Grain crops that remain productive for two or more years without replanting
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.
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Cultivation often has a negative impact on provision of [ecosystem] services. For example, cultivated systems tend to use more water, increase water pollution and soil erosion, store less carbon, emit more greenhouse gases, and support significantly less habitat and biodiversity than the ecosystems they replace
 The Millennium Ecosystem Assessment
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:
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.
Accelerated domestication.
Accelerated domestication (also called "de novo" domestication) of perennial wild plants provides another avenue for perennial crop development. This approach involves selection of wild herbaceous perennials based on their domestication potential, followed by artificial selection for agronomically important traits like yield, seed shattering (the tendency of seeds to fall off the plant or stay attached until harvest), free-threshing seeds (the tendency of seeds to easily detach from the chaff) and plant height. Pipelines for domestication, like those developed by researchers at The Land Institute, have established criteria for evaluating the potential of candidate species to be successful for domestication programs—e.g. high variability and heritability of agronomically important traits—and also guide what traits should be focused on during breeding efforts. Extensive lists of potential candidate species can be found in Wagoner &amp; Schaeffer  and Cox et al.
Domesticating new perennial species has a couple of major drawbacks. For one, wild perennial grains have very low yearly yields compared to domesticated annuals so breeding efforts have to make up a lot of ground before perennial grains are commercially viable. This problem is exacerbated by the fact that many candidate species are polyploid (i.e. they have extra sets of genetic material). Polyploidy makes it harder to breed undesirable alleles out of the population and create uniform plants that grow and mature simultaneously for easy harvest.
Genetic methods.
Several genetic methods can help the perennial crop development process. Genomic selection, a method of predicting plant traits based on analysis of their genome, shows promise as a method to accelerate selection of plants in domestication programs. If adult plant phenotypes can be predicted from the genomes of young plants, plants can be artificially selected at an earlier age, reducing time and resources needed to identify individuals with desirable traits. Transgenics and gene altering can add or target “domestication genes” and their orthologs (genes with similar sequences and functions) in perennial plants. Domestication genes have known effects on traits that are relevant to domestication, and have been discovered in annual crop species. Genome sequencing indicates that many orthologs also exist in perennial species that may be useful targets for genetic alteration.
Current applications of genetic manipulation are limited because the genomes of many candidate species have not been sequenced. Furthermore, methods of genetic manipulation have not yet been optimized in most candidate species. Despite these limitations, there have been rapid gains in the development of genetic techniques and these methods are likely to be a useful aid for the development of perennial crops in years to come.
Advantages.
Several claims have been published: 
Perennial grains in the marketplace.
Kernza, an intermediate wheatgrass, has been under development for use as a grain crop since the 1980s. Since 2001, the nonprofit organization The Land Institute's Dr. Lee DeHaan has led development of the crop, coining the trademarked name Kernza in 2009.
Recently, work on Kernza has rapidly expanded to include more than 25 lead scientists in diverse fields working on three continents. This international team has developed growing techniques and dramatically improved traits such as shatter resistance, seed size and yield, enabling the crop to now be produced and marketed at a small scale. US Institutional Kernza research partners now include the University of Minnesota, the University of Wisconsin, Madison, Cornell University, Ohio State, Kansas State, and numerous international universities in Canada and Europe, including the University of Minnesota, Lund University, and ISARA.
As the first perennial grain crop grown across the northern United States, researchers hope that Kernza will help dramatically shift agriculture practice, making croplands multifunctional through the production of both food and ecosystem services.
The Land Institute developed the registered trademark for Kernza grain to help identify intermediate wheatgrass grain that is certified as a perennial using the most advanced types of "T. intermedium seed".
The cultivar of perennial rice 23 (PR23) is used for a new rice production system that is based on no-tillage.
References.
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