Agriculture and climate change: Breeding for the Ides of March

But cut to the second half and we had near-heat wave conditions, with maximum temperatures in and around Delhi touching 40 degrees Celsius towards the month-end, levels last recorded in 1945.

Such extreme oscillations within a single month – beyond what even the Soothsayer in Julius Caesar would have imagined – has obvious implications for wheat, the country’s second biggest crop after rice.

Farmers sowed a record 31.56 million hectares (mh) under wheat this time, up from 29.25 mh in 2015-16 and the normal area of 30.17 mh. That was largely courtesy a good monsoon, which helped recharge the soil moisture and aquifer levels. The crop’s overall growth — from germination, seedling development and tillering (production of multiple side stems from the initial parent shoot), to earhead emergence, flowering and early grain development — was also excellent, till the spike in temperatures from around March 21.

For the wheat sown by November 15,

the main grain-filling or ‘dough’ stage happens roughly after 120 days. During this phase from mid-March that can extend to 15 days — when starch matter from the leaves, stems and earhead gets transported and accumulates in the kernels — day temperatures should ideally be in the early-thirty degrees range. Once that’s over, the normal heat build-up from April dries up the moisture, making the grain hard and ripe for harvesting.

But the mercury’s sudden and sharp rise during the critical grain-filling period this time — day maximums in Delhi surged from 24 degrees to 37.4 degrees between Holi and March 23 — has raised concerns over the size and weight of the kernels that would be harvested towards mid-April. The impact may not be much on the wheat that was planted by November 15, as temperatures really soared only after March 21 — by which time a significant part of the ‘dough’ stage would already have passed. This may not hold, however, for the wheat that was sown, say, in December and where grain-filling wouldn’t even have commenced when the heat wave struck.

Either way, one thing is clear: Given the increasing unpredictability of March temperatures and the transition to summer happening sooner than later — call it the effect of climate change — the future for wheat in India lies in advancing plantings to late-October or even earlier. But that’s easier said than done, for three reasons.

First, wheat sowing takes place only after harvesting of paddy in early to mid-October. In between, there is a period of field preparation that involves at least a couple of disc harrow, cultivator and planking operations. Further, the field has to be irrigated once and left for about 10 days, so that the seed bed has optimal moisture to allow sowing. All this makes October planting rather difficult.

Second, even if the farmer manages to sow in October, the germination and crop stand establishment tends to be poor, as the relatively high temperatures during this time causes moisture depletion in the upper soil layer where the seed is planted. “Germination can happen even at 35 degrees. The problem isn’t high temperature per se, but lack of moisture for the seeds to germinate,” explains Rajbir Yadav, principal scientist at the Indian Agricultural Research Institute’s (IARI) Division of Genetics.

Third, the wheat sown in October has a tendency to “come to heading” — i.e. for the earheads to emerge fully from the tillers — much earlier than desired. This transition from the vegetative to the next reproductive phase — when flowering and pollination starts — may happen in 55-65 days, as opposed to 80-90 days if sowing had been in mid-November. Early flowering, which cuts short the period of tillering and vegetative growth ultimately affecting yields, has to again do with high temperatures. These average over 25 degrees in October, while being only 20 degrees or so in mid-November, rendering it the most favoured time for planting wheat.

IARI scientists led by Yadav have bred a wheat variety HDCSW-18, which has a mild “vernalisation requirement” or the need for a certain minimum period of low winter temperature for initiation of flowering. This variety, simply put, does not come to heading without its basic vernalisation requirement being met during December and early January. So, whether sown in October or November, flowering — transfer of pollen from the male anther to the female stigma part of the same flower — will not take place before 85-90 days and the plant’s vegetative/tillering growth isn’t, therefore, compromised.

The major gain, though, is in the longer window now available for grain-filling. If sowing is on October 20 and the earhead emergence is completed by January 20, followed by five days of flowering/pollination and another 25 days of early kernel formation (‘milk’ stage), the ‘dough’ process can commence from around February 20 itself, as opposed to mid-March at present. It, then, considerable reduces the risk of terminal heat stress that results in shrivelled grains. “HDCSW-18 basically has a Vrn allele (variant of a gene) conferring partial vernalisation requirement. This allele is already there in PBW-343 (a once popular wheat variety), which we have used as a parent,” notes Yadav.

But how will the issue of topsoil moisture loss, hampering germination and stand establishment, be addressed in the case of October planting?

“HDCSW-18 has been specifically bred for conservation agriculture. If farmers sow wheat using Happy Seeder (a tractor-operated zero-tillage machine, which allows the seeds to be directly drilled in the fields even with the standing stubble or loose straw from the paddy that is combine-harvested), they will save field preparation time. Moreover, the crop residues on the field will provide a mulching cover, helping not only to moderate the soil temperature, but also retain moisture. Since there is no moisture loss resulting from both higher temperature and loosening of soil due to repeated tilling, you can comfortable plant in October,” claims Yadav.

IARI has already supplied the seeds of HDCSW-18 to select farmers for planting in the current rabi season. “Most of them are getting over 70 quintals per hectare and have, in fact, harvested their crop by March-end without being affected by the heat wave,” he adds.