AS THE SUMMER months keep getting extended and temperatures rising at an alarming rate, people are not the only ones affected. Based on 50 years of weather and rice-yield data from farms in the Philippines, it would appear that warming temperatures negatively affect rice yields.

Recent varieties of rice, bred for environmental stresses like heat, gave better yields than both traditional rice varieties and modern varieties of rice that were not specifically bred to withstand warmer temperatures. But the study found that warming adversely affected crop yields even for those varieties best suited to the heat. Overall, the advantage of varieties bred to withstand increased heat was too small to be statistically significant.

The study utilizing farm-level data of rice yields and area weather conditions in four-to-five-year increments over the 50-year period allowed the researchers to examine the relationship between rice yield and temperature in actual farm environments.

The rich data set allowed the scientists to see what was actually happening at the farm level, rather than only observing behavior at higher levels of aggregation like in provinces or districts. The study examined three general rice varieties planted during those 50 years – traditional rice varieties; “early modern varieties” planted after the onset of the Green Revolution, which were bred for higher yields; and recent modern varieties bred for particular characteristics, like heat or pest resistance, for example.

As expected, the study showed that in the presence of warming, recent modern varieties had the best yields when compared with the early modern and traditional varieties, and that early modern varieties outperformed traditional varieties. Interestingly, some of the early modern varieties may have also mitigated heat challenges given their smaller “semi-dwarf” plant architecture, even though they were not bred to specifically resist heat.

Taken all together, there are two main implications here. The first is that, at the farm level, there appears to be a “yield gap” between how rice performs in breeding trials and on farms, with farm performance of recent varieties bred to be more tolerant to environmental stresses not being statistically different relative to the older varieties.

The second is that rice breeding efforts may not have reached their full potential such that it may be possible to produce new varieties that will statistically perform better than older varieties in a farm setting.

This study has implications for other rice-breeding countries in Asia because the timing of the release of various rice varieties is somewhat similar to that of the Philippines. Plant-breeding institutions can learn from this type of analysis, too. It provides guidance as to where research funding may be allocated by policymakers to further improve the high temperature tolerance of rice varieties available to farmers. Further studies are needed on other agricultural practices and innovations that affect crop yields, including an examination of cover crops, or plants grown on cropland in the off-season that aim to keep soils healthy, to gauge whether they can mitigate the adverse impacts of a changing climate. By Manny Palomar, PhD (EV Mail May 22-28, 2023 issue)