Deepak Ray and Jonathan Foley from the University of Minnesota used an annual national-level cropland harvest frequency (CHF) metric, defined as the ratio of the annually harvested cropland to the total standing cropland. For example, a country that harvests all its cropland exactly twice a year has a CHF of 2.

The researchers found that, globally, the average CHF was 0.9 in 2011, up from 0.8 in 1961. This means there has been an extra crop harvest in every 10-year period compared to the 1960s.

"We are harvesting our crops more frequently, and we believe there is still scope for increasing this further," Ray told environmentalresearchweb. "We realise that increasing the number of harvests is not possible for every country, but there are some areas, such as countries in Africa and also some high-income countries such as South Korea, where the CHF is decreasing, and this trend should be reversed."

As well as areas where the CHF had decreased, Ray and Foley also found several regions, including Brazil, India and China, with significant increases in harvest frequency. "The increase in areas such as Brazil is encouraging because it means that, faced with a decision to either increase cropland by clearing forest or harvest the crops again on current croplands, farmers are starting to choose to harvest their crops a second time," said Ray. "However, the CHF for Brazil is only 0.9, which is very low for a tropical country that could theoretically harvest twice per year."

In theory, if the temperature is more than 10°C all year round, a country should be able to harvest its crops twice per year. The reasons for the lower CHF in countries such as Brazil are often complex and related to socioeconomic and/or biophysical factors.

"Deciding whether to single- or double-crop can often come down to the state of the transportation networks, farmers’ access to credit and even connections to the global supply chain," said Ray. "For many countries, increasing the frequency of crop harvests is not sustainable, given the potential degradation of soil and environmental conditions, but our research has shown that closing global harvest gaps could increase agricultural production by more than 44%, at least in the short run."

Ray and Foley, who reported the study in Environmental Research Letters (ERL), point out that their calculations are estimates and that further research is needed to estimate harvest gaps more precisely and explore whether changing harvest frequency is sustainable and appropriate in different countries.

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