Date Published: October 17, 2018
Publisher: Public Library of Science
Author(s): Alexey Morgounov, Kai Sonder, Aygul Abugalieva, Vijai Bhadauria, Richard D. Cuthbert, Vladimir Shamanin, Yuriy Zelenskiy, Ronald M. DePauw, Aimin Zhang.
Wheat yield dynamic in Canada, USA, Russia and Kazakhstan from 1981 till 2015 was related to air temperature and precipitation during wheat season to evaluate the effects of climate change. The study used yield data from the provinces, states and regions and average yield from 19 spring wheat breeding/research sites. Both at production and research sites grain yield in Eurasia was two times lower compared to North America. The yearly variations in grain yield in North America and Eurasia did not correlate suggesting that higher yield in one region was normally associated with lower yield in another region. Minimum and maximum air temperature during the wheat growing season (April-August) had tendency to increase. While precipitation in April-August increased in North American sites from 289 mm in 1981–1990 to 338 mm in 2006–2015 it remained constant and low at Eurasian sites (230 and 238 mm, respectively). High temperature in June and July negatively affected grain yield in most of the sites at both continents. Climatic changes resulted in substantial changes in the dates of planting and harvesting normally leading to extension of growing season. Longer planting-harvesting period was positively associated with the grain yield for most of the locations. The climatic changes since 1981 and spring wheat responses suggest several implications for breeding. Gradual warming extends the wheat growing season and new varieties need to match this to utilize their potential. Higher rainfall during the wheat season, especially in North America, will require varieties with higher yield potential responding to moisture availability. June is a critical month for spring wheat in both regions due to the significant negative correlation of grain yield with maximum temperature and positive correlation with precipitation. Breeding for adaptation to higher temperatures during this period is an important strategy to increase yield.
Wheat and wheat products provide about 20% of protein and 20% of calories consumed per capita . Global wheat area was estimated to be 225 million ha and can be classified into several environments depending on the growth habit, temperature, and moisture availability: spring wheat irrigated/high rainfall, hot and humid; winter wheat irrigated/high rainfall, semi-arid and low rainfall winter and spring wheat . Low rainfall spring wheat is largely produced in high latitude regions of Canada, USA, Kazakhstan, and Russia (above 45° north) with a continental climate. It is a short season crop, growing for approximately 100 days from May to September. Yields are relatively low (1.5–3.0 t/ha) due to the limited growing season, moisture availability, and the impact of abiotic and biotic stresses. Nevertheless, this production environment plays an important role in global food security and wheat prices since most of the grain is traded. The World Wheat Book provides detailed description of spring wheat production and breeding systems in Canada , USA , European Russia , North Kazakhstan and Siberia . Spring wheat production systems of North America and Eurasia are defined by their ecology, climate, technologies, crop varieties, and marketing systems. The hard red spring wheat region in Canada and the USA is situated at slightly lower latitudes than the Eurasian regions of western Siberia and northern Kazakhstan. The North American region is also slightly warmer and receives higher precipitation. Spring wheat varieties in Canada are developed, grown, and traded in eight market classes . These market classes are segregated based on gluten strength, protein content, kernel hardness, and seed color to fit end-user specifications, domestically and internationally. The market classes for spring wheat in Eurasia are very similar to North America and utilize the same criteria. However, production is mainly limited to hard red spring bread wheat. An important difference in the two production systems is a much wider application of zero tillage and conservation agriculture in North America compared to Eurasia. During 1975–2007, the share of wheat in the crop rotation declined while the share of legumes and oil crops increased substantially, and summer fallow acres declined in North America . Zero tillage is gaining popularity in Eurasia where on-farm trials have revealed that it can be a successful dryland cropping method under climate change .
We selected 19 spring wheat breeding locations across Canada, USA, Kazakhstan, and Russia (Table 1; Fig 1) that represent the diversity of high latitude spring wheat production environments in North America and Eurasia. For all 19 sites, three important weather parameters–monthly average daily minimum temperature (Tmin), monthly average daily maximum temperature (Tmax), and average monthly precipitation–were extracted from the CRU TS3.1 dataset (http://www.cru.uea.ac.uk/) developed by the University of East Anglia  from 1981–2015 and used to calculate seasonal and yearly averages. Weather data used in the study are not the actual physical data but approximations based on recordings from nearby weather stations. Evidence of climate change was evaluated for each site by comparing the mean weather parameters values for 1981–1990 versus 2006–2015, and by calculating coefficients of determination (r2) of Tmin and Tmax on years during 1981–2015. We assumed that significance of r2 indicated significance of weather change over time and thus, evidence for changing climate. The direction of the change was estimated by calculating the regression slope. A similar approach was previously used by Morgounov et al.  who analyzed climate change at 35 global winter wheat sites.
This study evaluated spring wheat production area across North America and Eurasia totaling more than 22 million ha (Table 1) and contributing up to 10% of global wheat production. These regions are extremely important for food security because most of the wheat produced is then traded. In 2017, wheat yields in North America were 2–3 times higher than in Eurasia, despite record overall wheat production in Russia. There are several reasons for this difference including environment, overall economic situation, production technologies, and input applications, as well as climate change. Table 2 compares average wheat yields from 1981–1990 and 2006–2015 for both on-station trials and the regional/provincial average. On-station yields in North America demonstrated good progress, increasing by 6.9–69.7% across sites. Farmers production yield in 2006–2015 compared to 1981–1990 demonstrated high yield gain across provinces and states varying from 31 to 65%. Significant yield gain in 35 years was also reflected by significant r2 of production yield on years. The yield gap between farmers production and research stations yields narrowed from 1981–90 to 2006–2015 for all the states and provinces of N. America indicating that farmers yield grew faster compared to stations. In Eurasia, by contrast, on-station yields increased by a maximum of 23% at individual sites, and even decreased at two sites. No significant yield gain in production fields was recorded with exception of Saratov. The yield gap in the recent time even widened compared to the 1980s. Overall, both on-farm and on-station historical yield performance in Eurasia was much poorer compared to N. America.
This study analyzed data from 1981 to 2015 to assess the global impacts of climate change. However, comparison of two decades (1981–1990 versus 2006–2015) may be affected by specific factors influencing wheat production. In North America, the 1980s witnessed below long-term precipitation, while 2010–2015 had above average precipitation. Precipitation increases in the 2000s may therefore indicate cyclical change rather than long-term climate change. In Eurasia, wheat production and agriculture underwent the introduction of intensive technologies in the 1980s. The collapse of the Soviet Union and economic crisis that followed greatly affected farming, which experienced a sharp decline and did not recover until the mid-2000s. Other drivers such as wheat prices, biotic and abiotic challenges, government policies, etc. may have also affected wheat production. However, comparing these two decades is still relevant as it considers two different climate scenarios. It is also reinforced by the regression analysis of 35 years of data for key weather and agronomic parameters.