Urea as a Foliar Spray to Kikuyu and Ryegrass Trial Findings

A five-year local trial focussing on the benefits of applying urea as a foliar spray to kikuyu and ryegrass pastures compared to granular applications was completed in early 2020.  The trial partners, North Coast Local Land Services, Far North Coast Dairy Industry group, and Norco Milk are thankful to the Wollongbar Agricultural Institute and dairy farms in the Lismore region for hosting sites for the trial.

The trials aimed to gather local data to verify similar research conducted in southern Australia and New Zealand.  This previous research indicated that foliar applications of urea were more efficient than typical granular applications. Improved efficiency may be due to urea being absorbed directly into the leaf rather than plant roots via the soil.

One study in New Zealand claimed that the application rate of urea could be reduced by 60% using foliar applications compared to that of a granular application. This would mean significant saving to farmers and the environment.

The final report on the North Coast trial has been submitted to the CSIRO Journal of crop & pasture science. Findings will also be made available in a detailed fact sheet in the coming months.

In the meantime, a summary of the findings can be found below.

Experiment 1:

A replicated grazing study on a site with high soil Nitrogen levels (75 mg NO₃⁻/kg).  Urea was applied as a foliar spray at 40kg/ha per month, and the granular plots received 100kg/ha per month in attempt to replicate the claims that foliar applications could be reduced by 60% of the granular.

Findings:

• Yes, it worked, the pasture yield response to a foliar spray rate 40% of the granular rate was not significantly different, meaning it was pretty much the same. However, the soil nitrogen levels fell from 75mg NO₃⁻/kg to 23mg NO₃⁻/kg under the foliar treatment and only fell to 60 mg NO₃⁻/kg for the granular treatment.
• This means that the inadequate nitrogen application in the foliar treatments forced the plants to draw down heavily on the available nitrogen in the soil. This was confirmed by leaf analysis which showed the same leaf nitrogen levels between the foliar and granular plots. This was consistent in the ryegrass and kikuyu phase.

Experiment 2:

A replicated plot cut study on a site with a low plant-available soil N (8.7 mg NO₃⁻/kg) this was to look at what would happen when the soil has virtually no plant-available nitrogen for the plants to draw upon. Urea was applied at the same rates as in experiment 1. We also had additional treatments of foliar applications at the same rates of urea but included a wetting agent to determine if it would assist in improving absorption.

Findings:

• The yield of all the foliar treatments was well below the granular application. There was insufficient nitrogen applied via a foliar application, and as there was virtually no plant-available soil nitrogen for the plants to draw upon it meant that the plant could not compensate in terms of yield response.
• The nitrogen use efficiency (kilograms of extra feed produced per kilogram of nitrogen applied) and associated cost of the extra feed produced per kilogram of urea applied were similar between foliar and granular applications. Meaning there was no yield increase or financial saving as a result of the foliar treatments on both kikuyu and ryegrass.
• About 75% of the foliar-applied urea was absorbed within 7 hours by both kikuyu and ryegrass. The addition of a wetting agent did further increase the rate of absorption. This was important to understand in deciding when to spray in relation to forecast rainfall.

Conclusion:

The lack of benefit of applying urea as a foliar spray in a subtropical environment is, in part, related to the low tiller density of subtropical pastures compared to perennial ryegrass pastures in more temperate areas, this in turn is related to the amount of leaf available to catch the foliar spray in the period after grazing.

In this project, the ryegrass tiller density was 560 tillers/sqm. In NZ tiller density varies from 2,500 to 5,500 tillers/sqm and in Tasmania from 4000 to 7200 tillers/sqm. The reason for this is that diploid perennial ryegrass, which is commonly used in temperate dairy areas, have higher tiller densities than short-rotation tetraploid ryegrass.

On the north coast, ryegrass is over sown into a base pasture of Kikuyu, as such, any urea not captured by the new regrowth leaves which are the most efficient absorbers of Urea, is caught by the dormant kikuyu plants which are unable to respond during winter.

For further information to fully explain the context around these summarised findings please contact Nathan Jennings SLSO Agricultural Advice 0437 083 147 or submit an online enquiry form Contact Us requesting a copy of the factsheet.