Just as grass lawns regrow after they are mowed, rice fields can regrow after they are harvested. This second harvest – known as a ratoon crop – has traditionally yielded only a small fraction of the first harvest. But farmers in China are now benefitting from laboratory and field studies conducted by the Joint FAO/IAEA Division that used nuclear techniques to determine the best rice varieties and the best fertilizer regimes for increasing second harvest yields, often resulting in second harvests as large as the first – meaning the farmers who ratoon correctly are doubling their yields – and their income.
The word “ratoon” meaningfully can be traced to two Latin words: retonsus which means “to cut down” and retono which means “to thunder back”. It’s meaningful because when ratoon is used in relation to agriculture, it means both: a crop is “cut down” at harvest, but its roots are left behind and from that stubble, a second crop “thunders back”. Although, realistically, it’s only been recently – thanks to an increased focus on fertilizer management and plant breeding – that the second “ratoon” crop has returned with any kind of thunderous energy.
Not all high quality crops can regenerate. For example, maize cannot produce a second crop, but sugarcane, sorghum, pigeon pea and, importantly, rice can. The Joint FAO/ IAEA Division began working on improving the outcome of rice ratooning – also called “stubble cropping” – in China’s Fujian Province in 2012. This included studying fertilizer and water management practices for Jiafuzhan, an early maturing rice variety developed by Chinese plant mutation breeders.
In addition to working with China’s mutation breeders to develop and promote climate-resilient early-yield varieties, the Joint Division used nitrogen-15 stable isotope tracing to determine the optimum application rate for nitrogen fertilizers. Both the nitrogen-15 technology and the mutation induction for developing improved varieties were monitored and fine-tuned in the Joint Division’s Agriculture & Biotechnology Laboratories in Seibersdorf, Austria, and then applied in the farmers’ fields.
The nitrogen-15 technology was able to discern how well the main crop had absorbed the fertilizer, how much was left in the field after the first harvest, and how much more would be needed for the ratoon crop. A major goal is to avoid over-application, which, in addition to being an unnecessary expense, could have environmental implications if the fertilizer is converted as a greenhouse gas and emits into the atmosphere or is washed away and becomes a water pollutant. In this study, the optimum fertilizer application needed for their ratoon harvest was 150 kg of nitrogen per hectare.
Efficient and economic ratooning requires developing and adopting crop varieties with high ratooning capability plus following the fertilizer management practices prescribed by the study. Success depends on the two being combined. The study found that farmers who adopted the combination of improved varieties and fertilizer management saw yields almost double from 6.7 to 12.3 tonnes per hectare. The only expense they incurred for the second crop was buying the required amount of supplemental fertilizer and the only labour required was having to spread the fertilizer, no replanting was needed.
Once the results were known, other farmers were eager to adopt the improved varieties and fertilizer management practices pesticide herbicide, and now ratooning is underway on 42 000 ha. In addition to nearly doubling rice yields, the farmers in Fujian Province saw their profits increase by USD 3 260 per hectare, which, combined with a 30 percent decrease in the cost of fertilizer, has proven extremely beneficial to the province.
In the past, many upland farmers had ratooned their rice crops, based solely on their awareness that a second harvest was possible. But their second harvest was much smaller than the first, sometimes dropping from 3 tonnes per hectare to 0.5 tonnes per hectare. At that time, they were happy even with a small second harvest. But now they know that with proper management, their ratoon crop can be almost equal to their main crop.
The success in China has indicated that improving ratooning yields also has potential for other rice-producing countries in Asia. They know what they will need: a rice variety bred for high ratooning capabilities, and nitrogen management which uses tracing studies to determine the amount of fertilizer needed to support a high yielding second harvest.