Alfalfa Germplasm, Breeding, and Genomics
Alfalfa is widely
cultivated throughout the
From a research perspective, alfalfa is fairly easy to manipulate. It can be propagated vegetatively, crossing can be accomplished without too much difficulty, abundant wild germplasm is available for study, and a substantial amount of breeding has already been conducted. In addition, even though cultivated alfalfa is a tetrasomic tetraploid, diploid populations exist for genetic studies.
We are interested in exploring primarily three characteristics of alfalfa: YIELD, STABILITY, and PERSISTENCE. Here's why:
Yield is essential for inclusion of alfalfa in intensive farming systems. In the dairy industry, where much alfalfa is used, the alternative to alfalfa is corn silage. Though corn silage is good feed, harvesting of the plant removes all cover on the soil over winter, opening the path for extreme wind and water erosion. Thus, alfalfa must be high yielding, or it will be replaced in the cropping system, to the detriment of the environment. To improve yield, I believe that more emphasis needs to be given to heterosis and to breeding methods that allow its repeated capture and use. Most alfalfa breeding has been conducted by repeated rounds of recurrent selection within a broad-based germplasm. New germplasm is typically integrated into the bigger population during the breeding process, and little effort (actually, no effort) has been expended to maintain distinct germplasm sources. The same can be said for most other forage crops as well. As a result, breeders have little ability to realize yield gains from the expression of heterosis. I proposed a method of breeding that can capture partial heterotic yield gains, but for it to be successful, particular attention needs to be given to maintaining distinct germplasm pools--i.e., heterotic groups.
In addition to yield,
however, alfalfa must persist for multiple years. Major
determinants of persistence in
Finally, yield stability needs to be considered. Cotton, peanuts, corn, and soybeans are dependable crops: farmers can expect certain results from them. Alfalfa must be dependable as well, so that yields do not vary widely among years or over locations. Because alfalfa is often grown on more heterogeneous soils and in more diverse environments than the major row crops, and because alfalfa is often in intercrops with grasses, stability needs more attention than it is often given.
Other aspects of alfalfa improvement are also important, including disease resistance and forage nutritive quality. However, among the common perennial forage crops, alfalfa has sufficient quality for most situations. Some have even argued that its quality is too high, necessitating other forms of fiber to be included to balance the livestock ration. Recently, alfalfa has been promoted as a bioenergy crop, and modifying the cell wall to make it a better source of energy through burning, gasification, or fermentation may be possible. We're working on a project toward this end currently. Historically, much research on alfalfa improvement has focused on resistances, with diminished examination of yield per se. Nematode resistance, particularly on the south Georgia coastal plain is of major importance, however.
Yield stability may be improved by maintaining genetic diversity in alfalfa cultivars. Although the diversity-stability debate continues to rage in ecology, sufficient evidence is available to suggest that genetic diversity can contribute to stability of production. With the advent of transgenic technologies, and with the potential need to incorporate a few genes in commercial germplasm, a potential result could be the loss of diversity. Research into alternate heterotic groups could help forstall the diminuition of diversity, particularly if semi-hybrid cultivars are produced. In reality, very little research has been conducted on the diversity needed for stable alfalfa cultivars, and measures of diversity within and among various germplasms are poorly documented.
