DR. Don Research Update
Intercropping corn and legumes increases economic yield in tropical small-scale agriculture.

By Don Lotter

April 19, 2004: Intercropping corn and legumes in tropical systems has been the subject of a number of classic studies, many done in the 1970’s. These studies were instrumental in demonstrating the wisdom of many peasant agricultural systems that had come under criticism at the height of the green revolution for being “inefficient” because they didn’t use capital-intensive inputs.

The Land Equivalency Ratio (LER) was developed for evaluating crop polycultures. The LER basically sums the value of all of the crops in a polyculture and divides that sum essentially by the value of the main crop in monoculture.

Researchers in Thailand, writing in the Japanese journal Plant Production Science, have recently validated many of the findings of earlier intercropping research – that crop polycultures of legumes grown with corn give about half again the value compared to corn alone.

However, a newer version of the LER was used in this research, called the Area Time Equivalency Ratio (ATER). The ATER takes into account not just the land use, but also the time that the intercrop takes beyond the monoculture. Legume intercrops generally grow for some time past the corn harvest, and can take land away from the tightly scheduled sequential cropping typical of Asian agriculture. The ATER takes this into account.

In the Thai research, corn, peanut, soybean, and mungbean were grown in monoculture and as corn-legume combinations. Corn was planted at 53,000 plants per ha in both monoculture and polyculture. The legumes were planted at 200,000 plants per ha in monoculture and 133,000 when sown with corn. Nitogen, phosphorus and potassium fertilizers were applied at a basal rate of 23, 9, and 18 kg per ha, respectively.

Corn yields averaged about 4.1 metric tons per ha and were not significantly different between any of the treatments – either in monoculture or any of the polycultures. Yields of peanut, soybean, and mungbean were reduced 28%, 39%, and 51%, respectively, by intercropping each with corn. The number of pods was the yield component most affected.

The LER was 1.66, 1.60, and 1.48 for peanut, soybean, and mungbean, respectively. This means that the value of the harvest was 66% higher for corn and peanuts together than for corn or peanuts alone.

The ATER was 1.58, 1.52, and 0.96 for the same crops. The lower ATER reflects the time taken by the legume to grow after corn harvest, which prevents immediate sowing of a follow-up crop. Mungbean can be double cropped (mono-cropped twice in succession) after corn in Thailand, plus, it is a shade intolerant crop, thus the ATER below 1. Peanut and soy are more shade tolerant.

Several reports show that corn yields increase by about 20% over monoculture when grown with legumes such as soybean. These cases may be where soil nitrogen is more limiting. Corn, with its high nitrogen needs, would have benefited from the legume nitrogen contribution in these cases.

It is clear here why intercropping is increasingly being practiced by organic farmers in the U.S., especially with vegetable crops. Forms of intercropping that are compatible with mechanized agriculture are also being researched – mostly in the form of relay cropping.

Polthanee, A. and Trelo-ges, V. 2003. Growth, yield, and land use efficiency of corn and legumes grown under intercropping systems. Plant Production Science. 6(2):139-146

Don Lotter has a Ph.D. in agroecology and has worked in sustainable agricultural development in North America, Latin America, and Africa over the past 25 years. He can be contacted via his website www.donlotter.com

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