Chem-fallow is an effective method of reducing tillage, reducing erosion and conserving moisture. Under dry conditions, wheat protein levels on chem fallow are not generally a problem. However, under good moisture conditions, often the protein content of wheat grown on chem fallowed cereal stubble is much lower than wheat grown on summerfallow or on a pulse stubble, even with recommended fertilizer rates applied. This problem with wheat protein creates a bit of a dilemma for those producers who wish to have some fallow in their rotation for risk management. While chem-fallow will continue to be an excellent soil conservation practice, other low input options need to be explored.
Greenfallowing, also known as green manure, has made some inroads in farming systems over the past 20 or 30 years although its growth in popularity hasn't been as great as chem-fallowing for that same time period.
A greenfallow crop is an annual legume crop that is allowed to grow for only part of the growing season, then terminated. Traditionally, this termination usually involved tilling the crop under, but in more recent times, herbicides have been used to kill the crop. The benefits of greenfallowing include nitrogen fixation that will be available for the crop the following season. Since the crop is terminated early in the growing season, it also allows soil moisture recharge from precipitation. These two factors provided producers with a reduced tillage option before it became popular. Annual legumes are effective as greenfallow crops because of their ability to fix nitrogen and their root system is concentrated in the upper two feet of the soil profile allowing soil moisture recharge from precipitation.
At first, field peas or lentils were used as greenfallow crops, then crop breeding with greenfallow specifically in mind produced annual legumes that grew quickly and had high nitrogen fixing capabilities. Indian Head lentils and more recently, AC Greenfix (a chickling vetch), are examples of varieties developed specifically for greenfallow.
Greenfallowing is a practice for farmers in the Brown and Dark Brown Soil Zones who want to have a certain amount of fallow in their rotation for risk management. In addition, there are also many farmers who are renting land that have a requirement that fallow be part of the rotation. For those producers, greenfallow can have a good fit in conjunction with chem fallow.
Discing in the greenfallow crop will result in maximum benefit of the nitrogen fixed by the crop. However, desiccating a greenfallow crop results in about only a 10-20% drop in the soil N as compared to discing once. Dessication has the advantage in that it leaves 20-30% of the residue on the surface for erosion control.
Greenfallowing does improve the soil quality and organic content of the soil as compared to chem fallow and especially when compared to summerfallow. Studies by Biederbeck et al have shown that wheat grown on greenfallow stubble increases wheat yield, improves grain quality with higher protein content and larger kernel size. These qualities affecting the cereal re-crop can certainly overcome the problem associated with chem-fallow. However, in a drought year such as encountered in 2001, the differences between greenfallow and chem fallow would not likely be significant. For ideal greenfallow production, the rule of thumb is, "If it's a dry year, seed early. If it's a wet year, seed later." In a dry spring you want to seed early to get the crop established to take advantage of the available soil moisture. In a moist or wet spring, you may want to seed later so you can spray out that first flush of weeds just prior to seeding your greenfallow crop.
The only problem with this rule of thumb is that if your crystal ball for predicting the weather is like mine, it's rather difficult to take advantage of until it's after the fact. However, if it is very dry at the time you wish to seed your greenfallow crop you may well be further ahead to chem- fallow instead.
AC Greenfix has proven to be one of the best varieties to use as greenfallow. It produces the greatest top growth and fixes the most nitrogen as compared to the only other registered greenfallow variety (Indian Head lentils) or other pulse crops. Greenfix also has better heat tolerance and drought resistance as compared to other pulse crops.
A general rule of thumb for estimating N production of AC Greenfix is that for every 1000 lbs of top growth, about 40 lbs of N is gained in total by the soil when it is disced in or desiccated. Plot studies at Swift Current showed that with timely rains, Greenfix can fix as much as 210 lbs/ac of N after seeding into wheat stubble.
The nitrogen fixed by greenfallow has considerable value, even without considering the higher grain quality and protein of the cereal re-crop. Assuming 46-0-0 costs $299/tonne and that a greenfallow crop fixes 50 lbs/ac N. This would amount to a value of $14.75/ac of N fertilizer. 75 lbs of fixed N would provide $22/ac value and 100 lbs/ac of fixed N would provide $29.50/ac value. Although there are costs to seeding the greenfallow crop, how high these costs are depend on the cost of the greenfallow seed and inoculant. The goal, of course, is to keep the input costs as low as possible.
An annual legume study conducted by Bryan Nybo with Wheatland Conservation Area at Swift Current, found that under average to well above average precipitation years (1998-2000), the average forage yield of some annual legume crops was between 1.9 and 3.3 tons/ac forage yield and relatively high levels of protein. If the total amount of N is accounted for, this means that there would be approximately 113 to 182 lbs/ac of N. During the severe drought of 2001, Dr. Biederbeck evaluated the performance of AC Greenfix (chickling vetch) at six locations across southwest Saskatchewan. The precipitation from April 1 until the sampling date (approximately two months later) ranged from 1.5 inches to a high of 3.5 inches. Even so, the forage yield taken from this greenfallow crop at these sites ranged from 720 to 1600 lbs/ac, averaging 1200 lbs/ac over the six sites. If the greenfallow crop was desiccated and using the 1200 lbs/ac value, it would provide the soil with an average of 53 lbs/ac of legume N. Not bad for a drought year and low forage production. This additional N would likely be more than enough for a cereal crop the following year.
Although the producer will obtain the greatest N supply from greenfallowing by discing it in or desiccating the crop, removing it for forage is another use for greenfallow. While this would remove about 75% of the N that could be supplied by the greenfallow crop, it is still worth evaluating.
Removing the crop as forage provides an additional supply of good protein feed that the producer can use for his own livestock operation or sell. The provincial cattle herd is expected to dramatically increase over the next eight years so forage supplies will also need to increase. As noted above, the potential of forage yield can be very good under average to above average moisture conditions. But even under a severe drought year as in 2001, forage yields can be worthwhile. A greenfallow study at Aneroid, Saskatchewan, found some surprising forage yields (Figure 1). However, it should be noted that the greenfallow treatments were not seeded until late May and in combination with the drought, the forage was not removed from the plots until early August. In addition, there were a lot of weeds in the treatments, especially in the Greenfix plots. In hindsight, the greenfallow crops should have been seeded much earlier to take advantage of soil moisture that was available in late April and early May.
Although the farmer will often need to add some fertilizer when seeding the cereal or oilseed crop the year following greenfallow when the top growth had been removed for feed, don't get too hung up that this is another input expense after all the hassle of growing a greenfallow crop. Remember that that forage you removed has value, as does the nitrogen that was fixed in the soil.
Figure1. Greenfallow Studies at Aneroid, Sask. 2001
If the greenfallow forage yield was 3000 lbs/ac (1.5 tons/ac), there would be about 26 lbs/ac N in the soil from the greenfallow treatment. In most cases, the farmer can use a narrow single shoot opener and still be in the safety zone when applying 25 lbs/ac N with the seed of the cereal crop. This may be very important if the farmer cannot or doesn't wish to convert to a double shoot opener enabling him to apply high levels of fertilizer.
Although AC Greenfix has the highest N fixation and top growth production of the specific greenfallow crops, it has not been recommended to be used for cattle feed. The seed of this chickling vetch contains a neurotoxin. Although one would take Greenfix for forage at flowering stage, there is still a concern in using this crop for feed. The USDA National Grazing Research Trials are presently evaluating the safety and feed value of AC Greenfix. NDSU Carrington Research Extension Center in Carrington North Dakota have been evaluating AC Greenfix as part of their annual forage as hay trials. Their results from 1995-2000 indicated that Greenfix had excellent forage quality with crude protein ranging from 22-26%.
Greenfallowing can offer producers more options in a direct seeding system. It can certainly result in reducing tilled summerfallow acres. Desiccating the greenfallow crop has the potential of providing large amounts of N for the cereal or oilseed crop the year following. Even in dry years, providing the greenfallow crop was seeded early, there can be significant amounts of N fixed with relatively little cost.
Other benefits to a greenfallow crop are the increase in soil quality and organic matter and the reduced risk of erosion. Using the top growth as a potential feed source is another benefit.
In addition to grain quality, kernel size has also been shown to be larger on greenfallow than on chemfallow. This is not to say that greenfallow will completely replace chem-fallow, but it has tremendous potential as a rotational crop for those farmers that require a form of fallow in their management system.