Using Nitrogen to Boost Pasture Growth: When, Why and How?
Nitrogen fertiliser can be a powerful tool for boosting pasture growth, particularly during periods of feed shortage. While rising fertiliser costs have sharpened the focus on value for money, nitrogen often remains cheaper than buying and feeding out hay or grain—provided it is used at the right time and managed well.
Why Nitrogen Works
Nitrogen plays a critical role in pasture growth. It increases photosynthesis, allowing plants to produce more carbohydrates for both shoot and root growth. It also stimulates grasses to produce new tillers. Since tillers naturally die off over time, ongoing tiller replacement is essential to maintain—or even improve—pasture density and long‑term productivity.
Although fertiliser is important, most nitrogen (and other nutrients) actually comes from the breakdown of soil organic matter such as old roots and pasture litter. This process, known as mineralisation, is fastest when soils are warm and moist—typically after the autumn break and during spring.
Clover and Nitrogen Supply
Clovers meet their own nitrogen needs through nitrogen fixation. Rhizobia bacteria in root nodules convert atmospheric nitrogen into forms the plant can use. However, for annual clovers, nitrogen is only released to the soil once plants die off in spring and their roots and shoots decompose. As a result, grass pastures can still be nitrogen‑deficient in winter, even with 30–40% clover content.
When Pastures Respond to Nitrogen
Nitrogen responses are strongest when soil nitrogen supply is low or cannot keep up with plant demand. This commonly occurs in winter and early spring. After the autumn growth flush, available soil nitrogen is often depleted, and cold temperatures slow mineralisation. As temperatures rise from mid‑August, plant growth accelerates and nitrogen demand increases faster than soil supply.
Urine patches are a useful visual indicator of nitrogen responsiveness. These patches grow more vigorously due to their higher nutrient levels, although potassium can also contribute, so soil tests remain important.
Rates, Timing and Management
To capture the full response, nitrogen should be applied soon after grazing, as the first two weeks post‑grazing are when plants draw most heavily on nitrogen. Adequate spelling is essential, particularly in winter, where cold temperatures slow growth and spells of five weeks or more may be required.
A practical rule of thumb is:
- Winter: Apply ~1 kg N/ha per day until grazing (minimum 25 kg N/ha)
- Spring: Apply ~2 kg N/ha per day, with a maximum of 60 kg N/ha per application.
| Season | Additional kg DM grown for every kilogram of applied N | Typical response times |
| Winter | 5-10 | 35 – 90 days |
| Spring | 15-20 | 14 – 28 days |
Typical responses are around 5–10 kg of dry matter per kg of nitrogen in winter, and 15–20 kg DM per kg in spring. Annual ryegrass shows the strongest response, followed by perennial ryegrass, phalaris and tall fescue, with grass‑dominant pastures responding more than clover‑dominant ones.
Minimising Losses and Improving Value
Urea is the most common nitrogen fertiliser, but is prone to volatilisation losses when applied to moist soils and partially dissolves but not fully when temperatures reach 18 degrees. Ideally in spring, apply before 5–10 mm of rainfall to wash nitrogen into the soil within two days. Liquid nitrogen products can reduce losses in warmer conditions and work well when applied alongside gibberellic acid.
Gibberellic acid can be a cost‑effective option for growing winter feed more quickly, although responses are limited if nitrogen is deficient. Used together, nitrogen and gibberellic acid can have additive effects on pasture growth.
Final thoughts
Nitrogen is a valuable tactical tool, but recurring winter feed shortages are best addressed through a broader strategy that balances feed supply and demand. Effective nitrogen decisions come down to four key questions: can you use the extra feed, will the pasture respond, are conditions favourable, and is it cheaper than buying and feeding out supplements or hay?
