Improving yield and persistence of alfalfa cultivars in low pH and high aluminum soils of the southeast

Summary

Development of alfalfa cultivars tolerant to low pH and high aluminum soils

Situation

An increasing number of bermudagrass growers across the southeast have been incorporating alfalfa into suppressed or closely clipped bermudagrass pastures. Most of these growers were attracted to alfalfa because they have seen enormous improvement in the quality of their hay (90-110 RFQ for bermudagrass hay vs. 170-200 for alfalfa-bermudagrass mixture), and they were able to save on the application of nitrogen fertilizer. They harvest nearly pure alfalfa forage early in spring and later in fall, and a grass-legume mixture in summer. Studies evaluating the sustainability of the production of alfalfa interseeded in bermudagrass showed that alfalfa competes well with bermudagrass, even in drought conditions. The major challenges to alfalfa production in the Southeast are low pH soils and aluminum (Al) toxicity. The prevailing soils in the region are the highly weathered Ultisols, spreading across the entire region. These soils tend to have low pH and are, therefore, poorly suited to alfalfa production without lime and fertilizer amendments. One of our breeding goals is to develop varieties that will tolerate low pH soils and reduce the likelihood of Al toxicity. Low soil pH reduces the availability of macro- and micronutrients, reduces beneficial microbial activity in the rhizosphere, and enhances the solubility of phytotoxic metals such as Al, manganese (Mn), and iron (Fe). Plants readily absorb soluble Al, which in high amounts will inhibit root growth and development, making the root system inefficient for water and nutrient uptake. Alfalfa requires a soil pH between 6.5 and 7.5 for optimal production. Productivity and persistence of alfalfa are affected by Al toxicity in acid soils because of the reduction in root growth and inhibition of nitrogen fixation. Optimum growth and production of alfalfa on the acid soils of the Southeast is contingent on the addition of lime. The addition of lime raises the establishment costs and also does not reduce soil acidity in the subsoil, which can substantially lower alfalfa yields as alfalfa has the potential to develop a deep root system in unrestricted soils, which benefits the plant in times of drought. However, the deep roots will eventually encounter acid soil zones even when the upper soil layers are correctly limed. Therefore, the most effective option to improve alfalfa production is the development of varieties with improved tolerance to Al in acid soils.

Response

Despite the efforts made over the past decades to understand the mechanisms and genetic factors underlying low pH and Al stress in alfalfa, nothing has materialized into varieties with acceptable yield under field conditions. The lack of progress is most likely due to the limitations of the procedures used to identify germplasm with tolerance to Al and low pH. Previous attempts to evaluate Al tolerance in alfalfa were based on greenhouse or lab conditions rather than field conditions and real production environments. Because of the limited success of greenhouse and lab screening approaches in identifying alfalfa germplasm with acceptable forage yield under field conditions, we shifted our focus in the forage breeding program at the University of Georgia (UGA) to direct field evaluation and selection in low pH and high Al soils. We initiated in 2016 the evaluation of 1200 alfalfa accessions from the U.S. National Plant Germplasm System (NPGS) collection at the Iron Horse Farm, in a soil with a pH of 4.9 and aluminum content of 178 pounds per acre. The average annual rainfall in the region is around 50 inches. The seed was directly planted in 5-foot rows and the accessions were evaluated for three years. In fall 2020, we selected based on phenotypic data a panel of 427 entries that were replicated by cuttings for further field evaluation in low pH and genome-wide association studies. From a previous evaluation of 1570 accessions in low pH soil at Tifton, a second cycle of recurrent selection for yield was done at the UGA J. Phil Campbell Research and Education Center near Watkinsville in a normal pH soil. The top 116 entries that exceeded the checks in biomass (100 to 137 percent of yield) were selected and crossed in bee cages in the greenhouse. Replicated half-sib progeny rows were planted at the JPC in the spring of 2020 with three replications at a pH of 5.1 and two replications at a pH of 6.8 for performance comparison. The top entries will be seed increased and evaluated on a large scale and for persistence under grazing for release of elite cultivars adapted to low pH and high Al environments. This panel is being used as a graduate student research to conduct a genome-wide association analysis to identify chromosome regions associated with low pH and AL tolerance and develop genomic resources for marker-assisted selection of these two traits in alfalfa germplasm, in addition to training future alfalfa breeders.

Impact

The successful completion of this project will enable expanding the adoption of alfalfa in Georgia and the Southeast. With the shrinking acreage of alfalfa in California and the western US due to competition from almond hulls and water shortages, the southeast offers the biggest opportunity for expanding alfalfa acreage nationally, whether as a pure crop or as a companion in bermudagrass pastures and hayfields. The development of grazing-tolerant varieties adapted to the Southeast was another factor raising interest in growing alfalfa on the Coastal Plains. Alfalfa offers producers a significant profit potential through selling hay, or by grazing or conserving the surplus as haylage. Improving alfalfa cultivars for tolerance to low pH soils and aluminum toxicity would have a great impact on the sustainability of livestock operations in the Southeast. The high rainfall and mild winters in these environments would allow growing non-dormant alfalfa year-round in addition to savings on nitrogen application when alfalfa is interseeded in Bermudagrass pastures. Based on the bermudagrass acreage in the region, there is a potential of over 20 million acres for the production of alfalfa as a companion crop across the Southeast. It is our ultimate goal to make low soil pH tolerant alfalfa varieties a reality for growers in the future.

State Issue

Plant Production

Details

• Year: 2020
• Geographic Scope: Multi-State/Regional
• County: Oconee
• Location: J. Phil Campbell Sr. Research and Education Center, Watkinsville
• Program Areas:
  • Agriculture & Natural Resources