Dear Enric,

be or not to be!!! Legume or grass cover crops???

It is very difficult answer to this question, but several suggestions could be obtained from different experimentations around the world.

However, I can give you my personal suggestion, based on my experience, which can accepted and/or contested based on the different point of view.

First of all, what is really important is to know the environment where farmer are working, in particular weather condition contribute to choose which cover crop species should be adoped with high chance of success. After that you can elaborate the cropping systems and its rotations. Although, we have carried out several experimentations with cover crops, only few experiments there were mixture legume/grass. Based on the result of these experiment, we noted that is possible to perform better some aspects on the following summer vegetable crops, such as reduced nitrogen leaching, better weed management etc...

However, in our enviroment (Mediterraean climate) the cover crop species production (Biomass) was subjected to seasonality because they can grow in bad weather condition. This made more difficult decide the right mixture to adopt. For this concern we prefer to use only a cover crop that assure us the best performance (biomass production, nitrogen, etc.). However, from my point of view, how manage the cover crop aboveground biomass no-tillage, minimum tillage, conventional tillage can play an important role in the nitrogen and allelophatych release and thus in crop performance.

Best regards

Emanuele

Excellent points Emanuele.  What works in one location and year may not in another, especially with mixtures.  There are many different approaches and clearly mixtures are some of the more complex.   We have a lot to learn about mixtures.  These are exciting times for cover crop research and it is fun to be able to play a small part in these efforts.   Happy cover cropping and happy juggling too (see link below).  Eric

https://www.youtube.com/watch?v=Z8yVDphBm78

Yes. Its true. Legume-cereal cover crops will prove beneficial as per our current understanding on the nutrient demand and extraction from legumes and cereals. In southern parts of  India, organic farmers mix the seeds of nine crops (legume and cereal), sow and plough in situ as green manure to enrich the soil fertility (This is an age old practice).

Hi Rolland, 

We did assess N mineralization during several weeks following cover crop incorporation.  Figure 11 in the first publication link below shows this for the legume-rye mixture versus non-legume cover crops.  The second linked paper below from this same trial, also showed that legume-cereal mixtures had more N accumulation than non-legume cover crops.  While having more N accumulation in a legume-cereal mixture might seem like a good thing, it can be challenging to time the N mineralization of this residue with the N demands of the subsequent cash crop.  We are still working on completing analysis of yield data from subsequent vegetables, so stay tuned.  It is also important to keep in mind that the seed costs for the legume-cereal cover crops (at weed-suppressive rates) were nearly 10 times higher than the cost of seed of non-legume cover crops (see table 1 in  biomass paper, third link below). Eric

Article Winter Cover Crop Seeding Rate and Variety Effects during Ei...

Article Winter Cover Crop Seeding Rate and Variety Affects during Ei...

Article Winter Cover Crop Seeding Rate and Variety Affects during Ei...

Eric

Not only N availability is important in fitting covercrop with crop rotation. Another important impact of legumes on the mitigation of soil diseases as sclerotinia. For this reason, organic farmers with vegetables rich crop rotation  (it is the case in your study)  absolutely avoid the use of legumes in covercrop mixtures. They prefer to use plant such Phacelia wich is favorable to P availability.

Daniel

Dear Eric and others

As already discussed, the optimal choice of cover crop depend on many factors, the growth conditions of the region, the time of year and how long time the cover crop can be allowed to grow, soil diseases and many others, so local experience is needed. None the less, mixtures of e.g. hairy vetch and barley or rye have been shown to be successful in many experiments under very different conditions in many countries.

I think that especially in vegetable rotations, the soil N level is often high, and then cover crops without legumes are often the best, while mixtures with legumes, or even pure stand legumes can be the best choice when soil N supply is low. In a diverse rotation, this would ideally mean that non-legume cover crops should be grown after vegetables to take up the excess soil N, whereas mixtures with legumes could be grown before vegetable crops, to improve the supply N if needed, such as in organic crop rotations.

I always wonder about this discussion of the need for synchronization of N release from cover crops with the N demand of the following crop. When just stated, it seems correct, but what exactly do we mean by synchronization, and why is it really important? We often need an early N release in order to supply the early N demand of the crop, but this is normally not a problem if the C/N ratio of the cover crop is kept low, which can be achieved by the correct choice of cover crop species and management.

Cover crops tend to increase early N supply for the succeeding crop (sometimes too much), but then often to reduce the later supply. This is mainly due to the fact that N availability after cover crops is not only affected by the dynamics of N mineralization from the cover crop residues, but also by the spatial redistribution of N in the soil caused by cover crops. After a cover crop, more N is normally available in the topsoil, but less in the subsoil, as compared to plots where no cover crops were grown, i.e. cover crops concentrate the available N in the uppermost soil layers. While the following crop is young, and have its root growth mainly in the uppermost soil layers, their N supply is enhanced by the increased N availability there, but later during the crop growth this can be reversed. When roots grow deeper, crops grown after a cover crop will find little N there, whereas crops grown where the soil has been left bare, will continue to find N resources in deeper layers as the roots reach these layers.

We have shown the effect of cover crops on the depth distribution of available N in the soil in a number of experiments, and also shown how this interacts with root development of the succeeding crops, I can recommend these papers dealing with this topic, also in vegetable rotations:

Thorup-Kristensen, K., Dresbøll, D.B. and Kristensen, H.L. (2012) Crop yield, root growth, and nutrient dynamics in a conventional and three organic cropping systems with different levels of external inputs and N re-cycling through fertility building crops. European Journal of Agronomy, 37, 66-82

 Thorup-Kristensen K, Salmeron M, Loges R 2009. Winter wheat roots grow twice as deep as spring wheat roots, is this important for N uptake and N leaching loss? Plant and Soil, 322: 101-114

Thorup-Kristensen, K. (2006) Effect of deep and shallow root systems on the dynamics of soil inorganic N during three year crop rotations. Plant and Soil, 288: 233-248

 Thorup-Kristensen, K. (2006) Root growth and nitrogen uptake by carrot, early cabbage, onion and lettuce following a range of green manures. Soil Use and Management, 22, 29-38

Kristian

This answer is from a farmer so it will be less scientific. We wanted a green manure that also brought income to the farm. So, very early in the spring we sowed a field of wheat, hull-less oats, yellow peas and a few other early maturing crops like radish to provide feed that broiler chickens could self-harvest. Once the crops matured we mowed a new section each week and fenced the chickens on it. The birds were four weeks old at the start of the trial and 12 weeks old when we harvested them. We supplemented their diet with store bought feed at 25% of a full diet. We plan to reduce that next year and are investigating what other seeds we might include in the mix. There were enough seeds missed by the chickens that germinated after we moved them to the next plot to re-green the previous field and provide well rooted soil cover for over winter. When we till this field for vegetables next spring we believe the combination of roots, straw, and chicken manure will be superior to green manure alone.

Eliot

Hi Eliot

Very interesting and judicious trial I think.

Working with poultry organic farmers, I am interested in information's on how many acres you need for how many broilers harvested at 12 weeks old (considering that you feed them at 25% of a full diet).

I am wondering one more question : you need 2 month to grow the broilers, at wich stage of maturity do you  start with mowing the crop and how long is it possible to stock the seed crop on field after maturity in your climatic conditions ?. Perhaps you need to sow the crop at multiple date to cover the hole period of 2 month mature seeds availability  ?

Thanks in advance for further details on your innovating trial

Daniel

Dear Kristian (and others)

thanks for your thoughtful comments on my question with many excellent points, and the publication list.  I was unaware of the 2012 paper and am really enjoying reading it;  this shows the value of Research Gate in helping us keep up to date on each others research)  I liked your point on page 80 of your 2012 paper (European Journal of Agronomy, 37, 66-82) "Such results point to fertility building crop strategies being much more important than conventional vs. organic farming systems in optimizing N utilization and reducing N leaching losses."   I also really appreciated that you had several different organic systems in your study and clearly showed that they were quite different in many regards.  Michel Cavigelli and I gave a presentation titled (Organic vs. Conventional Comparisons:  A Devil without Details)  at the American Society of Agronomy in 2014 where we discussed organic vs. conventional comparisons that may interest you and others;  it's available at the first YouTube link below.  Your 2012 paper does a nice job of providing lots of details on the various systems you compared (this is critical to help the rest of us understand your data).  

Do you have problems with weed growth and seed production in cover crop mixtures in the systems where you work?  As I pointed out in the conference presentation where I questioned if these mixtures are a "good fit' (see 2nd YouTube link below), weeds grow extremely well in legume-cereal mixed cover crops in California unless they are planted at quite high seeding rates  (rates that can cost  up to 10 times more in seed cost than cereal cover crop seed).  Furthermore the legumes in these systems often struggle to compete with the cereals which limits' their ability to fix N.

Eric

https://www.youtube.com/watch?v=os-aM4FqPUA

https://www.youtube.com/watch?v=WREmHa-jFbc

We do not have serious problems with weed seed production in cover crops, but our winter seasons in Denmark are also very different from the conditions in California.

With more legume seed in the mixtures, the weeds will clearly grow better, we would rarely use more than 50% legumes in mixtures.

Also the soil N availability is important, when it is low, legumes can be quite competitive, but when it is high, they are weak competitiors agains weeds or cereals, in genereral agains most non-legumes. When weeds grwo so well in your system, I assume there is a lot of N available for them in the soil. As I wrote in my first answer, I would reccomend cover crops without legumes in this situation,

Kristian

Dear Kristian,  thanks for the response on the weeds.  This is helpful to understand how different the cover croppnig situation in in Denmark versus the central coast region of California.  The legume cereal mixtures used here typically have 90% legume seed and even with that percentage the legumes can struggle to compete.  I believe that you are correct that part of this struggle may have to do with relatively high soil N left over from previous vegetable crop.  Perhaps farmers should use quick N tests before planting fall cover crops to determine the most appropriate type of cover crop to plant.  

We have done some research with mixtures with lower proportions of legumes (i.e. 60%) and found that often the % legume biomass at season end was only 1% of total cover crop biomass (see mixes 4 and 5 in figure 8 in the 2009 paper link below).  In that paper the Salinas site was a lower fertility site than the Hollister one which likely explains why, in general, the legumes did better in Salinas, although this varied by year.   As I pointed out in my presentation questioning the use of legume-cereal mixes, the survival of legume biomass often seems linked to weather conditions (growing degree accumulation early in the fall during the first 30 days after planting);  See figure 12 in the 2012 paper link below which was based on 8 winters of cover crop research at the same site.  We clearly have much to learn about the complexity of these mixtures, but long-term research is really helping to improve our understanding.  This is an exciting are of research.

Eric

Article Comparison of Rye and Legume-Rye Cover Crop Mixtures for Veg...

Article Winter Cover Crop Seeding Rate and Variety Affects during Ei...

You would expect legumes to flourish when and only when they have an advantage over the competing species. They are no dependent on soil N but are highly dependent on soil P which is difficult to supply in organic systems. Which plants, weeds, crop of legumes dominate in your system depends on some of these basics. Unless your soil is low in N and high in P the weeds and the crop will dominate and you will get little N benefit from the legumes. Less crop will only mean more weeds unless the environment is such that it suits the legumes more than the others. N fixation comes at a high cost to the legume in terms of energy.

Jeff Hoffmann

Hi Daniel Jamar,

Sorry for the delay in replying. In early April of 2014 we sowed about 1/2 acre and used 50 birds for this first trial. We did rough calculations of potential production based on average yields in our area. We raised the young chicks on short grass pasture for four weeks until the seeds matured around August 1. The birds were moved progressively down the home grown feed field for the next 8 weeks and were harvested at 12 weeks. The breed we used were of similar genetics to the Label Rouge birds in France.

We only sowed the seeds once and they stood for the whole 8 weeks. For next year , however, we plan to experiment with sowing another section of field in June with less cold tolerant crops like millets, sorghums, buckwheat, etc. which are recommended for the "food plot mixes" used by American hunters to attract game birds. We will see which the broilers like best.

Eliot Coleman

Dear Eric,

Of course they are. In fact, this is one of the most recommended strategies to improve soil quality, nitrogen fixation and crop yield in a natural way.

It is commonly used, for example, in extensive and organic livestock production systems based on pastures, were some crops are used to be eaten directly by animals. Such crops are based on legume-cereal (mainly Vicia sativa-Avena sativa)

Regards,

Alfredo J. Escribano.