There are some places across the Canadian Prairies that are seeing above average rain fall and therefore some laying water in the fields. Rain is almost always a good thing, but there are times when it can be detrimental to yield.
The first thing excessive rainfall does, as many are aware, is cause leaching of soil mobile nutrients. The 3 most mobile nutrients being Nitrogen, Sulfur and Boron. The lighter (sandier, lower CEC) the soil, the more nutrients will move. For example, for every inch of moisture accumulated, nitrate nitrogen (plant available) can move downwards of 10” in the soil profile of a sandy soil! This tends to be only about 3” for every inch of rain in a clay loam soil.
Another problem you run into is denitrification. This is caused by conditions where there is a lack of oxygen in the soil (anaerobic conditions), so microbes essentially begin to breakdown and use the nitrate. Once soils get very saturated these organisms tend to already be active and begin the process of denitrification more readily the next rainfall. This on top of leaching can cause for large amounts of Nitrogen losses.
When it comes to herbicide breakdown there can be issues with excessive moisture as well. Usually, we see a more rapid breakdown of residual herbicides under good moisture conditions, but to much moisture can cause herbicides not to be broken down and put you into the same situation you might run into under excessive drought situations. This is due to soil microbes that break down residues not being active under flood situations, essentially the microbes start to drown. This can keep residues around longer and cause some injury to your canola after Pursuit (imazethapyr).
Anaerobic soil conditions lead to other issues in many crops as well. In your pulse crops you will see a significant reduction in nodules on your roots, therefore a reduction in nitrogen fixation (lots of moisture steals nitrogen a number of ways!). This is again due to the bacteria not being able to survive under excessive moisture. You will also notice decreased root hairs and mycorrhizae fixations in crops like flax which will hinder yield.
The next thing you will see is typically a yellowing and purpling of the plants whether it be canola, lentils or wheat. The reason for this is due to plant stress and because roots struggle to grow and take up nutrients, so partly what you are seeing is some early macronutrient deficiencies such as phosphorous, nitrogen or sulfur.
Even though most organisms tend to be negatively affected by excessive moisture, you will see root rots and diseases caused species such as pythium and rhizoctonia. These diseases prefer wet conditions and can cause significant yield loss.
After all of this you end up with a weaker plant and therefore a plant more susceptible to other types of disease (rusts, white mould etc) and conditions that are favourable for their development. Even once it dries up a bit you still are not out of trouble!
Canola is typically seen as the best at surviving flood situations. In my experience Invigor varieties seem to be even more superior than RoundUp Ready. I have seen canola be under water for up to 5 days without significant yield loss, and even seen upwards of 10-14 days, but this is where you begin to see significant yield reductions (40%+). Wheat is very good at tolerating the water as well with minimal reductions in yield even at 2-4 days under water, beyond that I have not experienced, but can imagine yields would begin to drop off. Barley is slightly worse than wheat tolerating upwards of 3 days. Flax is next followed by lentils and peas which are very sensitive to flooding, seeing yield reductions even after 24 hours. If you get into your crops that prefer warmer temperatures then you run into even more issues, like with corn, dry beans and soybeans. The rain typically means cooler temperatures, less sunlight and cooler soil temperatures which these crops extremely dislike. Corn can see yield reductions of up to 35% in under 72 hours.
At the end of the day I really like Luke Bryan’s song “Rain is a Good Thing”, but in some instances it most definitely is not a good thing.