WORDS AND IMAGES SUPPLIED BY ABIE HORROCKS, FOUNDATION FOR ARABLE RESEARCH
Ryegrass seed crops are commonly followed in the rotation by an autumn cereal such as wheat. However, this option is growing increasingly challenging in some areas due to herbicide resistance in ryegrass. An alternative approach is to maximise the benefits of the ryegrass seed crop by using the re-growth as the basis for a winter feed crop.
In 2022/23, FAR investigated the viability of sowing legumes into ryegrass re-growth to a) extend the value of the crop by providing high quality feed and b) reduce the N fertiliser spend for the next year (nitrogen fertiliser costs frequently make up more than half of farm expenses).
Methods
Two trials were used to assess drilling legumes into ryegrass re-growth.
1. The Chertsey Establishment Trial (CET) measured biomass and N supply from adding faba beans (Vicia faba L.) to ryegrass re-growth under different tillage and irrigation scenarios.
2. A trial at Kowhai farm compared establishment of small and large seeded legumes over-sown into ryegrass re-growth (biomass and quality were measured).
At the CET, harvest was completed by 2 February 2023 in the dryland ryegrass plots and by 12 February in the irrigated plots. Faba beans were direct drilled at 60 kg/ha on 3 March into ryegrass re-growth. The trial was grazed by autumn born R1 dairy calves for 4 days from 27 April.
At Kowhai, four legume species were over-sown into the ryegrass re-growth on 6 April 2023. Three of the varieties were small seeded (<100g per 1000 seeds); Berseem (Trifolium alexandrinum L.), Balansa (Trifolium michelianum) and Hairy vetch (Vicia villosa)) and one, Faba bean was large seeded (500–700 per 1000 seeds). They were sown at 7, 4, 40 and 40 kg/ha, respectively. Biomass assessments from each plot were carried out 16 August. The four legume plots were compared with controls of annual (cv. Tama) or perennial (cv. Base) ryegrass re-growth.
Results
There were no significant effects of tillage or irrigation on ryegrass seed yields (average 1660 kg/ha). Faba beans were direct drilled into the ryegrass re-growth during very dry conditions. This was noticeably problematic in the dryland replicates and no-till plots (desired seed depth was 5 cm but actual depth was variable, 1-3 cm).
Three months after grazing, there was significantly greater faba bean re-growth in the irrigated plots (48%) compared with the dryland plots (17%).
There was very little soil mineral N in the top 50 cm after the ryegrass harvest (7.4 kg/ ha; range=5–13 kg N/ha) and soil mineral N remained low three months after the grazing event, averaging 3.8 kg N/ha (range=3–6 kg N/ ha). Despite low soil mineral N, there was 39 kg N/ha in the above ground biomass pre-grazing, on average. Some of this would have been removed by grazing, some would have remained un-grazed, and some would have been returned via dung and urine. Three months after grazing there was on average 41 kg N/ha in the above ground biomass.
Given that no fertiliser N was applied, and that soil mineral N was low at drilling, this N came from further mineralisation and N fixation generated from the faba beans.
At the Kowhai trial site, the large seeded faba bean established well, but the small seed legumes (vetch, balansa and berseem) failed to establish. As the ryegrass at this site had re-grown, this lack of establishment is likely to have been caused by either reduced soil/seed contact (due to the difficulty in adequately drilling the smaller seeds into the soil) or the reduced ability of legume seedlings to compete with the already established ryegrass. Dry matter quantity and quality (e.g., crude protein) were greater for the faba bean/ ryegrass mix compared with the ryegrass control.
Conclusion
Over-sowing post-harvest ryegrass re-growth with a legume resulted in greater biomass and improved quality (compared with re-growth without a legume), however the timing of drilling and soil conditions at sowing are important.
Dry and firm soil conditions may affect success of over-drilling larger seeds, while smaller seeds may struggle with competition from the ryegrass regrowth. Further work on the impacts of drilling the smaller seeded legumes earlier is required to better understand these dynamics.
Residual soil mineral N was low after the ryegrass seed crop was harvested but continued mineralisation of N carried the grasses through, while the legumes were expected to fix most of the N they needed for growth. Little is known about the benefit of N fixation to the companion crop; this is another area that requires further work.
As the legumes decompose in the subsequent spring crop further N advantages are expected and soil testing (PMN) would be recommended to ensure this is capitalised on.
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