WHITE CLOVER
(Trifolium Repens)
The recognition of the high forage quality and the nitrogen (N) fixation benefits of grass-white clover pastures has led to a resurgence of interest in its use as a means of reducing the economic and environmental costs in livestock agriculture.
The popularity of white clover fell out of use to some extent in the latter half of the 20th century due to availability of synthetic N fertiliser and broadleaf specific herbicides.
The key to white clover’s survival and productivity is its multi-branched creeping stem, called a stolon which provides sites for new leaves, roots and flowers. The stolon stores carbohydrates and proteins meaning that the plant can overwinter and regenerate in the spring.
GROWTH STAGES
There are 3 stages of white clover growth from germination to full establishment.
1. Rosette phase - reliant on a central taproot, few branches & small spread. (Plant size 10-20cm). Phase lasts approximately 3 months and does not fix N at this stage. Important to graze during this phase to promote growth.
2. Expansion phase - the plant is reliant on a central taproot, rapid expansion of up to 15 branches (25/30cm in size). Initial rooting is poor on the stolons and requires careful grazing to avoid damage. The taproots starts to die 12 months post sowing and only at 12-18 months does nitrogen fixation begin.
3. Clonal phase - no taproot and reliant on adventitious roots which form at the nodes of the stolons. The clover is actively fixing N. The stolons last for 12-18 months and new stolons are produced at the terminal bud which then become independent plants and this cycle continues each year.
BENEFITS
Benefits of white clover tend to occur from May onwards as the sward white clover content increases.
1. Increased herbage quality compared to grass only swards. The D Value is typically two to three points higher under grazing.
2. Increased DM intakes in summer and autumn.
Higher milk production and increased LW gain in beef and lamb systems.
3. Nitrogen fixation. Trial work (2013-2020) at Moorepark & Clonakilty in Cork has shown annual herbage production at 13.5T DM/ha with grass-white clover at 150kg N/ha, the same as grass only with 250kg N/ha but increased milk solids with the clover sward. Similarly lamb production at Athenry on a grass-white clover sward (12.9T DM/ha) with 90kg N/ha has outperformed grass only with 145kg N/ha (12.6T DM/ha) and lambs finished in 186 days against 200 days on the grass only.
NITROGEN FIXATION
The quantity of N fixed in a grass-white clover sward depends on some key factors:
1. Sward white clover content - N fixation increases as clover content increases up to an optimum level of about 30% of DM content of the total sward. At this level the white clover can fix up to 180kg N/ha/year. (The sward should appear to have 50/60% clover at its peak growth in August). Clover content is half the amount of what it looks like.
2. N fertiliser application rate - N fixation declines with increasing N application.
3. Soil temperature - N fixation increases as soil temperature increases.
4. Solar radiation (sunlight) – more sunlight, more N fixation.
SOIL FERTILITY & OPTIMUM GROWING CONDITIONS
Soil pH is critical for white clover development and a target level of 6.3 to 6.5 should be achieved at the outset of establishment. At these levels other nutrients become more available, particularly phosphate where soil index 3 is desirable (9.5-13.4mg/lt) and index 3 potassium (141 – 200mg/lt). Good levels of calcium and sulphur are desirable for rhizobia (N fixing bacteria) survival.
