Crop stress nuts and bolts 101
By John Dietz
Understanding how the crops you manage are coping with stress is critical. It can help you maintain yield and quality.
Dr Abdel El Hadrami taught a generation of agronomists about the physiology of plants at the University of Manitoba. Today, he's director of research and development for Omex Canada in Winnipeg.
Recently, he reviewed the basics of crop stress, and went a little deeper on some points:
Q. What causes plant stress?
A. There are two categories of stress: abiotic or biotic. Abiotic is any physical condition that is not linked to living organisms. Biotic is anything linked to living organisms. In Western Canada, we're dealing with cool and wet soils at seeding. That's abiotic stress. But also with diseases, which is biotic stress.
Q. Is there much difference in sensitivity?
A. Definitely. It is mainly dictated by genetic make-up. The rule of thumb is, the smaller the seed the greater the sensitivity to environmental stress. Soybeans are an exception. They are big seeds that are a little finicky. They are very sensitive to temperature. Soybeans need soil at 9 to 10 degrees. That's why they're seeded late and that's why we have issues with them if the harvest is early. The stage of growth and development also is key. The smaller the crop, the more sensitive it is.
Q. What happens with canola?
A. When we get cool springs, canola can stall for a number of weeks. The reason is, cold soil affects phosphorus, zinc, manganese and potassium availability. When the soil warms up, the roots start to grow and start absorbing those nutrients. Then you see the plants reviving and growing.
Q. What happens with wheat?
A. It handles the cool conditions at the beginning of the season very well. If it goes into cool soil, the seed has enough reserves to grow under those conditions. Yet, supplemental phosphorus, potassium, zinc and manganese could make a difference under cool conditions.
Q. What are other types of abiotic stress?
A. Salinity, wide fluctuations of temperatures, and excesses in moisture or dryness. Hail and frost also are very common abiotic issues in Alberta and Saskatchewan. Pesticide injuries are another.
Q. What's an example of biotic stress?
A. A number of diseases, in the soil or that come with the seed, would prefer a weaker crop to one that is healthy. Differences between crops are tremendously important. There even are differences between genotypes or cultivars. This year, Fusarium graminearum in Saskatchewan wheat is a particularly good example. Our cultural practices have changed. This is creating a high disease pressure. The same is observed on several other crops.
Q. What do cultural practices have to do with crop diseases?
A. We've moved from tillage to zero or minimum tillage, and we've been running tighter crop rotations. We don't allow residues to decompose much in the soil. We don't bury them because we don't do any tillage. Then we run wheat-canola-wheat. So, you are dealing with tan spot or fusarium on wheat, and with sclerotinia and blackleg on canola.
Q. Were you saying fusarium is increasing in Saskatchewan?
A. Yes. This year's seed sample survey results range between six percent and 56 percent having Fusarium graminearum. That's very scary. I haven't seen samples before like this. Up to 30 percent was considered high. This year, a couple wheat samples were at 50 percent and the highest was 56 percent infected with the fusarium that produces vomitoxin. It is now seriously present in Manitoba and Saskatchewan and knocking on Alberta's door.
Q. How do crops cope with stress?
A. Plants cope with stress in three ways: avoidance, tolerance or acclimation. For example, in the genetic makeup a seed will avoid stress by staying dormant until conditions are favourable. If stress is exerted for a long period, plants can change their physiology or acclimate. Under cool conditions, plants preserve their energy for the day that conditions are much better. And, if we can do something about how long stress is experienced, we can help a plant tolerate the stress.
Q. What can we do to improve cold tolerance?
A. When it's cool, plants won't grow. If we spray them with a foliar fertilizer that has a high level of phosphorus, and other micros such as zinc and manganese, we help them grow the root system and explore other horizons in the soil. That enables the roots to pump more nutrients into the plant. These processes seem to be very complex. As an industry, we look for ways to help plants to protect themselves from stress.
Q. Can we do anything about hail damage?
A. Hail causes wounds. On canola, those cells are leaking. You can spray boron as a foliar to help heal those wounds. Boron brings the sugars to the wound so it starts generating new cell walls to heal itself. Canola is a very plastic crop. If hail comes in July, it is able to create new stems and re-flower and produce more pods. In Alberta in 2012, we had customers who harvested 25-bushel canola – after hail – while nearby canola was mostly written off. They had applied boron to heal the wounds.
Q. Can we reduce damage from heat blast?
A. Maybe. When flowering plants try to cool themselves because the weather is very hot, they circulate water through the leaves. Boron moves with the water to the leaves, but it stays there when it's really needed in the flowers to control fertility. We end up having abortion of flowers. If we spray boron when it's hot, we have evidence of fewer symptoms of heat blast. We're running experiments on that.
Q. So what else are you trying to understand?
A. We started one of the largest multi-variety, multi-crop trials in Manitoba in 2012 and will continue it this year. We did replicated trials with 11 varieties of wheat, 13 varieties of canola, 14 varieties of soybeans and four varieties of oats, at one location that was nutritionally balanced and very rich. There was no limiting factor for yield. We experimented with mitigating drought stress using a seed primer first and a foliar treatment at herbicide time.
Q. Any preliminary results?
A. The treated crops yielded better. We didn't have the extra bushels; we just protected them from being lost. It was a way to check whether there was a major varietal response, or whether the treatment dictates the response of the variety. From what we saw, nearly all the response was across the varieties. We correlated the response to the treatment, not the variety. This year we want to authenticate that result at two locations with different conditions, and see whether those varieties were responding to the drought stress in 2012.
Q. What did the drought do to your trial?
A. The primer helped, where it was applied, to establish a good root system early in the season while there was enough moisture. Later, we applied a herbicide tank-mixed with a foliar that includes our stress-relieving technology. When the drought hit, the differences became serious compared to the ones that weren't protected. The primer has NPK and Zinc. The seedling treatment is a basic 6-12-6 analysis of NPK with our proprietary stress-relieving technology. Stress such as drought affects many nutrients. Zinc for instance controls over 400 plant enzymes involved in growth and development. Most soils in western Canada are zinc deficient; that's one reason we're having issues with delayed maturity. In fact, we've noted differences in maturity between our treated and untreated plots.
Q. So what's the bottom line?
A. Try to have a comprehensive understanding about causes of plant stress and, in advance, work through your best response scenarios. You may not have much time during the growing season, but if you're ready you can mitigate many stress factors that affect crop yield and quality.