Annual Legume Forages in a Direct Seeded System

Eric Oliver, P Ag

Conservation Agrologist, SSCA

Swift Current, SK

Chem fallow has become a very 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, it is not unusual that the protein content of wheat grown on chem fallowed cereal stubble is often 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.

Perennial legumes have played a role in crop rotations for some time, most notably in the Black Soil Zone. However, in much of the Dark Brown and Brown Soil Zones, perennial forages tend to be used primarily for long-term hay production and not in rotations with annual crops. The technique of greenfallowing has also been around for many years and has been an integral component of organic farming systems. 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.

What is Greenfallow?

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 this practice 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. As Biederbeck and Looman concluded in a study in 1985, "To be effective for fallow replacement in a semiarid environment, an annual legume for green manure or forage production must meet certain criteria, such as fast emergence to provide early ground cover; high rate of nitrogen fixation and biomass production; high water use efficiency (i.e. equal or better than durum wheat); resistance to insects and diseases; and high potential as an emergency source of protein-rich forage in dry years." 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. Although cereal crops could also be used as greenfallow, they have the distinct disadvantage of not fixing their own nitrogen.

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.

Unfortunately, greenfallowing has nearly always involved tillage and its role in a direct seeded system has generally been overlooked. Initially, I had concerns about plugging when seeding into greenfallow residue that had been desiccated and this concern may well be a big reason why it has not caught on more with producers. However, since there is little lignin produced in the plant at the time of desiccation, the plant breaks down much faster than if it had been terminated in late August when it contains maximum lignin. As a result, producers who have desiccated their greenfallow crop in July (at flowering stage) have not reported much problem with plugging during the seeding operation the following spring.

Many producers in a direct seeded system are continuous cropping with a diverse rotation and see no need for greenfallowing. This practice is not aimed at them. However, in the Brown and Dark Brown Soil Zones particularly, many farmers still 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.

Greenfallow as an Option

Discing in the greenfallow crop will result in maximum benefit of the nitrogen fixed by the crop. However, desiccating a greenfallow crop results in only about a 10-20% drop in the soil N as compared to discing once. This difference between discing in the crop and desiccating it is due to ammonia volatilization losses from the top growth in hot weather. The losses may be more if there is prolonged heat and wind.

Even though discing in the greenfallow crop will provide slightly more available soil N the following year than desiccation, it also creates other problems, such as initiating weed growth and a higher risk of erosion. In any case, studies do show that greenfallowing does improve the soil quality and organic content as compared to chem fallow and especially when compared to summerfallow. Studies 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, there is a rule of thumb, that being "If it's a dry year, then seed early. If it's a wet year, seed later." This makes a lot of sense in that 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.

Figure 1. Average forage yields 1998-2000 of an Annual Legume Study conducted by Wheatland Conservation Area, Swift Current, SK.

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. Remember, greenfallow is just another tool and an option that producers can utilize in their operation.

The potential for greenfallow to provide nitrogen is impressive. 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 (Figure 1) and relatively high levels of protein (Figure 2). 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. AC Greenfix has proved 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. Table 1 indicates the forage yield and the N production of AC Greenfix from a survey of fields across Southwest Saskatchewan from 1998-2000. 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.

Figure 2. Average protein content of forage 1998-2000 of an Annual Legume Study conducted by Wheatland Conservation Area, Swift Current, SK.

Table 1. Average and range of plant density, top growth, nodulation, N concentration, and N production by AC Greenfix from 1998-2000 at various locations¹ across Southwest Saskatchewan.

¹ Soils ranged from loamy sand to loamy clay with most being quite sandy.

The nitrogen fixed by greenfallow has considerable value, even without considering the higher cereal re-crop grain quality and protein. 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.

Another Greenfallow Option

Although the producer will obtain the greatest N supply from greenfallowing by discing it in or desiccating the crop, there is another option available to the producer that should be explored. That is to remove the greenfallow crop for forage. While this would remove about 75% of the N that could be supplied by the greenfallow crop, it is still worth evaluating if it has a fit in your operation.

Removing the crop as forage can still have benefits for the producer. First of all it will provide 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 in Figure 1, the potential of forage yield can be very good under average to above average moisture conditions. But even

under a severed drought year as in 2001, forage yields can be worthwhile. A greenfallow study at Aneroid, Saskatchewan, found some surprising forage yields (Figure 3). 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.

Figure 3. Forage yields of a Greenfallow Study 2001 conducted by SSCA and Wheatland Conservation Area at Aneroid, SK.

The concern when removing the top growth of the greenfallow crop for forage is that you have only about 25% of the potential N of the total N accumulated by the crop, which remains in the nodules and the roots. This will obviously not provide enough N for the cereal crop the following year. However, it occurred to me that neither will the chem fallow. There will still be a requirement for fertilizer to be applied to cereal stubble chem fallow. Because we are removing the top growth of the greenfallow crop, we will need to apply fertilizer to the subsequent cereal crop. However, lets look at an example of a field pea crop that produces about 2000 lbs/ac (1 ton/ac) forage, which is not a big yield. Assuming it contains about 17% protein, the total N that the crop could supply (top growth as well as roots and nodules) would be about 68 lbs/ac N. If the top growth is removed for forage, that leaves about 17 lbs/ac N in the soil. If we added 25 lbs/ac N in the form of commercial fertilizer, we total 42 lbs/ac, plus whatever N is available in the soil already. Although we are still adding some fertilizer to the system, it does provide the producer with a few more options. 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. 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.

1998 produced more or less "average" moisture conditions at Swift Current. The average forage yield of Grande peas in Mr. Nybo's Annual Legume Study was 4091 lbs/ac with 17% protein. A Grande/Barley mixture produced 3736 lbs/ac forage yield with 14.3% protein. The N produced by the roots and nodules of these two crops would amount to approximately 35 lbs/ac N for the Grandes and 27 lbs/ac N for the Grande/Barley mix. Even without the top growth included in the system (which would provide an additional 111 and 86 lbs/ac N respectively), the greenfallow can reduce the fertilizer input enough to allow single shoot application of the fertilizer to be applied, and still get the advantages of higher soil organic matter and soil quality.

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%. Dr. Biederbeck's survey of Greenfix on test strips on organic farms near Frontier, protein content averaged 25.4% in 1997 and 24.3% in 1999.

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 the forage you removed has value, as does the nitrogen that was fixed in the soil. My earlier example using field pea (Grande) with the top growth removed, left about 17 lbs/ac of N in the soil from the roots and nodules. If 46-0-0 cost $299/Tonne, this 17 lbs/ac N fixed in the soil would amount to a value of about $5/ac. 25 lbs/ac of N is still needed, so if you used 28-26-0 to bring up your phosphate levels, it would cost about $13/ac. If there was a lot of N in the soil from the greenfallow crop, one could likely only need some 11-52-0 to ensure adequate phosphate levels. Feed values this year have gone through the roof, but let's the feed had a value of only $35/ton and the forage produced on the greenfallow was 1 ton/acre, amounting to $35/ac. Even with the fertilizer cost, you still have a gain of $22/ac. Of course there are also costs associated with greenfallowing, such as the cost of seeding and inoculating the greenfallow, as well as a burnoff with glyphosate, desiccation, and any subsequent burnoffs after the desiccation. At the end of the day, however, the economics don't look that bad.

Summary

Greenfallowing can offer producers more options in a direct seeding system. Once again, this practice is intended for those producers who want to have some fallow in their system for risk management. 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 with relatively little cost. With granular nitrogen fertilizer prices around the 29 or 30 cents/lb, greenfallowing can make a major cost saving contribution to the farming operation. Since the greenfallow crop breaks down very quickly after desiccation, there are not major problems seeding into it the following year. AC Greenfix is a variety of chickling vetch that has the fastest growth, highest N-fixation, and most drought and heat tolerance as compared to other pulse crops. Studies and field surveys have shown it to be an excellent variety for greenfallowing. However, even field peas can provide good results and the seed availability and cost can be significantly less than Greenfix.

In addition to the value of the nitrogen fixed by the greenfallow crop, there are other factors that can make this practice attractive to producers. There is the increase in soil quality and organic matter. Even if the greenfallow crop does not fix all the nitrogen requirements of the following crop, the amount of N required is usually low enough that a narrow single shoot opener can be used by the producer. This in itself can be a major benefit for many producers that don't have a double shoot seeding system and are hesitant about the capital cost involved in switching to a double shoot system.

Another option with greenfallowing is to use the top growth as feed. Even though the benefits of N-fixation are less because your are removing about 75% of the N that greenfallow crop has fixed, there are still benefits to the producer. For one thing, the top growth of an annual legume has about the same protein content as alfalfa. The feed also has value and the amount of fertilizer required for the following crop will still be less than compared to chem fallow. The provincial cattle herd is expected to significantly increase in the next eight years or so, and so will the demand for forage to feed this increased herd. Even if the producer uses the greenfallow as a source of emergency feed on occasion, significant benefits can be realized. One thing to consider when deciding to take the top growth off for forage, you may want to cut the forage a little higher off the ground than you would if you were cutting alfalfa. This is to leave some stubble residue from the previous crop to reduce the risk of erosion and to trap snow.

The nitrogen that comes from greenfallowing is much less susceptible to losses than commercial N fertilizer and is also slowly released so there are more chances of cereal protein boost. In addition to the grain quality, kernel size has also been shown to be larger with greenfallow than on chem fallow. This is not to say that greenfallow will replace chem fallow, but the practice can eliminate the problem associated with low wheat protein grown on cereal stubble that is chem fallowed. In addition, if it turns out to be a dry year and you have not seeded the greenfallow early, then you may be further ahead in chem fallowing.

All in all, greenfallowing has tremendous potential to lower nitrogen input costs for producers, reduce tilled summerfallow acres, reduce erosion risks, and improve the yield and quality of the following crop, while improving the soil quality and organic matter.