High pasture quality requires high nitrogen

This applies to native grass, tropical grasses or temperate species like phalaris, cocksfoot, ryegrass or fescue.

After breaking rains following a long dry period for much of the state this past November, it showed how important high soil fertility, especially nitrogen, has been for pasture recovery.

Native and tropical grass growth and quality reflected soil fertility, as well as other factors, such as previous grazing management.

Tamworth NSW Department of Primary Industries research has shown just how important high soil nitrogen is for grass pasture recovery following good rains after an extended dry period.

Premier digit grass for example tested 17.5 per cent protein where soil nitrogen fertility was high.

In contrast, low soil nitrogen grass protein, where nitrogen deficiency was not corrected, tested less than 9.1pc.

Similar digestible energy differences occurred.

From a productivity perspective, high soil nitrogen treatments grew at over 100 kilograms per hectare dry matter per day.

Meanwhile, low nitrogen treatments grew at 35kg/ha day.

These differences in quality and quantity are enormous.

Not only do animals put on more weight, and faster, at the higher quality, high nitrogen treatments, but far more stock can be carried and far more opportunities occur for hay or silage making.

It is sound logic to aim to supply as much nitrogen as possible for grasses from the legume component of the pasture.

For example, in native grass, tropicals or temperate perennials, this can be done via annual legumes such as sub clover, medics, biserrula, rose clover serradella or similar species.

Pasture's legumes on average can build soil nitrogen by around 20kg/ha for each 1t/ha dry matter growth.

Legume growth within a pasture can range from 10t/ha to almost nothing, depending on the season and a host off management factors.

On average for many areas, 3t/ha legume dry matter per season is feasible.

That's around 60kg/ha nitrogen - generally sufficient for many pasture grass components.

That is provided suitable rhizobia have been introduced with the legume and plants produce healthy active nodules.

Past research has shown many pastures have poor rhizobia levels growing with their legumes. Clearly an important aspect to check on.

Even with good, well-balanced grass legume pastures, there is a role to supplement nitrogen supply with fertiliser applications like urea.

For example, many areas experience poor legume growth in the three-year drought from 2017-2019.

When the drought broke in January-February 2020, many grass pastures were low in available nitrogen because of three years of prior drought-induced poor legume growth.

It is difficult to assess nitrogen requirements of a pasture as much of the nitrogen is at various levels of unavailability, but much of this can become available during the months following a seasonal break.

Nitrogen soil tests are useful but a degrees of estimation is generally helpful.

In our own properties, we added urea at 100kg/ha to half our tropical grass area - four paddocks - just prior to what we hoped was the first breaking rains in early November.

We just scraped through with 5 millimetres of rain three days after application which is generally considered sufficient for nitrogen incorporation on a lighter soil.

Later rains consolidated the expected response.

Our assessment was that areas selected for top-dressing were more likely nitrogen low.

For example, one paddock was a new stand following cropping.

Another two paddocks had high grass levels going into autumn 2022, leading to substandard 2022 legume season; two years of poor legumes.

A fourth paddock had a previous history of ordinary legumes while all paddocks responded positively quality and quantity wise.

Next week: Fertiliser carryover from dry to the next year

Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact 0428 752 149.