Agronomy Library > Rotations

Match Crop Water Use to Available Moisture
Author: Helen McMenamin interview of Dr. Perry MIller from Montana State University
Date Created: April 19, 2002
Last Reviewed: January 15, 2009

Opportunism is the key to taking advantage of the variability of the weather in our region of the Northern Great Plains, according to a cropping systems specialist from Montana State University in Bozeman.  "We have to expect some dry years," says Perry Miller. "There's no way to know which, till well after seeding time. We know how to capture the moisture we get and protect whatever moisture is stored in the soil. Our challenge is to figure out crops and crop sequences that allow us to maximize returns from the moisture that's available. We have to stay flexible."

Low disturbance direct seeding and conservation of crop residues are the first tools for stretching soil moisture. Miller recommends early seeding and crop sequences that store and then take advantage of moisture. Fallow, the traditional moisture storage system, may be needed to keep risk manageable. But, in sequences with frequent (50%) fallow, salinity can develop as stored water moves in the soil profile. Miller advises including some medium and high water use crops - wheat, winter wheat, sunflower or mustard - in the rotation.

The other problem with fallow every two years is erosion, the old scourge of prairie farming. "Even with no-till, there's a risk of soil erosion with the broadleafs," he says. "You just can't build up a duff layer with so much fallow. And, you risk losing potential income in wet years. Perhaps a low water use crop like lentils or peas can fit in and pay a few bills when the moisture's available."

A system with less fallow can use shallow rooted and deep rooted crops to increase returns. Lentils, peas or a barley-pea mixture harvested as forage can leave some moisture at depth which winter wheat or sunflower can use. "I have some concerns about this sort of sequence," says Miller. "The low residue levels after the pulse crops are an erosion concern. But, the barley-pea mixture may be fostering cereal root diseases." 

 To achieve the healthiest possible crops, Miller aims for a wide or diverse rotation including crops of different types. These are cool season grasses - cereals; cool season broadleafs, such as canola, mustard or peas; warm season grasses, corn, millet or sorghum; and warm season broadleafs such as sunflower. 

Each crop type has its own suite of diseases and has different herbicide options so disease and weed pressures stay low when crop types are varied. This keeps crops healthy and minimizes crop protection inputs. Growing a range of crops can be a form of protection against unpredictable weather as well as providing rotational benefits. Miller had a dramatic illustration of this last year. He grew spring wheat, peas, flax, chickpeas and sunflower at five sites across Montana, all on wheat stubble, all with the same management. Spring wheat, the main crop for the state, produced the best returns at two sites. At the other three sites, rainfall patterns favored one of the other crops. 

Flexible management is key to balancing risk and returns in an environment with limited water, according to Miller. "We never know how much moisture we'll have for a crop," he says. "If we can flex the intensity, or water use, of our cropping sequence we may be better able to take advantage of the moisture we have and leave some moisture for the next crop." 

A high intensity crop sequence, which Miller defines as one with less than 20% fallow, may have the top water use efficiency, but if it's dry for two or more years, returns can be poor. Management flexibility for this type of rotation may mean taking crops off early to allow for some soil moisture recharge. Miller sees big potential for early harvesting of crops as a way to flex-manage crops and save moisture. 

Early seeding gives a crop a huge advantage in yield potential. In 2000, Miller harvested 50 bushels of green peas seeded April 11th, but under 30 bushels from a plot seeded May 1st. At a bushel a day in lost yield, it doesn't pay to wait for a flush of weeds. In southern Alberta, Ross McKenzie has seen even bigger differences in yields of yellow mustard seeded in April compared to early or late May. Canola also benefited from early seeding, but less than yellow mustards, possibly because drought, frosts and insect pressure made conditions very tough for canola in the trial years. The brown and oriental mustards (Brassica juncea) in the trials were intermediate in their response. 

Miller sees potential for fall seeding canola in his area. He's had yields over 40 bushels from crops seeded in snow in November compared to 32 bushels for crops seeded May 9th. Austrian winter pea may have potential as fall-seeded crops. The peas germinate very early and can take advantage of the brief moisture available in April. 

In a dry year, taking peas off as a hay crop can give better returns than combining the crop, according to Perry Miller. The strategy takes advantage of the rise in hay prices during a drought. Harvesting peas as forage 60 days after seeding can leave almost as much moisture in the soil as a fallow year. At one of Miller's sites, wheat grown on stubble from peas hayed 60 days after seeding made 12 to 16 bushels per acre more than when the peas were harvested as grain, and only about 5 bushels less than wheat on fallow. 

Haying even left more moisture than treating peas as green manure, terminating them with glyphosate 60 days after seeding. It seems the plants used some moisture after spraying, as they were dying. At three other sites, soil moisture following peas was the same whether peas were harvested as grain, hayed or used as green manure. Those plots were either completely droughted out or completely recharged. "Grain growers often don't want to know about baling off a crop," says Miller. "Cutting peas as forage after just 60 days is hard to justify when it doesn't give the maximum forage yield. But it limits the crop's water use and it pencils out quite well." 

Perry Miller had a couple of dramatic demonstrations of increased infiltration due to direct seeding during the summer of 2000. He and his field crew were combining peas on a direct seeded field when a thunderstorm that lasted half an hour dropped 1.1 inches rain on them. "That was in the afternoon," says Miller. "Later that day, we were able to finish combining peas without picking up even a trace of mud on the tires. Across the road, a conventionally tilled summer-fallow field sloughed most of the water into the ditch, along with several yards of topsoil." 

A month later, Miller drove to Pierre, South Dakota and arrived just after almost 5 inches of rain flooded the highways. The next day, he was able to walk in Dwayne Beck's low disturbance direct seeded fields without getting his shoes muddy. 

Where moisture is adequate, a layer of crop residues, or duff, can build up on the soil surface, insulating it and limiting evaporative moisture loss. Cereals contribute more than broadleaf crops because they break down more slowly as well as producing more residue.  Tall stubble can compensate to some extent for summerfallow, which destroys the insulating residue layer.