Novel strategies for genetic improvement of disease resistance in perennial ryegrass

Perennial ryegrass (PRG) provides the majority of forage for livestock in the UK. Drechslera andersonii or net blotch is a serious fungal disease of PRG, and the UK is at high or medium risk. Yield effects can be over 1t DM per hectare in the first cut (British Society of Plant Breeders - Recommended List). Few current diploid varieties have good resistance, and the UK bred high sugar grass (HSG) varieties show poor resistance. Fungicide use is not a viable option. Reduction in the use of HSG varieties will lead to decreasing sales and increase in imported varieties with higher resistance, but without the high sugar content, which improves ruminant nutrition efficiency. The project brings together the only UK owned forage grass and legume seed production and wholesale company (Germinal Holdings Ltd), and the major forage plant breeding and genetics research organisation within the UK (IBERS, Aberystwyth University) in a consortium that has the skills and commercial experience to deliver new net blotch resistant PRG varieties with HSG characteristics. IBERS, Aberystwyth University will breed net blotch resistant HSG varieties using a novel backcross strategy aided by genomic selection. This will increase the speed with which disease resistant hybrids recover previous values of agronomic and HSG traits. This has not been done before in a ryegrass breeding programme. New net blotch resistant HSG ryegrass varieties will help GHL maintain and increase market share in the UK and abroad. We have identified ecotypes with high net blotch resistance. They will initially be crossed with AberWolf, a high performing HSG varieties with low net blotch resistance. The F1 progeny will be assessed phenotypically for disease resistance, and genotyped with a 4000 molecular marker SNP chip. This allows us to identify quantitative trait loci (QTLs) for net blotch resistance. Secondly, prediction models have been developed to derive genomic estimated breeding values for agronomic and forage quality traits. This will enable us to select the best candidates for disease resistance as well as HSG traits. A polycross of the F1 plants will be performed to guarantee segregation of the disease resistance traits. By year 5 three further rounds of backcrossing with the recurrent HSG parent will have been completed. Seed will be set aside from year 3 for small scale field trials in Devon for the remaining two years. These field trials will also include a wide range of susceptible and resistant material with high or low water soluble carbohydrate (WSC) content for comparison and as controls. These rigorous trials will ensure that by the end of the project, we will have populations with much higher net blotch resistance in addition to the beneficial properties of AberHSG varieties in terms of yield, WSC and digestibility. A range of activities will be undertaken to publicise this work, and thus facilitate fast on-farm uptake of these new varieties. This includes communication with seed distributors, advisors and farmers. The technology will be demonstrated at various livestock shows, including demonstration plots, where possible. GHL will also produce easy to follow management guidelines for farmers.

Perennial ryegrass (PRG) provides the majority of forage for livestock in the UK, and delivers commercial returns to Germinal Holdings (GHL) through seed sales worth £8250K. Drechslera andersonii or net blotch is a serious fungal disease of PRG, and the UK is at high or medium risk. Resulting yield reductions can be very significant. Few current diploid varieties have good resistance, and the UK bred high sugar grass (HSG) varieties show poor resistance. Fungicide use is not a viable option. Reduction in the use of HSG varieties will lead to decreasing sales and increased imported varieties with higher resistance, but without the high sugar content, which improves ruminant nutrition efficiency (Lee et al. 2001, Anim. Res. 50, 441-449). A 20% reduction in HSG seed sales represents 400 t (£1000K), and has serious commercial implications for GHL. IBERS, Aberystwyth University will breed net blotch resistant HSG varieties using a novel backcross strategy aided by genomic selection. This will increase the speed with which disease resistant hybrids recover previous values of agronomic and HSG traits. New net blotch resistant HSG ryegrass varieties will maintain and increase GHL sales.

The livestock farmers will benefit from the new net blotch resistant HSG varieties by providing a high yielding high quality forage without the yield reductions resulting from net blotch attacks. The HSG varieties have also been shown to improve the efficiency of utilisation of plant protein by the ruminant animals.

The novel genomic selection-aided backcross approach described in the project will benefit the plant breeders and the wider scientific community by providing sufficient phenotypic and genotypic data to enable accurate mapping of the quantitative trait loci for net blotch pathogen resistance, and thus facilitate the elucidation of its genetic basis. Depending upon its genetic architecture it may also provide molecular markers with large effect, which could be used in future marker assisted selection programmes. The use of genetic selection to help providing faster genetic gain from backcross breeding will also benefit the scientific community by providing proof of principle of this approach, and a potential blueprint for applying this technology in other forage crop breeding programmes.

The development of new perennial ryegrass varieties in the UK will not only enable Germinal Holdings Ltd (GHL), who is the commercial partner in this project, to maintain or increase its share of the UK ryegrass seed market, but it will also benefit the UK balance of payments. Without the new UK developed varieties, a 20% (1000 t) increase in ryegrass seed imports, would represent a negative impact worth £2.5M for the UK. Conversely, a 5% increase in market share for GHL, brought about by the development of resistant Aber high sugar grasses (HSG) varieties, would mean an extra 500 t of seed worth about £500K/annum for GHL.

There is also a potentially positive impact on the environment from increased use of HSG varieties. Studies have shown that lambs fed on HSG produce 20% less methane than lambs fed on a control variety of forage grass, possibly due to their high energy content. In three zero-grazing trials involving early, mid and late lactation animals the amount of feed nitrogen lost in the urine was reduced by up to 24% from animals fed the HSG variety.