Phenology, Growth, and Yield of Three Wheat Varieties Under Late-Sown Terminal Heat Stress Conditions in Bangladesh and Implications for Variety Selection

In the rice-wheat farming systems of South Asia, the calendar is a matter of survival. For millions of smallholder farmers, the wheat crop depends entirely on the conclusion of the rice harvest. When the rice is ready, the fields are cleared, and the wheat is sown. However, climate change and shifting monsoon patterns have begun to delay the rice harvest, pushing the planting of wheat later and later into the winter. This delay creates a dangerous mismatch: the wheat is forced to grow during the coldest part of the year and then mature during the hottest. This specific type of pressure, known as terminal heat stress, occurs when high temperatures strike just as the grain is filling out. It is a silent thief of harvest, shortening the time the plant has to grow and robbing the developing seeds of the energy they need to become heavy and nutritious. Understanding how different wheat varieties cope with this squeeze is critical, because as the window for planting narrows, farmers need to know which seeds will still produce a meal on their tables.

Researchers in northern Bangladesh recently set out to test this reality in the field. They focused on three elite wheat varieties that are widely used in the region: BARI Gom 21, BARI Gom 26, and BARI Gom 27. Two of these, BARI Gom 26 and BARI Gom 27, were officially released with the label of being moderately tolerant to heat, while BARI Gom 21 is an older, well-established variety. The scientists wanted to see what would happen if these seeds were planted at the perfect time versus what would happen if they were planted late, under the severe heat stress that many farmers now face. To do this, they planted the three varieties at two different times: one group on November 25, which is the ideal window, and another group on January 5, a date so late that the crop would be forced to mature during the peak of the summer heat.

The results of this experiment were stark and immediate. When the wheat was planted late, the entire life of the plant was compressed. The time from planting to harvest, which usually takes about 112 to 115 days, was slashed to just 80 to 83 days. The plants were rushed by the heat, forced to mature before they had fully grown. This acceleration hurt every aspect of the crop. The number of plants that successfully established themselves dropped, the leaves became smaller and fewer, and the total amount of green material the plants produced was reduced by more than half. The plants themselves were shorter, having been denied the time to stretch their stems fully. In the late-sown plots, the wheat was essentially a shadow of its potential, struggling to survive a season that had been turned upside down by the delay.

The impact on the final harvest was even more severe. Every single measure of the crop's success declined. The number of grain-bearing heads per square meter fell, and the number of grains on each head dropped significantly. But the most dramatic damage occurred in the weight of the individual grains. The thousand-grain weight, a measure of how heavy and full the seeds are, collapsed. For one variety, BARI Gom 27, the weight of the grain dropped by nearly half compared to the early-planted crop. This happened because the high temperatures during the grain-filling stage disrupted the plant's ability to pack starch into the seeds, leaving them light and shriveled. Consequently, the total grain yield per hectare plummeted. Across all three varieties, the late-sown crops produced roughly 62 to 66 percent less grain than those planted on time.

Despite these universal losses, the researchers found distinct differences in how the varieties responded, which offers a clear path forward for farmers. The variety BARI Gom 21, the older of the group, proved to be the most reliable producer. Even under the severe stress of the late planting, it delivered the highest absolute amount of grain, producing 1,742 kilograms per hectare. While it suffered a significant percentage drop from its own potential, it simply started from a higher baseline and maintained a steady performance. In contrast, the variety BARI Gom 26, which was officially labeled as heat-tolerant, showed a different kind of strength. It did not produce the highest total weight, but it was the most resilient relative to its own potential. It lost the smallest percentage of its yield and maintained the most stable grain weight under stress. This suggests that while BARI Gom 26 is biologically better at handling the heat shock, BARI Gom 21 is the better choice for a farmer who needs the maximum amount of food from a difficult season.

The study also challenged a long-held assumption about the third variety, BARI Gom 27. Despite being officially designated as moderately heat-tolerant, it performed the worst under these extreme late-sowing conditions. It suffered the most severe drop in grain weight and the highest level of susceptibility to the heat. The researchers concluded that its official label of tolerance does not hold up when the planting is delayed as late as January 5. This variety, they found, is not suitable for the most extreme late-sowing scenarios that are becoming common in Bangladesh.

The findings of this study provide a clear, evidence-based guide for the future of wheat farming in the region. For farmers facing the reality of delayed rice harvests, the recommendation is to plant BARI Gom 21, as it offers the best chance of securing a substantial harvest even when the season is cut short. For breeding programs aiming to develop even more resilient seeds, BARI Gom 26 offers a valuable genetic template for stability under stress. However, the study also delivers a sobering reminder: no variety can fully compensate for a planting date that is too late. Even the most heat-tolerant seeds suffered massive losses when sown in January. The ultimate solution, the researchers suggest, lies not just in better seeds, but in fixing the root cause: adopting farming practices that allow the rice to be harvested earlier, so that wheat can still be planted in the cool, optimal window of November and December. Without this systemic change, the gap between the rice harvest and the wheat planting will continue to widen, leaving farmers with fewer options and smaller harvests.