New strains of clover developed by Teagasc are helping farmers improve yields, reduce costs and lower their environmental impact.
Research carried out by the State agency has shown that incorporating white clover into grazing swards provides additional nitrogen for herbage production and pastures receiving 150kg of nitrogen per hectare annually had similar yields to perennial ryegrass-only pastures receiving 250kg per annum.
The white clover also provides higher-quality forage leading to increased milk yields relative to grass-only swards. In addition, the incorporation of white clover into pastures has been identified as a greenhouse gas mitigation measure by Teagasc’s Mitigation Abatement Cost Curve (MACC) analysis. This makes the breeding of new clover strains important not only for farmers but also for the achievement of our national climate targets.
Teagasc has been breeding white clover at its Oak Park facility since the 1960s, according to Stephen Byrne, a research officer with the organisation’s crop science department.
“The programme started off with the evaluation of different strains,” says Byrne. “It went into the commercial development of new cultivars later. The first white clover cultivar was developed in 1983. Five new cultivars which have been commercialised in the last 10 years.”
Clovers have an important role to play in lowering the use of nitrogen fertilisers on grassland, he explains. “Clover is able to fix nitrogen due to the rhizobia bacteria in its roots,” Byrne adds.
In essence, this means that it takes nitrogen from the atmosphere and puts it into the soil making it available to the grasses growing there. “Clover can supply between 50kg and 200kg of nitrogen per hectare per year. That’s the advantage of getting white clover into grassland. We want to get one that contributes to the forage yield and persists throughout the year. And we want the minimum proportion of clover to grass delivering the maximum amount of nitrogen.”
That’s the goal, but attaining it isn’t easy. “Plant breeding is all about identifying the best plants to use as parents from the next generation,” Byrne explains. “We can identify those parents though field evaluation and we measure the performance of the next generation over a number of years.”
This is the typical breeding method for white clover. The genetic merit of parent candidates is identified through the performance of their progeny. The progeny plants are grown in field plots in combination with perennial ryegrass over at least three years following an establishment year under mechanical cutting and sheep grazing. Evaluation in swards is crucial for improving traits like forage yield and compatibility with perennial ryegrass. Once the best candidates are identified, plants are selected within these families as parents to produce an improved population. This process represents a single cycle of selection and takes at least seven years to complete.
However, that can now be shortened to just one year thanks to advances in genomics, a reduction in the cost of DNA sequencing and the availability of reference genomes for many plant species. The white clover genome sequence was published in 2019, for example. This makes it feasible to characterise genetic variation across large plant genomes.
Genomic selection is a form of DNA-assisted selection that uses genome-wide DNA profiles to calculate genomic estimated breeding values (GEBVs). Selection of plants to use as parents for the next generation are subsequently based on GEBVs, thereby avoiding the need to evaluate the progeny of these plants in field trials. The increased speed enables up to seven cycles of selection to be completed in the same time it takes to complete a single cycle with field evaluations.
“Genomic selection has a number of advantages as well as its speed,” says Byrne. “It improves the accuracy of the process and allows us to evaluate more plants. We can look at thousands of different plants quite easily. We are using this method to improve the accuracy and the rate of genetic gain. It is expected that we can more than double the rate of genetic gain for forage yield in white clover using the models developed at Oak Park. That means getting better varieties to the farmer. It will make it possible to develop varieties that will persist in the sward for three, four or five years and not just for a year or part of a year.”
And farmers interested in availing of the benefits of these new strains can be assured of their efficacy. “The Department of Agriculture puts candidate cultivars through trials in different areas of the country,” says Byrne. “Successful cultivars make it on to a recommended list and by using that list, farmers can be sure they are choosing cultivars which are suitable for Ireland.”