When to put urea on pasture: timing, rates and grazing

The short version

Urea only boosts a pasture that is already able to grow. Dairy Australia's best practice guide says to apply nitrogen only when the pasture is actively growing, soil moisture is adequate, rain is likely during regrowth and other nutrients are not holding it back.

• Spread it as soon after grazing as you can. Dairy Australia says you lose about 1% of the potential response for every day you wait.
• Do not spread on drought-stressed or waterlogged pasture (water running off the surface).
• Pick paddocks with good ground cover and plenty of ryegrass or kikuyu. Bare patches lose more N.
• Fix low phosphorus, potassium, sulphur or sour soil (low pH) first, or at the same time. A soil test tells you which.

Soil temperature and season

Dairy Australia says temperate grasses such as ryegrass generally respond when soil temperature at 10 cm is above 4°C, and subtropical grasses such as kikuyu and paspalum above 10°C. That is the average over the regrowth period, not just the day you spread.

• Winter: growth is slow, so the response is lower. NSW Local Land Services says nitrogen is best applied early in winter, while soil is still a little warmer.
• Spring: the fastest growth gives the best response.
• Autumn after a dry summer: soil N builds up with little growth to use it, so the response at the break can be poor. Dairy Australia suggests waiting until later in autumn, once deeper soil moisture is adequate and growth has resumed.
• Summer and autumn on dryland: only apply if the root zone is moist and more rain or irrigation is likely.

NSW Local Land Services uses a rough rule of about 10 kg of green dry matter (DM) per kg of N, so 60 kg/ha of urea grows roughly 300 kg DM/ha more feed. Treat all of these as guides, not promises.

How much per application

Dairy Australia says the most efficient response comes from 20–50 kg N/ha at one time. Urea is 46% N, so that is about 43–109 kg/ha of granular urea. NSW Local Land Services gives 20–60 kg N/ha (43–130 kg urea/ha), with 60–100 kg urea/ha common.

• Below 20 kg N/ha the response is unpredictable.
• Above 50 kg N/ha each extra kilogram grows less. Dairy Australia allows up to 60 kg N/ha for one grazing round on highly productive spring pasture.
• Leave at least 21 days between applications, and preferably 28 days at higher rates.
• Rule of thumb: rotation length in days × 1 to 1.75 kg N/ha per day. A 30-day rotation at 1 kg N/ha per day is 30 kg N/ha.

To work out cost per kg of N across products, see cost per kg of nitrogen. Browse all nitrogen fertilisers.

Rain, wet soil and nitrogen lost to air

Urea can lose N as ammonia gas (volatilisation). Dairy Australia says loss is highest in hot, dry, windy weather and in the first 48 hours after spreading.

• In the cooler, wetter months (May to November in southern Australia) urea does not need to be watered in if there is enough soil moisture, rain or irrigation.
• Dryland summer losses average around 10–15%. Spreading the day after rain in hot, dry, windy weather can push losses above 30%.
• Avoid warm (over 10°C) waterlogged soils, where N is lost as gas from wet soil (denitrification).
• On cold, wet soils use urea or ammonium-based products rather than nitrate-based ones.
• NSW Local Land Services also advises against spreading before heavy rain or on wet soils.

Dairy Australia says inhibitor-coated urea costs more per kg of N and seldom grows more pasture. It pays only if you cut the N rate by the loss you expect to save, mainly where high losses are likely, such as dryland pasture in autumn. Compare plain urea and an NBPT-treated urea, or see Green Urea NV. More detail is in how much N urea loses to the air.

Where sulphur is also low, sulphate of ammonia supplies both. Dairy Australia notes it is an expensive form of N and acidifies soil quickly with regular use. See urea vs ammonium sulphate.

Grazing after urea and nitrate poisoning

• For best use of the N, Dairy Australia says wait until ryegrass reaches the 2.5-leaf stage (or canopy closure), or 3 leaves for kikuyu and paspalum, before grazing.
• If nitrate poisoning is a concern, Dairy Australia says do not graze for 7–14 days after N, or while pasture is water-stressed.
• NSW Local Land Services says do not put stock on until 2 weeks after the first effective rain following application, and suggests locking paddocks up for about 3 weeks.
• Annual ryegrass and kikuyu can build up toxic nitrate at high N rates. Capeweed is another nitrate accumulator.
• Do not put hungry stock straight onto lush feed. Fill them with hay first and move them later in the day.
• In dry periods Dairy Australia allows spreading 2–3 days before grazing to cut ammonia loss, but stop cows eating lumps of fertiliser.