There is a wide divide in many parts of the world between the yields of crops that could be achieved and those that are actually harvested.
Drought-tolerant maize has improved yields in more than a dozen countries.
What is the yield gap?
It is the difference between yields of crops that are actually achieved in a particular locality and an estimate of the potential achievable yields using the right crop varieties and the correct agronomic practices and inputs. Yield gaps have been the focus of renewed interest in the face of stagnating crop yields for several reasons:
• the projected rises in food demands due to increases in global population and changes in diets;
• the effects of climate change on food production; and
• the environmental and ecological costs of further expanding land under cultivation.
This interest is reflected, for example, in such initiatives as the Global Yield Gap and Water Productivity Atlas project, set to provide estimates of yield gaps on existing farmland based on current climate and available soil and water resources.
The largest yield gaps are reported in Sub-Saharan Africa: average maize yields in 2000 were about one and a half tonnes per hectare, while potentially attainable yields across maize-growing areas were over three times higher. Yield gaps are also responsible for problems related to food distribution, since surplus grain production in developed countries is often exported to developing ones with low production, sometimes with negative effects on local food prices and/or on the functioning of domestic markets. Examples include legumes in South Asia, where demand is increasing but cannot be met by local production; and rice in Tanzania, where the abolishing of import tariffs nearly caused the collapse of the national rice-growing sector. Therefore, research directed to closing the yield gap in low-productive, rain-fed agricultural areas is likely to have the highest impact on future global food security.
How is the yield gap measured?
The short answer probably is that measuring yield gaps accurately is difficult since estimates of attainable yields always rely on assumptions that are hard to prove or disprove. This is especially true when extrapolating from the field scale to national and global scales, even more so in developing countries where farming conditions are vary considerably. In addition, climate change is likely to have a strong impact on attainable yields as global temperatures increase, the weather patterns become more erratic and extreme weather events more frequent. The impact of climate change on crop yields is expected to be greatest in the tropics and the effects will be most felt in the areas that already experience food insecurity.
Attainable yields are usually calculated separately for rainfed and irrigated farmland. One method of estimating the yield gap is to measure the yield of a crop variety adapted to the local conditions and grown using the best agronomic practices in the absence of pests and diseases, and to determine the difference with actual yields derived in a given area.
This method assumes that the management practices used in the experiment are indeed the best across all of the area where the crop is grown, and also the best for all the crop varieties used, neither of which may be correct. It also assumes that widely adapted varieties, the most common output of crop breeding programmes, are well suited across a varying landscape. For these reasons the best attainable yield reference is sometimes derived from farmers’ fields, statistically considered as the value at the 90th or 95th percentile, which is referred to as the locally attainable yield. This estimate is likely to provide a better indication of the yields that are achievable in the soil and weather conditions of a specific site using the best available and affordable technologies.
Another method for measuring yield gaps is the use of crop simulation models that take into account the factors likely to limit crop productivity and are sensitive to planting and harvest times, weather, soil conditions, and the crop variety used. The accuracy of these models is, however, again dependent on the validity of the underlying assumptions in the target areas, and whether all the relevant information is available for the models. This approach also requires verification of the simulated model outputs in the field.
In drought-prone areas, a key objective is to maximise water-use efficiency for better harvests.
Closing the gap
Despite the difficulties in estimating yield gaps – estimates for the differences between attained and potential yields for crops in rainfed areas range from less than 1 to more than 5 tonnes per hectare – there is a broad consensus that more accurate knowledge about these gaps and an increased understanding on how they may be closed is essential, particularly in developing countries. This information will help to prioritise research and guide plant breeding initiatives, and also identify information gaps in the less productive environments.
In drought-prone areas, a key objective is to maximise water-use efficiency for better harvests, both through breeding and by the introduction of management practices that improve water conservation in soils. Also needed is better understanding of how different constraints interact with each other to affect yield since most studies have primarily considered water and soil fertility in isolation.
Improving the fit
In train stations, the gap occurs when a straight train carriage stops by a curved platform: the fit isn’t right. Poor fits may also partly explain yield gaps in smallholder fields in developing countries. Widely adapted varieties may not be good enough in varying situations, so it may be needed to develop a larger number of locally suited varieties using participatory varietal selection methods that allow farmers to assess varieties in their fields and select the ones that perform best. In addition, a criticism of many yield gap studies is that they tend to focus on the role of the widely grown and traded grains, while the importance of underutilised crops both for future food security and for more sustainable agricultural systems is increasingly recognised.
An integrated solution
This blog has so far not mentioned what many studies on yield gaps also fail to refer to: the socioeconomic factors that contribute to creating and maintaining yield gaps. Yield gaps can also be viewed as poverty traps, and closing them will require much more than just new scientific knowledge and technologies. This will be the topic of my next blog.
Claudia Canales Holzeis is a plant molecular biologist with a near-decade of experience in plant genetics research. She previously worked as Senior Project Officer for the International Service for the Acquisition of Agri-Biotech Applications (ISAAA), based in the Philippines. A graduate of the University of Reading in Environmental Biology, Claudia Canales gained a DPhil. in Plant Genetics at Oxford.