Dr. Martin Entz, University of Manitoba
IN NATURE, PLANTS rarely grow alone. In some agricultural production, farmers have followed the lead of nature and grown two or three crops together in the same field. This practice is referred to as “intercropping,” and it was widely used in ancient agricultural systems. First Nations farmers, who grew corn, potatoes and squash in places like Netley Creek, MB thousands of years ago, practiced intercropping. Today, intercropping maize with pigeon pea is a popular practice in parts of Africa. An important incentive to intercropping is yield stability. If one crop fails, the other can fill in. Therefore, intercropping is a useful strategy for food security.
Intercropping is making inroads in Canada and farmers are now experimenting with a wide range of crop types. My first encounter with intercropping was “Peola,” the mixing of peas and canola, at the Melfort research station of Agriculture and AgriFood Canada in the 1980s. In the early 1990s, my then U of M colleague, Kevin Vessey, and his graduate student, John Waterer, conducted field studies on this crop mixture and confirmed its “over-yielding” potential in Manitoba. A few years later, former U of M weed scientist Rene Van Acker and his graduate student Tony Szumigalski went on to test peola and other intercrops under low and high crop input scenarios. They found that the pea/canola intercrop reliably resulted in yield increases, but canola/wheat and pea/ wheat intercrops were a bit less reliable. The first organic grain intercropping study in Manitoba was conducted by my graduate student Jackie Pridham in 2003 and included a wide combination of cereals with pulse and oilseed crops. More recently, excellent intercropping research has been conducted in Melita, MB by Scott Chalmers while Lana Shaw conducts intercropping research across the Saskatchewan border near Redvers. Scott and Lana deserve a great deal of credit for demonstrating practical intercropping systems to farmers.
Farmers have also been innovators. My research group has had the privilege of interacting with Saskatchewan farmer Colin Rosengren who has long intercropped canola and various pulses on his no-till farm. Other mainstream farmers have followed in Colin’s footsteps. Organic farmers have designed their own intercrop combinations. Intercropping and workshops and field days are very well-attended by farmers and agronomists.
Now that intercropping is becoming mainstream, it is important to understand the mechanisms involved. Chinese farmers have intercropped for centuries and Chinese researchers have more recently tried to understand the scientific basis for this practice. Perhaps the leading Chinese intercropping scientist is Dr. Long Li of China Agricultural University in Bejing. I have followed Dr. Li’s work for many years and I recently had the opportunity to sit down to discuss the topic with him. Other scientists, especially in Scandinavian countries, but also here in Canada have conducted intercropping research. Here are some points from the work that are relevant to the questions farmers are asking today.
DOES NITROGEN (N) TRANSFER OCCUR BETWEEN PULSE CROPS AND NON-N FIXING INTERCROP PARTNERS?
Yes. In the 1990s, researchers in Scandinavian countries established that N from peas and faba beans was transferred to cereals such as barley. The mechanism was first confirmed in greenhouse studies using a split-root technique. For example, in 1996, Jensen observed that barley received 19% of its N requirement from the pea intercrop. Other research measured the N transfer from grain legumes to cereals grains at 15%. Recent work in China confirmed this N transfer from legume to non-legume intercrop in the field. The primary mechanism responsible for this N transfer is the pulse crop sending some of its N into the soil rhizosphere (the soil immediately around the root) and then the intercrop partner scooping the N up.
Work at the University of Manitoba (U of M) by Sawatsky and Soper (1991) did not test the interaction, but they did observe that a significant amount of N from peas was indeed deposited into the rhizosphere. This confirmed that N leakage from pea plants also occurs in Manitoba. Some studies have shown that below-ground networks of mycorrhizal fungi can also transfer N from one plant to the next, but to date, this is reported mostly in perennial forage systems. What does this information mean for N fertilization in intercropping systems? It is safe to say that we need more research here. But on some commercial farms, N is mid-row banded between two rows of the non-legume, while no N is applied between the two rows of legume. In other cases, farmers avoid N fertilization of their pea/canola intercrops altogether.
DOES ADDING AN INTERCROP INCREASE N FIXATION IN THE LEGUME PLANT?
Yes. Pea/barley intercrop studies in Alberta in the 1990s showed that adding barley to peas increased the percent N fixed by pea from 62 to 82%. Studies on pea/canola in Manitoba and Saskatchewan showed an increase in N fixation in pea by about 10%. These pea/ canola intercrops received 10 kg N/ha as fertilizer. One lentil/flax intercrop study from Saskatchewan showed lentil received 77% of its N from the fixation of atmospheric N when grown alone compared with 85% when intercropped with flax. The reason for greater N fixation in legumes grown with intercrops is thought to result from less available N in the soil system, causing the legume to become more independent in terms of N fixation — it forces the legume to work harder. Szumigalski and Van Acker reported that canola/pea grown together resulted in a 15% boost in grain N compared with growing the crops separately. European research has shown that organic grain legumes growing in the presence of weeds sometimes become more efficient in terms of N fixation. It appears that the same mechanism is at work. Fertilizer rate matters too. It has been confirmed that as the rate of N fertilization increases, the N fixation in intercropping decreases.
WHAT ABOUT PHOSPHORUS (P)?
There is some evidence to suggest that intercropping can increase the P status of both intercrop partners. Using permeable and impermeable root barriers, Dr. Long Li found that maize overyielding, when intercropped with faba bean, resulted from its uptake of P mobilized by the acidification of the rhizosphere by faba bean roots. There was a 12 to 50% increase in maize P from intercropping with faba bean. Another of Dr. Li’s studies showed that chickpea allowed wheat access to more soil P. The mechanism was that chickpea increased the phosphatase enzyme in soil. Phosphatase is an enzyme that helps plants access P. Most phosphatase enzymes in the soil come from the soil microorganisms (e.g. bacteria) and not the plant. In one study, it was observed that when bacteria evolved in intercrops, production of phosphatase enzyme was increased 16% compared with bacteria from monoculture. At the Glenlea long-term organic crop rotation study, we have observed more soil phosphatase activity in the organic plots compared with the conventional plots.
ANYTHING ELSE ABOUT P?
Yes. A large proportion of phosphorous in prairie soils is in the organic form, called organic P or microbial biomass P. This P is not detected in most standard soil tests. Researchers in France found that microbial biomass P was significantly increased in the rhizosphere of intercrops, while sole crops did not show any significant increase in microbial P in the rhizosphere. The French example was with chickpea and durum wheat. Microbial P was higher in the rhizosphere of chickpea and durum wheat when the crops were grown together compared to when the two crops were grown on their own. As prairie soil health improves through several different beneficial practices that farmers are now using, I predict that biological nutrient sources within intercrops are going to become more important to understand and manage.
Many other questions need to be addressed so that farmers can intercrop with more confidence. One question regards insects. Evidence shows that certain insects are reduced with intercropping, but more research is required. Another question concerns diseases. If we intercrop, what are the consequences of root diseases? In other words, is it better to grow a crop like peas more frequently by intercropping, or should we respect the rotation interval of four (Canadian recommendations) or six (older European recommendations) years between pea crops? I know scientists are working on these questions now and I look forward to learning about their results.
One last point. We know that plants “talk” to each other and this makes future intercropping prospects exciting. One example of such “cross-talk” comes from California. Researchers conducted field experiments to test the hypothesis that wild tobacco plants become more resistant to stress when grown near a sagebrush neighbour that has been experimentally clipped. It worked. In the coming years, we will learn more about how plants communicate when grown in intercrops and I believe farmers will use that knowledge to optimize their intercropping systems.