Josh Lofton, LSU AgCenter, Field Crop Agronomist
Beatrix Haggard, LSU AgCenter, Soil Specialist
Ronnie Levy, LSU AgCenter, State Soybean Specialist
Dan Fromme, LSU AgCenter, State Corn and Cotton Specialist
Through most of this week, the state has experienced widespread showers and thunderstorms with varying levels of high wind and hail. The effects on the crop from wind and hail damage was documented through a recent article on the Louisiana Crops blog (http://louisianacrops.com/2014/05/14/damage-to-corn-with-severe-weather/). However, wind and sporadic hail were not the only problems associated with these recent storms. Many areas have experienced torrential rainfall events that have led to many fields being underwater for prolonged lengths of time. With this, many are trying to determine the influence of these rains to the field. As with saturated conditions on early-season corn, the overall impact will depend on the progress of the crop, saturated time, and environmental conditions.
As many experienced during the early part of this season, saturated conditions can be very influential on the corn crop, especially when the crop has just established. However, much of the corn across the state has either reached or is close to tasseling. With this more advanced growth stage, many of the plant roots are deeper in the soil where the roots will be less restricted by these near-surface saturated conditions and, therefore, limited yield impact can be expected. While most of the damage of the saturated corn will probably be limited, the effects on remaining soil nitrogen may be more exaggerated. As was noted in an article earlier this season (http://louisianacrops.com/2014/04/08/damage-to-corn-from-flooding/), the impact of saturated conditions directly following application can be limited, due to only a small portion of the applied nitrogen being in the nitrate form. However, as the season advances, more nitrogen is transformed into nitrate. This form can be lost during these conditions by either denitrification or leaching. As the corn crop approaches or passes these early reproductive stages, potential N losses due to high rainfall events can be more concerning due to dwindling soil nitrogen levels and corn still having a high N demand. Therefore, managers should monitor these fields that have become saturated, even for a short duration, for potential nitrogen stress. If an apparent nitrogen deficiency begins to emerge and corn has not or has just reached tasseling, an application of urea at or near tasseling can provide adequate levels of nitrogen for the corn crop to complete the season. However, these applications can be costly and, as such, should be used sparingly and only applied to fields that have the potential to “run out” of nitrogen while the corn still has a high demand.
While much of the corn crop across the state has reached later growth stages in which limited yield loss can be expected, this is not the case for many of the state’s soybean acres, especially the double-crop soybeans. When determining the impact of these rains on a soybean crop, managers need to look at the environmental conditions during these saturation events, length of the saturation, and stage of the crop. When looking at the crop stage, soybeans are more sensitive to saturation events during early vegetative (VE to V4) and early reproductive (R1 and R2). During later vegetative stages, the soybean crop can withstand longer periods of saturation with less effect; however, yield reductions can still occur. As with early-season corn, time of saturation and growing conditions during the saturation period can be critical. Most losses typically occur when the soybean plant has been in anaerobic (without oxygen) conditions for greater than 36 to 48 hours. However, the conditions during this saturation can influence how detrimental a short-duration saturation event can be. If optimum conditions are present (warm sunny days with warm evenings), the soybean crop can be more impacted than if little growth potential existed. Fortunately for most of the state, daytime conditions have been cloudy and cooler much of this week. Managers must also determine if any herbicides have been applied that may influence soybean growth. It is essential during these saturation events that the soybean crop be stressed from other sources as little as possible. Additionally, herbicides that may potentially only show minimal damage during non-saturated conditions may be more damaging during saturation events because the plant cannot properly metabolize the chemical.
When scouting for damage caused by saturated conditions, it is essential to wait multiple days to let the water recede as well as allow for conditions to return that would allow for soybean growth. Prior to this time, the crop conditions can look worse than they actually are. When scouting, diminished or potential stand loss is a primary concern. However, with soybeans, loss of nodules can also be a major concern. Therefore, along with stand evaluation, managers need to evaluate if nodulation has been diminished and if nodules are still active. To evaluate, soybean plants should be removed with a shovel (in an attempt to keep most of the root system intact) and roots should be washed. Nodules can be identified has little round balls on the root system.
Younger soybean plant’s nodules may still be developing and few may exist; however, older soybeans should have numerous nodules on a healthy root system. To evaluate if these nodules are still actively fixing nitrogen, cut the nodule in half. An active nodule will be pink to blood red when exposed to oxygen, but an inactive nodule will be brown, black or white. If numerous inactive nodules exist after optimum conditions have returned, the saturated conditions may have potentially disrupted nodulation and, therefore, the soybean plant may not be receiving the nitrogen needed for optimum growth. Careful examination and determination of these nodules should be taken; on average it is too costly to try to correct a nitrogen deficiency in a soybean crop.
While rainfall is always a welcome sign during the growing season, excessive rainfall can often be detrimental to the crop. However, corn, soybeans and cotton are typically more resilient than we think. Thorough scouting following a saturation event is critical to determine how detrimental these events can be. While immediately following a heavy rainfall event it may appear as an entire field may be effected, typically the field will drain quickly enough where only minimal impact can be expected. However, it is important if you believe that you have been adversely affected by these events to contact your local LSU AgCenter extension agent for consultation.
For further comments or questions, contact:
Josh Lofton, Field Crop Agronomist, firstname.lastname@example.org
Ronnie Levy, State Soybean Specialist, email@example.com
Dan Fromme, State Corn and Cotton Specialist, firstname.lastname@example.org