Sustainability
Pulses improve the sustainability of cropping systems


Growing and harvesting crops together as they occur in nature is very difficult, so farmers instead use crop rotation, the practice of growing different types of crops after one another. In Western Canada, a good crop rotation includes a variety of crops grown in sequence, including cereals (wheat, barley, oats), oilseeds (canola, flax, sunflowers), and legumes (pulses).

Since the innovation of the Haber-Bosch process to industrially produce nitrogen fertilizer, crop rotations have become less diverse and less reliant on pulses and other legumes. These non-diverse cropping systems are susceptible to pests and disease and can have a negative impact on soil and water1.

Introducing nitrogen-fixing pulses to crop rotations improves the yield and quality of wheat grown in rotation2, 3, 4. Although every crop in a diverse rotation is important and brings specific benefits, pulses have been shown to bring extra benefits to rotations.

Pulse crops bring an advantage to cropping systems by leaving nitrogen behind for the following crop in the form of crop residues5. But this is only part of the story. Much of the nitrogen fixed by pulses is taken away from the field when the high-protein seeds are harvested, and one study has shown that only 8% of the yield increase of wheat grown after peas could be attributed to the nitrogen benefit that the pulse provides6. So what is driving the remaining 92% of the yield increase? This is a complex story, but it involves two components that are essential for crop growth: soil and water.

References

1. Karlen, D.L., Varvel, G.E., Bullock, D.G. and Cruse, R.M. 1994. Crop rotations for the 21st Century. Advances in Agronomy. 53: 1-45.
2. Miller, P.R., Gan, Y., McConkey, B.G. and McDonald, C.L. 2003. Pulse Crops for the Northern Great Plains: II. Cropping Sequence Effects on Cereal, Oilseed, and Pulse Crops. Agronomy Journal. 95: 980-986.
3. Przednowek, D.W.A., Entz, M.H., Irvine, H., Flaten, D.N. and Thiessen Martens, J.R. 2004. Rotational yield and apparent N benefits of grain legumes in southern Manitoba. Canadian Journal of Soil Science. 84: 1093-1096.
4. Gan, Y.T., Miller, P.R., McConkey, B.G., Zentner, R.P., Stevenson, F.C. and McDonald C.L. 2003. Influence of Diverse Cropping Sequences on Durum Wheat Yield and Protein in the Semiarid Northern Great Plains. Agronomy Journal. 95: 245-252.
5. Miller, P.R., McConkey, B.G., Clayton, G.W., Brandt, S.A., Staricka, J.A., Johnston, A.M., Lafond, G.P., Schatz, B.G., Baltensperger, D.D. and Neill, K.E. 2002. Pulse Crop Adaptation in the Northern Great Plains. Agronomy Journal. 94: 261-272.
6. Stevenson, F.C. and van Kessel, C. 1996. The nitrogen and non-nitrogen rotation benefits of pea to succeeding crops. Canadian Journal of Soil Science. 76: 735-745..
Crop rotations for the 21st Century
This 1994 paper reviews the history of crop rotations, including the use of pulse crops and other legumes. It focuses primarily on US agriculture and discusses agronomic, policy and economic issues related to crop rotation. It was published in the peer-reviewed journal Advances in Agronomy.

The paper discusses how crop rotation promotes sustainable agriculture by improving soil fertility, reducing pest and weed levels, promoting efficient water use, and increasing crop yields. For thousands of years, legumes such as pulses were used to provide nitrogen to maintain soil fertility. Farmers began to abandon crop rotation after pesticides and nitrogen fertilizer became available, as many believed that they could replace the benefits of crop rotations. More recently, the general consensus among agricultural scientists is that no amount of fertilizer or pesticide can fully compensate for the benefits of crop rotation. Characteristics of successful crop rotations are also discussed. For example, when legumes are used in crop rotations, it is important to choose a subsequent crop that effectively uses the leftover nitrogen.

In the future, using legumes in crop rotations may encourage the production of energy crops. The need for energy-intensive nitrogen fertilizer is the major limitation in using crops as an energy source; however, using nitrogen-producing legumes in rotation with energy crops could solve this problem. Future crop rotation research is needed to more fully explain the advantages of crop rotation, as this may make farmers more likely to use this practice.

Pulse Crops for the Northern Great Plains: II. Cropping Sequence Effects on Cereal, Oilseed, and Pulse Crops

This peer-reviewed article was published in Agronomy Journal in 2003. The objective of this study was to compare the effect of three pulses (chickpea, lentil, and pea) on the next crop (wheat, mustard, canola, pea, or lentil). This study is unique because it considered the effect of soil texture (clay versus loam soil) on the crop rotation. The study was conducted in Saskatchewan during a three year period.

The study found that soil texture was important because the benefits of pulse crops were more consistent for the clay soil than the loam soil. Growing pea or lentil before wheat, mustard, or canola resulted in better yields, probably because pulses left more water in the soil for the next crop. In addition, pulses had better yields when grown after wheat than when grown after another pulse crop. This research demonstrates that using diverse crop types is beneficial to all the crops in the rotation.

Rotational yield and apparent N benefits of grain legumes in southern Manitoba

This study was published in the peer-reviewed Canadian Journal of Plant Science in 2004. The objective of the research was to compare the effect of four different pulse crops and a flax crop on the yield and nitrogen content of the next crop (spring wheat). The pulse crops studied were pea, chickpea, bean, and soybean. The research was done at two different locations in southern Manitoba over a three year period.

Of the crops studied, pea consistently provided the most benefit to the wheat crop. The other pulse crops had varying benefits depending on the location and their adaptation to the local growing conditions. For example, chickpea showed the most potential to provide benefits in dry conditions.

Influence of Diverse Cropping Sequences on Durum Wheat Yield and Protein in the Semiarid Northern Great Plains

The objective of this study was to determine if durum wheat yield and quality were affected by the crops grown in the previous two years. It was published in the peer-reviewed Agronomy Journal in 2009.

Three pulses (pea, chickpea, and lentil) and two oilseeds (mustard and canola) were grown prior to durum wheat. The protein concentration and grain yield of the durum wheat crop were measured. This study was conducted over a four-year period at two locations in Saskatchewan.

Results showed that yields and protein content increased when durum was grown after either pulses or oilseeds, but were highest after pulses. Durum yields were increased by 7% and protein was increased by 11% when durum was grown after a pulse rather than spring wheat. Approximately one-quarter of the yield increase was due to increased nitrogen and water remaining after the pulse crop was harvested. This study concluded that both pulses and oilseeds provide significant benefits when grown in rotation with durum wheat.

Pulse Crop Adaptation in the Northern Great Plains

Miller and colleagues reviewed the current research on the production of peas, lentils, beans, soybeans, and chickpeas in western Canada and the northern USA. Published in Agronomy Journal in 2002, this article summarizes how pulse crops affect environmental sustainability in terms of crop yields and efficiency of water use. Key areas for further research are also outlined.

Overall, research shows that pulse crops consistently provide a nitrogen benefit to wheat that is grown after a pulse. This nitrogen benefit is demonstrated by higher wheat grain yields and higher wheat protein content (nitrogen is a major building block of protein). This is important because nitrogen supplied by a pulse crop reduces the need for nitrogen fertilizer, an input that is energy-intensive to produce and is responsible for a large portion of the greenhouse gas emissions in agriculture.

Peas, lentils, and chickpeas were specifically highlighted as crops that efficiently use water. Research suggests that these three pulse crops respond to drought conditions better than spring wheat. By using less water, pulses conserve water for use by subsequent crops. This is particularly important because water is a major limiting factor in growing crops in the northern Great Plains.

The nitrogen and non-nitrogen rotation benefits of pea to succeeding crops

This peer-reviewed article, published in 1996 in the Canadian Journal of Plant Science, studied the benefits of growing a pea crop prior to spring wheat.

A special form of nitrogen, called nitrogen-15, was used to track the amount of nitrogen produced by the pea crop and its use by the following wheat crop. In addition to providing nitrogen, there are other ways pulses can benefit subsequent crops: reducing diseases and weeds, increasing the availability of other nutrients such as phosphorus, improving the soil structure, and releasing growth-promoting substances. The study also looked at these “non-nitrogen benefits” for the next crop. The study was conducted at three locations in Saskatchewan over a three year period.

This study found that when pea was grown before wheat, the wheat yield was 43% greater than if wheat was grown after another wheat crop. The yield benefit was consistently observed at all three locations. Of this yield increase, 8% was due to the nitrogen provided by the pea crop. The remaining 92% of the benefit from the pea crop was due to other factors, especially the reduction of wheat root diseases. This study demonstrates that pulses provide significant benefits that go beyond their nitrogen-producing ability.

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