Yield Gap

The yield gap (Y_g ) concept is based on definition and measurement of yield potential.  Yield gap may be defined as the difference between yield potential or water-limited yield and actual yields (Van Ittersum et al. 2012; Lobell, Cassman, and Fields, 2009); but Mueller et al. (2012) and Titonell and Giller (2013) define yield gap as the difference between ‘attainable yields’ and landscape-level observed yields.  Examining yield gaps is important as policy makers work toward ensuring food security at the micro and macro level in a sustainable manner.  Due to favorable climate, soil quality, and access to irrigation, some regions may have greater potential for sustainable agricultural intensification.  To understand the concept of yield gap, we will define some key components in the calculation:

1. Yield potential (Y_p ) – is the yield of a crop cultivar grown with no limitation of water and nutrients and biotic stress effectively controlled (Evans, 1996; Van Ittersum and Rabbinge, 1997).  Therefore for a given site when a crop is grown to achieve yield potential, growth rate is determined by; solar radiation, temperature, water supply, and genetic traits that govern length of growing period.  Yield potential is defined for irrigated systems where crops are given adequate water through the growth process.
2. Water-limited potential yield (Y_w ) for rain-fed crops – is similar to yield potential but crop growth is limited by water supply. It is therefore influenced by soil type (water holding capacity and rooting depth) and field topography (runoff) (Van Ittersum et al. 2012).
3. Locally attainable yield (Yl ) is the maximum yield achievable by resource endowed farmers in the most productive fields.  These yields are more conservative than absolute biophysical potential yields because  are achieved using current technology and management techniques (Titonell and Giller, 2012; Mueller et al. 2012).
4. Average yields (Y_a ) is the actual yield achieved in a farmers’ field.

Yield gap can be estimated as the difference between and Ya  and Y_a or Yl  or .  Lobell et al. (2009) identify 4 methods of estimating yield gap at local level that can be used to obtain the yield potential (maximum attainable yield) and these include: 1) crop model simulations, 2) maximum farmers’ yield based on surveys, 3) yield contest, 4) field experiments.  In this case we are going to assume that yield ceiling will be obtained from crop modelling or field experiments.

Yield potential is estimated mainly using crop models that assume perfect management using lack of all yield reducing factors.  A short coming of crop models is that they lack the sensitivity to short term abiotic stress and leads to higher estimates of potential yields than would occur in the field.  As for field experiments, the difficulty of achieving perfect yield conditions increases with plot size and with year to year climate variation, which may be large at a location therefore requiring a time series of experimental data to ensure a mean estimate that reflects a range of climates.  A combination of crop model simulation and field experiments is recommended to provide more robust estimates.  The use of maximum farmer yields may be used to estimate yield potential in locations where farmers intensively manage a crop with the possibility of achieving the yield potential.  Actual yield estimates in the field (i.e. crop cuts) are recommended to complement farmer reported values (Lobell, Cassman, and Fields, 2009).  Crop production capacity under rain-fed and irrigated conditions can be analyzed by estimating yield potential and water limited yields as a benchmark.  This is essential for sustainable agricultural intensification.