Green and Brown Manures in Dryland Wheat Production Systems in Mediterranean-Type Environments

In this review, we draw together research on the use of green and brown manures in wheat cropping systems in Mediterranean-type environments in the light of contemporary pressures on cropping systems including changing climates, increasing costs and declining profit margins. Green and brown manuring have been demonstrated to have benefits in terms of weed control, delaying the development of resistance to herbicides, reducing populations of disease organisms, altering soil water, soil quality and biology, erosion control, and contributing to the nutrition of subsequent crops. However, few researchers have attempted to measure more than one of these variables, which presents difficulties in both interpreting the causes behind results of field trials and in estimating the total benefit of manuring, and hence its consequences for profitability. Well-designed experiments have been reported on component mechanisms (such as weed numbers or N₂ fixation). However, these experiments are often not taken through to maturity of the crop following the manuring treatment. As a result, there is limited yield and grain quality data on which to base sound analyses of profitability. A few reports are available which present the impact of manuring on wheat yield and profitability in specific areas and systems but the results vary widely. For such reports to be of value, further research is needed into the factors inducing the changes in response (climate, soil type, or the specifics of the farming system at the time the treatments are imposed) and the mechanisms by which these act. Thus, research is needed into both the mechanisms and yield benefits that flow from the individual responses to manuring. Two further limitations to determining the economic benefit of manuring emerge. Firstly, impacts are primarily reported for a single year after a single manuring treatment. However, if measurements are made over a number of years, effects can often still be detected. More studies aimed to assess the longer-term impacts of manuring on soil health, disease prevalence, and weed populations are required. Secondly, there has been very little effort to explore the whole-farm impact of using manures. These impacts could include effects on other farm enterprises as well as business-level impacts such as potential changes in labor requirements, cash flow, and risk. The incorporation of manuring into wheat production systems may have multiple on-farm and off-farm benefits. However, there is a substantial research requirement before these approaches could be recommended. The highest priority is a sound demonstration of short- to medium-term economic benefits to growers. Without this, adoption can be expected to be poor.