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by R. Martin, Department of Plant Science, McGill University (Macdonald Campus)
reprinted from Macdonald Journal, February 1979
The basic agronomic reasons as to why crop rotation is advantageous are:
1. Because there may be toxic excretions from the roots of the crops or the decomposition of the stubble;
2. Because of the differences in the kinds and quantities of nutrient required by the plants;
3. Because there are differences in the feeding powers of plants for nutrients in the soil solution;
4. Because different crops are susceptible to different pests (diseases and insects);
5. Because of the different crop-weed associations.
In order to illustrate our discussion with some relative figures, we will start by looking at an experiment done at Woodlee in Ontario from 1947-1962 (Table 1).
The first aspect, namely the toxicity of root excretions and/or stubble decomposition is not--at least, has not been identified--as a limiting factor in corn production. Using the results of Table 1, we will discuss hereafter the four following factors.
If we look at the difference in yield between CC and COM with no fertilizer, there is a 36.9 bu/ac advantage of rotation over continuous corn. Accordingly, a good legume ploughed down may supply 60 to 80 pounds of nitrogen per acre. This could partly explain the difference observed as we know that corn requires a heavy dose of nitrogen fertilizers. And even when there is only one year of legume in the rotation, the gain in yield is 8.8 bu/ac (CS-CO).
It is also recognized that nitrogen fertilizers enhance maturity; therefore, a good supply of nitrogen would reflect not only on the yield quantitatively but also qualitatively. Since we are in a marginal area for corn growing, a few days gained in the fall by adequate fertilizer may prove to be critics' in some years. To this must be added the facts that low moisture corn is easier to harvest and that the drying process when the moisture level is relatively high impairs the quality of the grain.
This difference of 36.9 bu/ac cannot be attributed only to the nitrogen supplied by the legumes. The tap root of legume also improves the structure of the soil, which favours the development of roots and better uses of the soil fertilizer reserve as is discussed in another article. This can be illustrated by looking at the difference of COM and CC where fertilizer is applied. There is a 25 bu/ac gain of rotation over monoculture of corn.
Roots of different crops develop in different ways. This implies that by taking advantage of this characteristic of the plants a better use of the soil reserves can be obtained. Moreover, the exchange capacity of roots varies among various plants. The exchange capacity of the roots of dicotyledonous plants is much higher than that of monocotyledonous plants. The magnitude of the exchange influences the absorption of cations; therefore, those with high capacity absorb relatively more divalent cations such as calcium and less monovalent such as potassium. This latter aspect can explain the deficiency symptoms observed in fields that have been seeded to corn for several years. This is not the only reason; however, it could be one of several. Finally, there is association of micorhizal fungi with certain root systems which helps to mobilize some of the nutrients.
The first two aspects have a definite influence as we have seen previously but this could be minimized by the use of fertilizers. However, as shown in Table 1 even with fertilizers there is still a difference of 25 bu/ac between corn in a rotation system with corn in monoculture. Part of that difference can be explained by improvement of soil characteristics discussed in the other articles.
There are, however, two factors responsible for crop losses: diseases and insects, which are difficult to overcome by means other than by crop rotation. One can argue that the use of resistant varieties can be an alternate solution, but because of numerous diseases and insects that can affect a crop, it is difficult--and not yet achieved--to have a variety resistant to ail pests. The following Table illustrates the damage that can be caused to corn by the major Quebec corn pests, namely: Fusarium, eyespot, and European cornborer.
These major pests are responsible for 15 to 45 per cent losses in yield quantitatively depending on the intensity of the infestation. Among the conditions favouring infestations of these pests, monoculture of corn is an essential one. In the cases of Fusarium and eyespot, it ensures a build up of inoculum of the causing agents. Another disease with a higher incidence than in previous years is corn smut. Again, crop rotation should see a lessening of this disease. For European cornborer, the predominant damaging insect of corn, the pupae overwinter at the base of the corn stalk. Thus reseeding corn over corn favours the propagation of cornborer.
Though insecticide in this case can be effective on cornborer, the degree of effectiveness does not equal rotation, which depletes the food source of the insect.
Infestations of weeds that were, only a few years ago, found only occasionally in the field, are observed more and more. Yellow nutsedge and horsetail infestations are two clearly identified situations in fields that have been seeded with corn for several years. Why? The use of selective herbicides eliminated the natural competitors of these weeds and left open areas that they could invade. The exact losses directly resulting from these infestations are difficult to estimate. At Macdonald
College attempts to evaluate the losses caused by different weeds have been made and the preliminary results confirm that weeds, particularly species such as the two mentioned above, do cause substantial losses in crops.
There are different associations of weeds and crops, and long-term experiments in Rothamstead have shown that crop rotation is an effective way of preventing predominancy of one or a few species of weeds. Even though there are herbicides available that could clear the land, it is only postponing the issue. Atrazine resistant lamb's quarters that have been identified in some corn fields in Ontario illustrate the threat of depending solely on short term solutions.
The data in Table 1 illustrate that crop rotation, even as simple as the two years--one of corn and oats --improves corn yield. Not only does it improve yield, it also ensures a better utilization of the soil as a basic resource of crop production. At the same time, it offers a certain stability of yield by offering some control over crop enemies diseases, insects, and weeds.
Nevertheless, we understand the difficulties that crop rotation brings to the farmer: diversification of machinery, feed demands of the herd, etc. However, the benefit that crop rotation offers, we feel, overcomes these difficulties. In order that the farmer gets the best of crop rotation, however, research has to be done to find the best system suited for our type of agriculture
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