilizer and fall burndown applications were completed. As spring has started to turn around and we get busy with ammonia and dry fertilizer applications and planting our crops, there seems to be very little time for preemergence soil residual herbicide applications. This leads to issues with season long weed control. If the preemergence residual does not get applied and we rely on postemergence herbicide applications, we can end up with bigger challenges as bigger weeds are harder to control.
University research has demonstrated that a failed attempt at a preemergence residual application or the reliance on a postemergence application only has an adverse effect on yields. University of Guelph found that for every inch of weed growth in corn there is a 4 bushel per acre yield loss due to weed size/competition (Figure 1). For soybeans, where herbicide options are slimmer, Southern Illinois University found that for every inch of weed growth, there is a 0.7 bushel penalty per acre due to competition (Figure 2). These findings reinforce the importance of preemergence residual herbicides on reducing weed competition and protecting crop yields.
Figure 1 (below). Corn yield (bu/a) in response to weed growth (inches). Corn yield decreases as as weed size increases at time of application. (Source: University of Guelph; Dr. Peter Sikkema via Twitter)
Figure 2 (below). Soybean yield (bu/a) in response to weed growth (inches). (Source: Southern Illinois University Belleville Research Center; Ron Krausz)
Planting a crop without a preemergence residual herbicide will result in weeds emerging at the same time as the crop and competing for nutrients, water and sunlight. On corn, preemergence residual herbicides can also be applied as “delayed-pre’s” shortly after the crop has emerged. However, for soybeans, a fair amount of preemergence residuals can no longer be used after crop emergence. For example, PPO inhibitor residuals (Authority, Valor/Fierce and Sharpen containing products) can no longer be applied to soybeans once they have cracked through the soil surface. If used, severe crop injury or plant death can result. The same is true for Photosystem II inhibitors like metribuzin or metribuzin containing products. Once soybeans have emerged, a few options to consider would be products that have burndown activity on the emerged weeds and a residual (group 15 products) or (Group 15 + fomesafen). Apply these products as soon as possible to control the emerged weeds and prevent additional weeds from emerging. Follow up with the planned post emergence application a few weeks later, particularly to overlay the residual.
Even in a compressed spring season, it is imperative to apply preemergence residual herbicides and do so in a timely manner. This will limit early season weed competition, minimize yield loss and set the field up for successful season long weed control. Always read and follow the herbicide label to avoid crop injury. For more information on weed management and residual herbicide pre and post programs contact your local FS crop specialist.