Flooded fields following heavy rain can take toll on crops

After a number of years of generally wet conditions across the Prairies, last year provided a break with a widespread drought.

This spring continued with dryness across much of the region, but recently we have moved into a “more normal” weather pattern with most of the rain falling as a result of thunderstorms. These are typified by a localized downpour of 90 millimetres or more in one swath, with little or no rain only kilometres away.

After the heavy downpours, I have had a questions from farmers about the “yellow patches” that begin to appear.

So, what is the cause of this phenomenon?

Generally, the yellowing is a result of an oxygen deficiency. Just like animals, plants need oxygen to function. Research in flooded soils has shown that the oxygen concentration approaches zero after 24 hours. Without oxygen, the plant cannot perform critical life-sustaining functions such as respiration, water uptake and root growth. Water logging also leads to accumulations of compounds such as carbon dioxide, which are toxic to plants in high concentrations.

Unless you are growing rice, your crop can’t tolerate flooding, which depletes soil of oxygen, increases disease infections and creates nitrogen losses.

Weather conditions following flooding are important to overall plant survival.

Cool, wet conditions favour disease development. Acting as opportunists, many diseases are able to attack weakened plant tissue, including leaves and roots.

Very hot, windy conditions may dry soil too quickly, causing stress and restricting plant growth. Early in the season this situation might also cause crusting of soil and emergence issues.

Crop responses and conditions may not be apparent for several days after flooding, requiring patience in replanting decisions. Producers should check plant meristems to assess damage and recovery potential.

Replanting should be a strictly economic decision that considers the yield potential of the reduced stand versus that of a later-planted crop minus replant costs. However, experience tells me that by the time you are able to access the drowned out area of the field with equipment, it will be too late to replant effectively.

Some crop species survive flooding better than others. Beans and peas may not survive one day of flooding, while cereals usually can withstand three to four days. Soybeans generally survive flooding better than other crops, but diseases are increased and symbiotic nitrogen fixation is diminished, which often results in reduced yields.

Some degree of damage to the crop is usually inevitable when seeded fields are flooded by excessive spring rains. The extent of the damage depends on the specific crop, its growth stage, the duration of flooding and temperatures during flooding.

Yellowing can appear anywhere water can pool for as little as a day. Cereals can often stand three to four days of wet feet, but peas and lentils can take much less, while soybeans are more tolerant of water logged soils. | Thom Weir photo

Damage occurs because flooded soil is quickly depleted of oxygen, and most crops require oxygen for normal metabolism, growth and development.

Flooding frequently results in higher levels of plant diseases that reduce stands and yields.

As well, flooding usually causes soil nitrogen losses, which can severely limit yields if nitrogen is not replaced. These losses occur from denitrification, nutrient runoff and leaching. Denitrification losses can occur at a rate of more than four to five percent per acre per day. This equates to 25 percent losses in five days and 50 percent losses over 10 days.

Plant stomata close under flooded conditions and may remain closed for long periods, resulting in reduced respiration, transpiration and photosynthesis. Plants may be slow to recover when water recedes.

Long-term impacts on the crop are often related to disease infection and retarded root development that limits access to available subsoil moisture later in the season.

What remedial actions can be taken to help the crop recover? There are no silver bullets.

The application of a fungicide may help reduce diseases moving into a weakened crop, but the idea that strobilation fungicides can miraculously turn a damaged crop green are over-promoted.

An application before flooding might help, but a post-flood application probably is going to give marginal results. In addition, the application of a magic elixir of macro and micro nutrients is not going to turn a sow’s ear-like crop into a silk purse.

However, the application of some good old fashioned nitrogen may compensate for the losses that have occurred. Remember, though, that this crop is in a weakened state and any foliar burning from the application of any fertilizers will be amplified.

In summary, the major factors that will impact the damage from crop flooding are:

  • type of crop
  • duration of flooding
  • growth stage of crop
  • disease prevalence and varietal resistance
  • weather (temperature and wind) during and following flooding

Thom Weir, PAg. is an agrologist with Farmer’s Edge. He can be reached by emailing thom.weir@farmersedge.ca.

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