Breeding effects on corn hybrids

Background

Corn hybrids are the result of crossbreeding two genetically distinct inbred lines, combining desirable traits from both parent lines to produce a hybrid that exhibits improved performance, such as higher yields, better stress tolerance, and enhanced disease resistance. This process, known as hybridization, has been a cornerstone of modern agriculture, allowing farmers to grow crops that are more productive and resilient.

Pioneer-brand corn hybrids have been developed and sold to farmers in the central-US Corn-belt continuously since 1926. Over nearly 100 years, thousands of Pioneer hybrids have been released for commercial sale, many of which have proven to be popular among customers.

Interested in understanding the basis of improvements in their commercial products, Pioneer maintains seed stocks of all commercial parental inbred lines and re-creates several of these historically important hybrids for annual field testing and evaluation.

Commonly referred to as ERA hybrids, key representative hybrids from each decade (1930’s to current time) are evaluated in field trials to explore various hypotheses regarding yield improvement and product performance. The first Pioneer ERA studies were made public in the 1970’s (Duvick, 1977, Maydica 22:187-196).

ERA hybrids have been used to test a wide array of hypotheses, including: (i) maize grain yield over time and estimates of overall yield gain, genetic gain, and agronomic gain, (ii) changes in stress tolerance, such as high plant density stress, water deficits, and nitrogen deficiency, (iii) anatomical and morphological changes, (iv) physiological processes, such as grain filling duration, radiation use efficiency, harvest index, nitrogen use efficiency, nitrogen uptake, plant biomass partitioning and metabolism, (v) genetic changes, and (vi) integration of crop models and genomic selection to improve the efficiency of breeder selection. These results have been summarized in different articles (Duvick et al., 2004, Plant Breeding Rev. 24:109–151; Messina et al., 2024, J. Exp. Bot. 74:4847-4861).

Collectively, the vast array of work with ERA hybrids has identified many direct and indirect effects of breeder selection for improved grain yield. These studies have provided insights into potential mechanisms of yield improvements in corn germplasm adapted to the central US Corn-belt. ERA studies have demonstrated significant changes in crop morphology and development, such as decreased tassel size, increased leaf angle, increased root branching, shortened anthesis-to-silking interval (ASI), and increased stay-green (Campos et al., 2004, Field Crops Res. 90:19-34). Experiments evaluating ERA hybrids have identified improved density tolerance, and general stress tolerance overall, as a primary agronomic mechanism to achieve higher corn yields.  Modern corn hybrids have higher rates of nitrogen uptake and nitrogen use efficiency, driven primarily by late season increased nitrogen uptake (DeBruin et al., 2016, Crop Sci. 57:1431-1446; Ciampitti et al., 2012, Field Crops Res. 133:48-67). Many of these results are consistently demonstrated in various sources of germplasm adapted to the central US Corn-belt and in germplasm adapted to corn-growing regions in Argentina, Brazil, Canada, and China (Tollenaar et al., 1999, Crop Sci. 39:1597-1604; Luque et al., 2004, Field Crops Res. 95:383–397; Borras et al., 2018, J. Exp. Bot. 69:3235-3243; Liu et al., Field Crops Res. 302:109065) 

Despite these advances, we still face many gaps in our understanding of the long-term impact of breeder selection on maize yield improvement. We are thus seeking new ideas/hypotheses that could be tested with these historical products from perspectives that have not been considered before.