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Potential to enhance N use efficiency with ProGibb SG

Published on 08/05/2021

It’s a question that Nufarm technical specialist Cynthia Christie has been asked more than a few times lately, as the countdown tightens on implementation of government’s Essential Fresh-water reforms.

From July, the amount of synthetic N applied to pastoral land cannot exceed 190 kg per ha per year. On some farms, in some parts of the country, that cap is significantly lower than what is currently being used. So where exactly might ProGibb SG fit in this context? 

Christie qualifies her answer by first pointing out there is no research or data on ProGibb’s potential use as a tool to reduce N applications. Nor is there likely to be any undertaken, given the cost and complexity of exploring such a hypothesis. “But it is a good question. It would be a good project for a PhD student!”

It’s also very important farmers understand that ProGibb should not be regarded as a substitute for N fertiliser: “We’ve seen what happens in this scenario. It doesn’t work.” In the absence of any new data about ProGibb’s possible role as a way to reduce existing N applications, however, she refers to what is known and has been repeatedly demonstrated with regard to this particular gibberellic acid formulation. The first point is that its ability to increase pasture production by 30-50 per cent more DM per ha in just three weeks does not hinge on extra N fertiliser. “Provided existing soil nutrient levels are sufficient to support plant growth, we’ve shown you don’t need to apply more N to achieve this extra growth response.

There is an additive response when ProGibb SG is applied with liquid N, but the two responses are separate, she says. The second point that has been clearly defined is that the herbage nutrient profile of ProGibb treated grass is very similar to, or the same as, that of non-treated herbage. “So we know that the extra pasture growth stimulated by ProGibb utilises available soil nutrients the same way ‘normal’ pasture growth does."

Put those two points together,and there is potential opportunity for ProGibb to enhance N use efficiency for existing soil N in pasture, without sacrificing DM yield. The principle here is akin to that which applies to catch crops, as studied by the Forages for Reduced Nitrate Leaching (FRNL) project, i.e. the faster and more actively a plant grows, the more soil N it absorbs to support that growth.

Catch crops are fast-growing species sown to utilise what otherwise might be surplus soil N remaining after a previous crop has been grazed, reducing the amount of soil N available for leaching. “If you’re growing 30-50 percent more DM per ha, and the nutrient levels present in that extra DM have not been diluted by the increased daily growth rate, the logical assumption is that you are utilising existing soil nutrients more efficiently,” Christie says. The proviso is that soils in ProGibb-treated pastures are not nutrient deficient in the first place, particularly for N, but this can easily be determined through soil or herbage testing, she points out.

And that in itself is not a bad thing.“Many arable farmers test their nutrient status frequently throughout the season, to make sure the fertiliser they apply is appropriate to crop demand. I’m not sure how many pastoral farmers would do the same. But the more informed farmers are about the N status of their pastures, across the whole year, the better they will understand the N balance on their farms.”

Using ProGibb to accelerate DM growth in the autumn has always been a useful, cost-effective tactic for dairy farmers in particular, to help build their feed wedge ahead of winter and support either extended lactation or improved cow body condition score. But autumn is also a time when excess soil N is not wanted. Normal pasture plant growth rates slow with cooler temperatures and shorter days, and the risk of N leaching increases. 

Christie says perhaps the best way to regard ProGibb in this newly N-conscious context is to concentrate on what it can do, and how in turn this might be helpful for farmers who are worried about loss of productivity (and income) under the pending N cap. “There is an alternative to N fertiliser for promoting grass growth. It can certainly support pasture yield on their farms.”

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