By Dr. Craig Drury
A long-term Ontario study breaks down the effects of crop rotation on yield, soil health and environment.
Research scientist Dr. Craig Drury with Agriculture and Agri-Food Canada discussed the results of long-term crop rotation studies in Ontario at the Top Crop Summit, Feb 23-24, 2021.
One of the longest crop rotation studies at the Harrow Research and Development Centre in Ontario was established in 1959. It consisted of continuous corn, with or without fertilizer, compared to corn in rotation with oats and two years of alfalfa, with or without fertilizer. The five-year average yield (2016-2020) for fertilized continuous corn was only 112 bushels per acre whereas the fertilized rotation corn was 68 per cent greater at 188 bushels per acre. In contrast, the yields for unfertilized continuous corn was not sustainable at only 17 bushels per acre because the nutrients were depleted in the soil.
Another study was set up in 2001 to look at the long-term effects of crop rotations on yield, soil health and environment. Seventeen crop rotations were established including continuous corn, continuous soybean, continuous winter wheat, winter wheat underseeded to red clover as well as various two-, three- and four-year rotations with those crops.
At the start of the study, the soybean yields were all similar, but after two years, continuous soybean yields declined and were always significantly lower than soybeans in rotations. Five-year soybean yield averages (2016-2020) showed that continuous soybean had a yield of about 42 bushels per acre. Moving to a two-year rotation resulted in higher soybean yields, but the highest yielding rotation was with a three-year rotation of soybean-winter wheat-corn with an average yield of about 63 bushels per acre, which was 50 per cent greater than continuous soybean.
A similar yield trend occurred with corn. Continuous corn was yielding only 118 bushels per acre. A corn-soybean rotation raised yields up to around 154 bushels per acre, and the highest yields were with a three-year rotation with winter wheat underseeded to red clover followed by corn and soybean at 169 bushels per acre, which was 43 per cent greater than continuous corn. The red clover cover crop in the winter wheat increased corn yields by five bushels per acre compared to the winter wheat-corn-soybean rotation.
The research looked at soil health benefits as well. Dr. Ike Agomoh, a postdoctoral fellow working with the soil team at Agriculture and Agri-Food Canada compared the effects of long-term crop rotation on many of the soil health indicators. Potentially mineralizable nitrogen (N), was found to be one of the most responsive indicators with over three times higher values for the more diverse crop rotations than continuous soybean. For soybeans, the potentially mineralizable N data followed the same pattern as yields.
The same trend was seen when looking at particulate organic matter carbon with the lowest levels in continuous soybean and almost double the amount in the soybean-winter wheat-corn rotation.
After 17 years of the rotation study, total soil organic carbon was also increasing. Continuous soybean had just under two per cent soil organic carbon while the soybean-winter wheat-corn had just over 2.25 per cent soil organic carbon – an increase of 16 per cent. Although changes in soil organic carbon can take time to be detected, some of the soil health indicators such as potentially mineralizeable N or particulate organic matter responded to changes in soil management practices more quickly.
Research was conducted by Dr. Lori Phillips to see how crop rotation alters the soil microbiomes. After 15 years of crop rotations, samples were taken in the spring and fall over several cropping years. Different fungal communities developed in different crops under continuous rotations. Fungal communities in continuous soybean were different than those under continuous wheat or continuous corn, and also different than those under perennial grasses. Some potentially pathogenic fungi, such as Volutella spp., were up to 16 times more abundant in continuous soybean than rotation soybean. However, potentially beneficial fungi such as Fusidium spp., which might be involved in the development of disease suppressive soils also similarly increased. Overall, with two or three crops in rotation, there was a greater diversity of fungal communities in the soil.
Data from our long-term Agriculture and Agri-Food Canada study in Woodslee, Ont., as well as Dr. Bill Dean’s long-term crop rotation study at the University of Guelph, was utilized in the 11 site North America study, which examined the effects of crop rotation diversification on agricultural resilience to adverse growing conditions. The probability of having an increased risk of crop failure was higher for simple crop rotations compared to a more complex crop rotation. When there was an increased opportunity for a bumper crop, complex rotations had a higher chance of producing high yields compared to simple rotations. On average over all growing conditions, corn grain yields were 28 per cent greater with the more diverse rotations compared to simple crop rotations.
All of these studies show the value of long-term crop rotation research on soil health and crop yields.