Environmental Benefits of Legumes
While cool season legumes directly contribute to the economy of the Palouse, the Northern Plains, and other U.S. production areas, they also contribute indirectly through their positive effects on other crops. Dry peas, lentils, and chickpeas are typically grown in rotation (i.e., alternating years) with cereal grains. In contrast to dry beans, the cooler weather preferred by legumes during the growing season, especially at bloom, fits well with the climates conducive to small grains like wheat or barley.
The crop rotation strategy boasts a number of advantages. The pairing with grains reduces the potential for diseases and helps to control weeds, insects, and other pests in both crops. The grains also benefit from the increased nitrogen and other nutrient values in the soil after rotation with lentils, dry peas, or chickpeas.
Legumes produce their own nitrogen from the atmosphere through a symbiotic relationship with a soil bacterium. As a result, commercial nitrogen applications are unnecessary or significantly reduced for the legume crops. The same is true with regard to the need for commercial nitrogen or other fertilizers in the grain crops, which can utilize the remaining nitrogen in the soil to reduce the input cost for the producer. Reduced nitrogen applications also reduce the use of the natural gas necessary to produce the nitrogen, thereby helping reduce greenhouse gas production. For both the producer and the environment, using nitrogen produced by the plant is a much better approach and promises a smaller ecological impact.
The Harvesting of Legumes
The harvesting of a cool season legume crop consists of a single pass with a combine, a mechanical harvesting device that integrates many operations. It cuts the plant from the ground, separates the seeds from the rest of the foliage, distributes the residue across the field, and transfers the resulting product to a storage bin via a truck. In the U.S., the harvest generally begins in August and is conducted through September.
Harvesting of lentils, dry peas, and chickpeas is carefully timed. Using the combine on the crop prematurely can result in an immature, underdeveloped crop. Harvest the crops too late and the excessive dryness can cause crop loss due to the shatter of the pods prior to or during harvest.
Moisture Content of Harvested Material
Ideal harvest timing includes waiting for moisture content to be acceptable for storage. Harvesting too early can result in a product with too much moisture content for storage, making it prone to spoilage.
The crop is usually cut when the product is not yet ready to shatter but dry enough to store without damage. No product is stored at more than 15 percent moisture, with 12 percent or less being the optimum moisture level for a harvested crop.
Differences in Harvest Times
Although dry pea, lentil, and chickpea crops are relatively drought-tolerant, both the timing and amount of precipitation affect growing duration and the scheduling of harvest times. In the Palouse, for example, late-season rains after July 15 will delay harvest, while early-season drought prior to July 1 will significantly accelerate harvest dates. Planting is typically done in April and May, with an August harvest.
In Idaho and Washington, dry pea crop maturity is reached about 100 days after emergence, with harvest starting in late July when pods are dry and seed moisture is less than 13 percent. The peas are harvested directly in the field. In the Northern Plains, the planting date and harvest dates are typically two to three weeks later because of colder winter soil temperatures. Harvest is typically starts in August and completes in early September.
Lentil crops also reach maturity about 100 days after emergence, with harvest in midAugust, when the crop is swathed and then combined. In Washington, lentils bloom approximately 60 days after crop emergence and all varieties are harvested in August, being cut and swathed into windrows approximately one week before combining.
Swathing of lentils in the Northern Plains in Montana and North Dakota is risky due to frequent wind storms, which would blow away the windrows. Lentils are cut directly by the combine in this region. In all cases, a timely harvest is critical to avoid seed bleaching, seed shatter and post-maturity disease. All of these issues degrade the quality of the crop and reduce the yield.
Once the pulse has been threshed, the seeds must be carefully stored prior to delivery to the processing plant. Excessive heat can lead to discoloration. Excessive moisture can result in mold and fungal problems. Clean, protected facilities and aeration during periodic transfers from one storage bin to another help guard against post-harvest pest infestation or damage.
When properly selected, legumes can be safely kept in storage for long periods of time without deterioration, allowing endusers to buy in bulk. With dry peas, the pea weevil can emerge during post-harvest storage, leaving damaged seeds that must be separated in processing. The pea weevil is most effectively controlled in the field during the growing season, helping prevent higher processing costs. Generally, all peas are held long enough to allow for the emergence of the pea weevil larvae prior to processing.
Seed moisture must be carefully watched when storing pulses to prevent disease or damage. Peas can safely be stored at 15 percent moisture, chickpeas and lentils at 14 percent. If moisture levels are too high, grain dryers are often used, though always with extreme caution as they can cause mechanical and thermal damage to pulse crops. Moisture is tested several times during the first few weeks of storage to maintain proper levels and to prevent seed sweating. Aeration is used to cool and dry the seed and to avoid storage complications.
A significant amount of the chickpea crop remains at least for a short time stored on the farm before being delivered to the processor. Once chickpea seed is harvested, its outside seed coat usually has a lower moisture level than the inside of the seed. But if left to sit in the storage bin, the moisture level can balance out (also called tempering or sweating), causing the overall moisture level to rise.
In this way, chickpeas that were harvested at a safe moisture level can just a week later exceed the recommended 14 percent level. Left untreated, the crop can spoil. For this reason, chickpea producers often store the crop in a hopper-bottomed bin that has aeration, which can help bring down the moisture level.
Lentils are also commonly stored on the farm for a time before delivery to the processor. As with chickpeas, it is most common for lentils to be stored in a hopper-bottomed bin with aeration. If the crop includes a great deal of green weed seeds, the lentils, though safely stored at 14 percent moisture, are typically cleaned or aerated as soon as possible after harvest to prevent heat damage. Lentil varieties with green seed coats will discolor with age, decreasing the grade and price of the crop.
If they are not kept in cool, dark conditions at a moisture content at or below 14 percent, those lentils with green seed coats can discolor as tannins within the seed coat oxidize. Other factors such as high humidity and high temperatures can also cause color change. In each case, such changes in color impact the grade and price received for the crop.
Moisture levels up to 16 percent and temperatures below 59 degrees F (15 degrees C) are considered safe for dry pea storage. If supplemental heat drying is necessary, air temperatures are kept below 109 degrees F (43 degrees C) to preserve germination. Temperatures up to 158 degrees F (70 degrees C) should only be used for drying feed peas. A great deal of respiration occurs in pea seed after it is placed in storage.
Dry peas, lentils, and chickpeas can be safely stored for three to four years. Storage lengths of this duration can, however, result in color loss, moisture absorption, and desorption as well as hardness or case hardness issues.