Complémentarité herbe-cultures dans les rotations

The complementary roles of leys and of arable crops in rotations

A specialization in the farming systems has been observed in France during the past 40 years. There are now purely arable farms, and, on the other hand, purely animal farms. As a consequence, farmers reduced the number of their crops, and hence the lengths of their rotations. On arable land, we now have mainly arable crops ; monocultures such as maize and winter wheat are more and more frequent. There are no more clovers or lucerne in the rotations, because of the absence of livestock to be fed on them. Besides, even on animal farms, leys are becoming rarer in the rotations.
Simplified rotations, and more particularly the extreme case of maize monoculture, have negative effects on soil fertility and on the environment. On the contrary, long rotations, that include leys, improve soil fertility, reduce weed infestation, decrease erosion, increase yields, and limit yield variability. The impact of long rotations on the environment is positive, because they require less herbicides and less energy input. Nevertheless, the introduction of leys raises the problem of nitrate leaching following the destruction of grass. To limit it, the best strategy is to use non-persistent total herbicides to kill the pastures, and use shallow cultivation for the following crop. The calculation of the N fertilizer rate will have to take into account the high level of mineralization of organic matter of the destroyed grass.