Breeding for yield potential and stability at CIMMYT

by Karim Ammar Principal Scientist, Durum Wheat Breeding

The International Maize and Wheat Improvement Center (CIMMYT) has been improving low-latitude spring durum wheat for more than 6 decades from its centralized Mexico-based program. Starting in the early 70s, it developed and globally distributed semi-dwarf, high yielding and widely-adapted germplasm, following in the footsteps of the “Green Revolution” bread wheats. Thanks to their architecture that was more suited to the intensifying and modernizing agricultural systems of the era, their earliness and resistance to major diseases (at the time), the CIMMYT semi-dwarfs durum, as well as those independently developed by Italian breeding programs, went on to revolutionize durum cultivation globally, effectively and irreversibly displacing old local landraces which lost their competitiveness in the continuously intensifying production systems.

CIMMYT-derived cultivars such as Mexicali 75, Yavaros 79 and Altar C84 were released in numerous countries besides Mexico, including in North Africa and Southern Europe, the Middle East and West Asia and the Indian Peninsula. They contributed to increasing national yields and enhancing food security in these regions. Post 80s, sustained efforts to increase yield potential and adaptation to a wide range of water availabilities contributed to the current dominating role of CIMMYT germplasm in the spring durum world. Among the cultivars released worldwide from 1994 to 2014, 70% were from crosses involving a parent from CIMMYT, with roughly half of those (35%) being direct releases. The direct releases during the same period is even higher when the data is segregated by regions such as North Africa and West Asia (51%), SubSaharan Africa (62%) and Latin America (68%). Today, CIMMYT is maintaining a breeding effort designed not only to sustain significant progress in yield potential and adaptation to a wide range of water availabilities and heat stresses, but also to protect yield gains from losses due to major diseases and genetically improve the industrial quality attributes of the durum grain to respond to global industry requirements. While we attempt to deliver these “full packages” of traits in our globally distributed germplasm, this article will focus exclusively on our strategy to increase yield potential and stability in low latitude spring durum wheat.

Grain yield is critically important in all durum wheat target environments and production systems. Genetically, it is a highly complex trait, controlled by hundreds of genetic factors, each contributing a small individual effect, to the trait. It is the accumulation of these individual effects and the interactions between them that determine the final genetic yield. Some of these factors may be always expressed, no matter the environment, while others may contribute only under very specific growing conditions, making final grain yield one of the most environment-influenced trait one can deal with.

Given the extreme diversity in growing conditions facing durum producers globally, the first step to any strategy to increase yield must be a careful characterization of the target environments in terms of the natural and production constraints under which yield is generally realized. Some 30 years ago, CIMMYT divided the low-latitude spring wheat world in 5 distinct, trans-border and often transcontinent, “Mega-Environments” (MEs), within which yield is achieved under the same or similar set of constraints and general conditions. More recently, the concept of MEs was refined into that of “Target Populations of Environments” (TPEs) which allows the same focused breeding approach. With regards to low-latitude spring durum wheat, the CIMMYT program addresses three MEs/TPEs, namely the fullyirrigated, high input, temperate environments (ME1/TPE1), the rainfed, drought-prone, temperate groups of environments (ME2+4/TPE2) and the irrigated but constantly heat-affected environments (ME5/TPE3). We have used this environmental characterization to design 3 distinct “breeding pipelines” to maximize opportunities for yield gains by focusing our crossing and selection on sub-traits needed specifically for each ME/TPE and matching our testing/evaluation conditions to those prevailing in each ME/TPE.

These are the generally fullyirrigated, dry desert environments with medium length cropping cycles, initiating with cool temperatures and heat stress settling at the end of the cycle, generally too late to affect yield potential. While relatively limited in area (1.3 million hectares worldwide), the high yield levels achieved in this MEs/TPEs produce quantities of grain that is important to food security of several countries, whether the grain is produced locally or imported from such environments. CIMMYT’s strategy to achieve yield gains in this ME/TPE is straightforward. Varieties or experimental lines identified as the most outstanding for yield potential under favorable conditions are intercrossed. Plants from the resulting populations are selected for visually detectable traits that are needed to realize high yield under intensive growing conditions, such as reduced height to avoid lodging, adequate earliness that takes most advantage of the growing season, outstanding spike fertility and grain fill. This process is repeated four times, using two full cycles per year, for a total of 2 years. By the end of the 3rd year after the cross is made, new experimental lines are genetically stable, enough to be evaluated in yield trials, usually for at least two seasons, before selecting those few that may reliably surpass previous yield potential levels and become candidates for global distribution. This last evaluation step is highly critical as it requires highly precise and reliable experimental conditions that minimize experimental confounding factors and uncontrolled variability in order to rapidly and reliably detect even small differences in yield potential. The CIMMYT experimental station of CENEB (Campo Experimental Norman E. Borlaug) near Cd. Obregon, Sonora, northeastern Mexico, is arguably one of the world’s best site to conduct such precision-requiring evaluations. Cradle of the “Green Revolution” wheats, and nowadays home of the largest CIMMYT field operation, this site sits in the middle of the Yaqui Valley irrigation district which provides water for one of the highest yielding durum wheat cultivation area in the world.

In 2022, the average yield of 219,000 hectares sown to this crop yielded an average of 7.4 tons/ha, with some of the best commercial farmers fields reaching the 10 tons/ha levels. More importantly for global relevance, based on data from our international yield trials conducted around the world, the performance of elite lines at the CENEB site is very well correlated with the performance of the same material tested in many key experimental sites in irrigated environments, from Egypt to India, Pakistan, Turkey and Iran. Simply put, the most outstanding varieties identified in Cd. Obregon are often also outstanding in other irrigated sites around the world.

The powerful predictive power and capacity to quickly identify even small increments in maximum yield potential makes the CENEB site a central part of CIMMYT’s strategy for improving yield in the most favorable environments of the world as well as in heat or drought stressed conditions as described below.

While one could argue that heat stress can affect yield almost everywhere spring durum is grown, it is important to differentiate between environments with occasional heat waves during the middle or end of the growing season from those where temperatures above the optimum for proper wheat growth and development occur consistently from the very beginning of the crop cycle and are sustained all the way to the end. This section refers to the latter ones, which require a very different/specific strategy for yield improvement. These extreme environments are limited to 0.6 Million hectares for durum wheat worldwide and are located primarily in Central and Peninsular India with a few areas in Sudan and Iran. CIMMYT’s breeding pipeline for increasing yield in this ME/TPE has depended on the evaluation of lines under continuous heat stress through a late-planting protocol implemented at the CENEB-Cd. Obregon site as a proxy for heatstressed environments. Lines are sown in early March instead of midNovember, when night minimum temperatures are always above 13 oC (typical of south Asian conditions) and maximum day temperatures are very often above 30-35 oC. These testing conditions expose plants to a continuous stress that first affects vegetative development and biomass accumulation, then pollen viability and spike fertility and ultimately grain size and physical quality. Lines from our program recently identified as outstanding under these experimental proxy conditions and those from our international trials evaluated in the actual heat-stress prone areas of Central and Peninsular India and found to be outstanding there, are inter-crossed annually. Plants from the resulting populations are selected for visually detectable traits, as described in the previous section, but with an added emphasis on earliness and grain size, two traits that are critical to achieving higher yields under excessive heat stress conditions. Resulting genetically stable lines are evaluated for two years in the latesown treatment before selecting for global distribution those few that may represent a significative progress in performance under heat stress. Thanks to this approach, over the last 10 years, progress in yield under heat stress has been continuous and the frequency of lines with outstanding performance under heat has increased significantly in our outgoing germplasm.

More than 75% of the durum wheat grown worldwide is produced in rainfed systems, with conditions in a single location varying in different years from high rainfall to extreme drought with every level of precipitation in between. These erratic and unpredictable water availability scenarios are typical around the Mediterranean Basin, where much of the world durum wheat is grown and consumed but are also prevailing in the rainfed areas of Ethiopia and parts of Latin America. This mega-environment

(ME2+4/TPE2) is by far the most significant one for global durum wheat production, covering at least 4.2 million hectares worldwide. Accordingly, the breeding pipeline addressing this ME/TPE is by far the most important one within CIMMYT’s durum breeding program.

To maximize yield under these drastically erratic and diverse conditions, the first yield facet to ensure is drought tolerance. Since it is situated in a desert with hardly any rainfall during the wheat growing season, the CIMMYT CENEB-Cd. Obregon site is ideal to evaluate yield under drought by applying restricted amount of water to simulate drought stress without the confounding factor of natural precipitation. A major investment has been made to equip more than 40 hectares of experimental land with a drip irrigation system that allows for the precise and highly uniform application of any amount of water, generating virtually any desired level of drought stress. This testing infrastructure is of utmost importance as data reliability always decreases with decreasing yield levels in any experimental trial. As important as this capacity to precisely and reliably identify drought tolerant germplasm is, it is only one part of the strategy

CIMMYT uses to address yield gains in this ME/TPE. These environments are also characterized by variably frequent but certainly occurring “good years” where water availability is not yield-limiting. Farmers take advantage of the “good” or “above average” years to accumulate income to enhance their survival capacity during the “bad” years.

But since it is impossible to predict ahead of sowing if the year is going to be bad, average or extremely dry, the only alternative for a farmer is to have a variety with the plasticity and the wide adaptation to significantly minimize yield reduction or the probability of total crop loss if the season turns out extremely dry while maximizing yield if the year turns out to be rather favorable. This combining, within a same line, of both high yield potential and drought tolerance has been, for the last 15 years, the cornerstone of CIMMYT breeding strategy to maximize yields in rainfed environments. Genetically, the factors or genes promoting high yield under favorable conditions and those enhancing drought tolerance are often different. Our approach to accumulate genetic factors increasing both facets of yield consists of testing all newly developed lines, from their first testing stage to the last, under full irrigation as described in section 5 and under drip-simulated drought as described above in this section, at the same time and with all other conditions being equal. Based on this systematic parallel evaluation in highly controlled setups, only those lines that are outstanding under both conditions are advanced to the next stage. With 2-3 years of such testing protocol, the finally selected lines are very likely to be capable of enhanced performance under a wide range of water availability, maximizing yield in favorable conditions while exhibiting an outstanding resilience under substantial drought stress. The success of this breeding pipeline is well validated by the frequent release by national programs and private entities of varieties that turn out to be outstanding either in typical rainfed drought-prone areas as well as in irrigated or high rainfall favorable ones.

Building upon its historical successes in raising the yield potential of durum wheat over a wide range of environments, CIMMYT’s strategy for enhancing yield gains globally relies on an extensive characterization of the main target environments and their classification in 3 main “Target Population of Environments” (TPEs) in which yield is achieved under similar natural and production conditions, regardless of countries and continent. For each of these TPEs, CIMMYT has implemented distinct breeding pipelines designed to maximize yield gains by focusing on the most important pathway to achieve yield under a set of conditions that are specific to each TPE. The choice of parental lines used in crosses, the emphases placed on different yield affecting traits during plant selection and finally the yield testing conditions focused on during the line evaluation phase are all determined based on the different TPE-based requirements for yield enhancement. A key element for the success of our strategy has been the availability and continuous improvement of the CENEB-Cd. Obregon experimental site, one that is ideal for high precision evaluation of maximum yield potential, the highly reliable estimation of drought tolerance through a uniform and precise dripsimulated water stress setup, and a useful proxy for selecting lines with good heat tolerance through a latesowing protocol. The strategy described herein, as well as other approaches implemented to accelerate yield gains (not discussed here), should provide CIMMYT’s durum breeding program with an ample opportunity to respond to the challenge of increasing future yields in all the environments we address, including those requiring resilience to drought or heat stresses.

Karim Ammar