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Stubble Retention

Jim Wright

Farmers Cowra

Soil is man’s most valuable asset, Without it he couldn’t exist but he has abused it (and still is) ever since he started cultivation.

Without doubt the most common form of soil abuse is over-cultivation. This is often confused by the term “over-cropping”; there is a big difference. I have found that you can crop on a continuous basis providing it is done with minimum soil disturbance and saving all the crop residue each year. When I talk about continuous cropping I don’t mean the growing of the same type of crop each year, but using every available crop which will grow in your particular area.

We have found with this practice you can even improve soil structure by build up of organic matter. Actually, the soil becomes much easier to work, water infiltration is improved and soil aeration is improved. Soil erosion, although not eliminated, is reduced to a minor problem.

Stubble Retention - The Problems

Why retain stubble when it is so easy to send it up in smoke? That is a good question,

  • It is difficult to handle, so is it worth worrying about when you have so much trouble with sowing machinery?
  • With direct drilling this makes it more difficult. If you could go in with a disc plough and chop it up two or three times it would be much easier.
  • A serious problem is the disease yellow leaf spot. This is the scourge of the stubble farmer.
  • Another problem is that because of the excess trash on the surface, pre-emergent herbicides can’t be used.
  • Yet another problem is wet weather because stubble, if turned in, is almost impossible to get tyned machinery through.

So there have to be many good reasons to bother with it.

Experiences with Stubble Retention

The two main advantages of stubble retention are:

1. improved soil structure; and

2. soil erosion control.

It was soil erosion which forced us into saving our stubble. We have undulating country which is very subject to erosion. We like to crop continuously, so in i976 we decided against burning stubble and we have burnt very little since.

At that time we had only conventional machinery. We didn’t want to buy special plant as we had no idea how it would work. We hooked two old disc ploughs in tandem and ran over the stubble, ploughing ‘only about 2” deep. We left the stubble in that condition until close to sowing time when we fire harrowed, which left some stubble; probably about half on the surface and some slightly buried, mostly in short lengths. This allowed conventional machinery to get through (after harrowing a few times). However this was not really satisfactory because of time, and still too many blockages with the conventional combine, and far too much soil bashing, Over recent years we have been knocking stubble down with a large set of disc harrows and chopping it at the same time. This is also done very shallow with minimum soil disturbance, but we are still not satisfied with this system. There is always a better system. We now have fitted an air seeder to a chisel plough which will sow into very heavy amounts of stubble residue,

Overcoming The Problems

Sowing Machinery

Direct drilling into stubble can be achieved by fitting culters in front of each tyne on a chisel plough and then following behind the tyne a press wheel, This will depend on soil type. Soils that are inclined to crust may be better not pressed, but as we proceed with direct drill and stubble retention our soil structure will improve to such an extent that a press wheel may be necessary in all cases.

Levelling off behind chisel ploughs is a major problem, We have tried almost everything from four row finger harrows to heavy stump jump harrows and nothing has been successful, A rod weeder does quite a good job but has problems with handling straw. A further hazard with chisel ploughs is the large sweep points. These, under moist conditions in a direct drilling situation, have a real disadvantage in that they slice through the soil causing long flakes of soil (still held together with old roots) to fall directly over the seed. This has the effect of smothering the newly- germinated seedling. This may not be easy to understand but I can assure you I have had some real disasters,

Another worry was the row spacing of 300mm or 12” which we consider too wide, However, this was easily overcome by dividing the flow of grain into a Y-shaped tube which spreads grain into 150mm rows. You don’t have to own a chisel plough to sow into a reasonable amount of stubble residue. It can be achieved in many and varying ways but probably the simplest and cheapest system is to remove the under-carriage from the conventional combine and buy some heavier tynes and space them to allow the maximum amount of stubble clearance. This type of machine would then be suitable for direct drilling only after chemical control of weeds as you would not have any overlap of your points to cut out the weeds. It is for this reason that I think a chisel plough is really a far better machine to handle heavy stubbles.

Stubble Bashing is Soil Destruction

If we have to disc stubble in and harrow several times, probably disc again, scarify, harrow again and then sow, the whole purpose of stubble retention is lost. The damage to soil by cultivation is much greater than the benefit we will get from the stubble. If you are doing this you are wasting your time, fuel and money. You would be far better off to use a match and burn. Then you could direct drill with a conventional seeder. This is only satisfactory for those without a soil erosion problem, but if you have long slopes you can run into severe erosion.

It is important to remember at all times that it is the soil we should be considering, It is number one as we want to continue to grow crops in the years ahead, This brings me to the question of how often do we want to cultivate our soil? If the answer is two years crop - two years pasture I would then say it is a doubtful proposition to worry about saving your stubble, providing that you direct drill or,at the most, one cultivation prior to sowing,

Perhaps some of you ~ realise that the root system of a crop makes up a large percentage of overall residue, So It is of great importance to disturb these roots as little as possible, because this will help water infiltration. We find that early-sown cereal crops produce a tremendous root system, and is a very valuable tool in improving soil structure.

Once we get this build - up of root matter in the soil we must not destroy it. This is not easy as many of you know that an oat crop that was heavily stocked last winter (on heavy country) is almost impossible to direct drill. Under these conditions it is almost always desirable to have one cultivation; this allows sowing to be done at a much more even depth. It is advisable to move stock off crops during very wet weather (if possible) as stock pugging up soil is much worse than cultivation.

Sowing Depth

Sowing depth, I believe, is critical in the establishment of a good healthy crop. This undoubtedly is the greatest problem associated with direct drilling especially into stubble as you have the unevenness of header tracks plus the fact of stock tracks which tend to follow wheel tracks. If you set your machine to penetrate the hard patches you are sure to go too deep in the soft areas between wheel tracks. This is a problem which is very difficult to overcome without cultivating prior to seeding. I believe there are two ways of overcoming this obstacle. The first and probably the most practical is to sow at a different depth allowing the seed to be placed at the optimum depth. This can be achieved by taking the sowing tube off the sowing boot and extending it to the required distance behind the tyne so as to allow the seed to fall in after some of the soil has settled behind the tyne. This is not easy as soil type, speed of travel and size of working point will greatly influence where the correct distance is. We will be working on this system next year as we have not got anything that is really satisfactory. The other system which I have tried is furrow sowing. This is sowing in furrows without any harrows behind, the advantage here is that you can work very deep but the seed which is in the furrow (which is not filled in) has only a shallow depth to penetrate.

Crop Disease

Now, after all this trouble to protect our soil, which is our main objective (even though there are some other advantages), we are still plagued with cereal diseases. The principal one is yellow leaf spot. This disease causes more loss on our property than any other and I think wheat breeders could do a lot in this field as we notice some varieties are much more susceptible than others. I might also add that crops that are nitrogen deficient or chemical affected appear to be more susceptible to yellow leaf spot. This is simply my own observation and in no way conclusive.

To minimise disease we can adopt a few practices such as crop rotation and burn stubble if the previous crop was infected. Do not allow any self-sown crops to mature during the summer or autumn as these can be a real source of infection.

Crop rotation is obviously the best way to control disease. The type of “disease breaking crops” as we call them, will depend on what is most suitable to your area. We find in the Central and Southern parts of the State

we can rotate such crops as wheat, oats, lupins, rape, barley, triticale, field peas, rye corn. It is obvious we have a good range of alternative crops and also good defence against weeds in all of these crops with the wide range of chemicals available. The limiting factor is the market for the individual crop. It would be advisable to make sure of a market before selecting the type of crop to sow.

Having machinery available and a variety of alternative crops, we can safely start direct drilling into most stubble residue, but it is not to be taken lightly as many farmers have had very disappointing results. It is of the utmost importance to know your paddock history, especially the nitrogen level, as the incorporation of stubble will certainly require extra nitrogen while it is being broken down.

Probably the most important thing to remember is why we are going to the trouble, and this is to improve our soil structure and control soil erosion. In many cases, we may have to expect lower returns per hectare, simply to save our soil but in the long term I am sure if we keep our soil in good order it will give us better returns.

In conclusion let me point out it is not easy for us to accept change, but we must, for the benefit of future generations. When one looks at the deterioration in soil structure and the increase in soil erosion over the past 20 years, we cannot allow this to continue and I feel we have a moral obligation to “save our soil".

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