A perennial grain is a grain crop that lives and remains productive for two or more years, rather than growing for only one season before harvest, like most grains and annual crops. While many fruit, nut and forage crops are long-lived perennial plants, all major grain crops presently used in large-scale agriculture are annuals or short-lived perennials grown as annuals. Scientists from several nations have argued that perennial versions of today's grain crops could be developed and that these perennial grains could make grain agriculture more sustainable.
Rationale
The 2005 Synthesis Report of the United Nations' Millennium Ecosystem Assessment program labeled agriculture the "largest threat to biodiversity and ecosystem function of any single human activity." Perennial grains could reduce this threat, according to the following logic:
Most agricultural land is devoted to the production of grain crops: cereal, oilseed, and legume crops occupy 75% of US and 69% of global croplands. These grains include such cereal crops as wheat, rice, and maize; together they provide over 70% of human food calories. All these grain crops are currently annual plants which are generally planted into cultivated soil. Frequent cultivation puts soil at risk of loss and degradation. This "central dilemma" of agriculture in which current food production undermines the potential for future food production could be escaped by developing perennial grain crops that do not require tilling the soil each year. No-till technology enables short-lived (annual) crops to be grown with less intense tillage, but perennial plants provide the most protection for the soil.
Crop development
The current agricultural system is predominantly composed of herbaceous annuals. Annual systems depend heavily on tilling and chemical applications, like pesticides and fertilizers, and thus contribute to sustainability issues like erosion, eutrophication and fossil fuel use. The development of perennial grains could improve the sustainability of agriculture. In contrast to annual systems, perennial systems involve plants with deep, long lived roots. A perennial system is not dependent on tilling and could reduce the dependence on chemical applications, build soil health, and sequester carbon. Annual crops have been domesticated for nearly 10,000 years, whereas no commercial perennial grains have yet been developed. It is unclear exactly why perennial grains were not domesticated alongside annual grains during the agricultural revolution. Annuals may have been more predisposed to domestication for several reasons. For one, wild annuals were likely easier targets for early domestication efforts because they generally have greater single-year yields than wild perennials. Because the fitness of annual plants depends on the reproductive output of a single year, annuals naturally invest heavily in seed production (typically the product of interest for agriculture). In contrast, perennials have to balance seed production with overwinter survival in any given year and thus tend to produce lower yields per year. Second, annual plants have a shorter generation time, facilitating faster gains through the artificial selection process. Third, early agriculture used tilling to clear fields for the following year's crop and the practice of annual tilling, which clears the soil of existing plants in preparation for new ones, is not compatible with perennial grains. Finally, once annual grains were domesticated there was a reduced incentive to pursue the domestication of new perennial grains. If the limitations of early domestication efforts explain the lack of perennial grains, there may not be an insurmountable physiological barrier to high yielding perennials. For instance, the trade off between survival and yield in perennials should primarily be observed in the plant's first year when they are establishing root structures. In subsequent years, perennials may actually benefit from having a longer growing season and greater access to soil resources due to pre-established root systems ( which can also reduce reliance on fertilizer). However, even if physiological limitations limit resource allocation to seed production in perennials, their yields may still be comparable to or exceed annual grain yields due to improved resource acquisition and higher overall biomass. While perennial crop domestication could alleviate some of the sustainability issues caused by reliance on annual crops, the gains may still be fundamentally limited by general agricultural practices. Producing grain on scales large enough to meet the world demand depends on the conversion of massive tracts of native grassland to agriculture, regardless of the perennial or annual nature of the crop.
Methods To capitalize on the potential benefits of perennial crop domestication, the domestication process needs to be accelerated. Serious efforts to develop new perennial grains began in the 1980s, largely driven by Wes Jackson and The Land Institute in Salina, Kansas. Approaches to perennial crop development generally fall under three main methods: perennialization, de novo domestication, and genetic manipulation. These methods are not mutually exclusive, can be used in tandem and each present their own challenges.
Perennialization Hybridizing existing annual crops with perennial wild relative is a common approach to perennial crop development. This approach aims to conserve the important agronomic traits that have been developed in annual grain crops while converting the plant to a perennial life cycle with well-developed long-lived root systems. However, perennialization is not without challenges. For one, plants produced through hybridization are often infertile so successful breeding of plants beyond the F1, or initial hybrid generation is rare. Second, perennial traits are often polygenic (controlled by multiple genes) so conferral of a perennial lifecycle to domesticated annual crops depends on a full suite of genes being transferred to the hybrid offspring from the perennial parent. In contrast, yield traits are generally less polygenic so single genes can have positive effects on yield. Thus, perennial crop development through hybridization may be more effective if the goal of hybridization is to introduce increased yield to perennials rather than introducing perenniality to annual crops.
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