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Old premise, new research in plant density tolerance

Martin O. Bohn Published on 29 January 2015
Different corn hybrides

The world population reached seven billion people in 2011. By 2050, it is projected that a global population of nine billion people will need to be fed.

In addition, meat consumption in developing countries with a rapidly growing middle class (like India and China) is rising, resulting in a further increase in demand for corn grain as a major source of livestock feed.

Despite this growing demand for corn grain worldwide, the possibility of developing new agricultural land is limited.

Increasing grain demand combined with limited land availability suggests that production per unit of land has to grow. This can be accomplished by increasing the number of plants grown on a given unit of land (i.e., plant density) while maintaining “per-plant” yield.

Achieving yield increases by improving plant density tolerance has historically proven to be successful. While old hybrids and new hybrids yield similarly at low density, new hybrids have a distinct yield advantage at high plant densities.

This suggests an increase in plant density tolerance rather than in yield potential per plant as the main source of yield increase.

High plant density causes increased competition for light, water and nutrients. As plants are increasingly crowded, shading within the canopy occurs. In addition, the root systems of the plants are in closer proximity at higher plant densities, meaning roots from multiple plants are now competing for water and nutrients.

The ability to tolerate abiotic stresses is evidenced by the plant continuing to progress from vegetative to reproductive stages. Characteristics such as ear shoot development, synchronous silking and standability are vital to the goal of maintaining per-plant yield at high plant densities.

Due to the complex nature of yield and considering the importance of the various traits likely to impact yield under the high plant density stress (barrenness, rows per ear, tassel branch number, etc.), researchers at the department of crop sciences at the University of Illinois at Urbana-Champaign (UIUC) have taken a top-down approach to explore this complex trait, as there are many possible interactions between traits underpinning biomass production and grain yield.

These various traits that contribute to grain yield under stress will be simultaneously studied to determine relationships between the traits and the overall impact on plant productivity.

Recently, a research group (UIUC Department of Crop Sciences) evaluated the plant density response of 32 hybrids derived from crosses between 12 elite inbreds which represent key heterotic subgroups of the U.S. Corn Belt germplasm.

The plant density survey performed at six plant densities (19,000 to 54,000 plants per acre) found six inbreds that produced top-yielding hybrids at a high plant density. Even at the maximum planting density, these hybrids did not reach a yield plateau.

This initial survey also identified 20 plant characteristics either directly or indirectly associated with grain yield at increasing plant densities.

This study clearly demonstrated that high plant density tolerance levels vary significantly among inbreds and their hybrids. However, to develop high-plant-density-tolerant corn hybrids quickly, we need to gain a better understanding regarding plant characteristics which contribute to the “crowding tolerance” of corn, and where the genes involved in these characteristics are located in its genome.

Supported by a USDA-NIFA grant, we made the next step toward this goal and developed new experimental populations by crossing the six parental inbred lines, which consistently produced top-yielding hybrids under high plant densities.

These unique populations of 320 hybrids each allowed us to determine genetic regions involved in the inheritance of all agronomic and morphological traits previously shown to be components of high plant density tolerance.

Our analyses revealed 246 genetic regions of interest, and a genome-wide association study identified 11 single-nucleotide polymorphisms with significant trait associations.

Finally, we are in the position to study high plant density tolerance at the gene level to determine how these genes contribute to maintaining high yields at high plant densities. We envision that this new knowledge will help us to accurately predict hybrid performance under high plant densities and accelerate the development of superior corn hybrids.

In the center of the U.S. Corn Belt, we are mainly concerned with maximizing grain yield: How far can we increase plant density before current corn hybrids respond by reducing yields?

Similarly, how far can we increase plant density before income per acre drops, in spite of increased yields? We are less concerned about the effects of increased plant density on dry matter yield or seed quality.

Our studies showed that corn hybrids adapt to increasing plant densities by altering plant architectural traits (e.g., leaf angle, leaf number, stem diameter, yield components). This might indirectly affect total biomass yield and composition.

To date, we have not investigated how high plant density affects biomass production and quality. Based on reports in the scientific literature, the following general trends emerge:

1. Dry matter yields increase with plant densities, but climatic conditions as well as hybrid-specific agronomic and morphological characteristics moderate this trend.

2. High plant densities seem to alter biomass composition, resulting in reduced corn silage quality and milk yields. In support of these trends, the highest milk yields were found at 35,000 plants per acre, whereas dry matter yields peaked at about 40,000 plants per acre.

Due to the complex interactions between corn hybrids and management systems, which includes row spacing and plant density as well as type and rates of nitrogen application, most studies tested only a small number of hybrids.

Our investigations demonstrated that corn hybrids show a highly variable response to increasing plant densities. Given this vast crop diversity, I am optimistic that in the future corn hybrids will become available that combine high dry matter yields with a balanced quality to maximize milk yield.

Even though our high plant density research in corn does not focus on dry matter yield and quality for milk production, we recently started experiments to evaluate a set of our elite maize hybrids for grain, cob and stover yields under low (20,000 plants acre) and high (50,000 plants acre) plant densities and to determine their biochemical composition as well as their “theoretical ethanol potential” and actual total fermentability.

We are confident the results of this experiment will enable us to assess how plant management strategies (i.e., plant density) and the growing environment (i.e., precipitation, temperature, soil type, etc.) affect the performance of maize hybrids with regard to biomass yield, biomass composition and its fermentability to liquid biofuels.

The outcome of these experiments might also provide useful information to all breeding programs directly focusing on improving milk yield instead of liquid biofuels.  FG

References omitted due to space but are available upon request. Click here to email an editor.

320 corn hybrids developed at the University of Illinois were tested for their high plant density tolerance at the Crops Sciences Research and Education Center (Urbana, IL) in 2013. Photo provided by Martin Bohn.

Martin O. Bohn
  • Martin O. Bohn
  • Department of Crop Sciences
  • University of Illinois