Pasture: Use it or lose it
By Chris Glassey, DairyNZ farm systems specialist
DairyNZ farm systems specialist Chris Glassey explains how better managing post-residual grazing can increase grass growth rates and improve profit.
The saying ‘grass grows grass’ is often misinterpreted. It should be used only in the context of keeping average pasture cover above minimum levels across the whole farm from start of calving to balance date. That’s when growth rate is more closely related to the days between grazing, not the intensity of grazing events. ‘Grass grows grass’ does not mean leaving high grazing residuals to promote pasture growth.
In fact, leaving too much grass behind after grazing can be wasteful and reduce subsequent growth rates. That’s because ungrazed grass blades die before the next grazing. This means, in conservatively grazed pastures, net dry matter (DM) accumulation is reduced because the amount of pasture lost to decay is greater than the rate of new emerging growth.
The purpose of a spring rotation plan is to ensure that area allocation of pasture (rotation length) from start of calving is planned at a rate that’s not too fast or too slow for retaining pasture cover, pasture growth and pasture quality. This should result in sufficient grazing intensity to set target grazing residuals at 3.5 to 4.0 centimetres (1500 kilograms of DM per hectare – or kg DM/ha).
Consistency is important too: don’t force cows to graze lower than at the last grazing. They won’t want to, and eating old pasture results in lower quality feed and likely reduced production.
Residual grazing: research
In the late 1990s the effect of leaving higher post-grazing residuals in winter on subsequent pasture growth rates was tested.
- In early and late July, pastures were grazed for two, four, eight or 24 hours, then protected from further grazing using Post-grazing residuals ranged from 864kg DM/ha to 1773kg DM/ha. Regrowth from each was measured over the next 52 days.
- Even though the 24-hour areas were grazed much harder than the two-hour areas, their accumulated growth after 52 days was 38 percent The area with the lightestgrazing had the lowest regrowth, although it maintained the highest pasture cover at the next grazing.
More extensive research in 2006 identified that grazing intensity resulting in a post-grazing residual of 3.5 to 4.0 centimetres (compressed height):
- maintains sufficient plant energy reserves
- maximises pasture regrowth and quality
- encourages tiller initiation (through increased light penetration)
- minimises pasture
Find out more about using the Spring Rotation Planner at dairynz.co.nz/SRP
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