Gut feeling or common sense
OPINION: Land use change is to the fore (again) because of headlines indicating the potential for growing rice, expansion of dairying in some regions, and ongoing concerns about carbon farming.
They appear under various names, including bioactives, biopesticides, biostimulants and biofertilisers. Bioactive is a catch-all term. Biopesticides are pest management products derived from natural materials such as animals, plants, microorganisms, and specific minerals. Biostimulants enhance plant processes and stress tolerance, without providing nutrients. Biofertilisers enrich the soil and supply nutrients to the plants through enhanced microbial activity.
The industry programme A Lighter Touch (ALT), involving partners HortNZ and MPI, with 13 product groups, and three research providers plus two chemical companies and a merchant company, has been active in investigating the potential for biopesticides in New Zealand. The programme website stresses that biopesticides are most effective as part of an integrated pest management programme. Google ‘a-lighter-touch. co.nz’ for the latest information.
In contrast, biofertilisers are usually promoted as replacing a certain amount of synthetic fertiliser. They act through mechanisms such as nitrogen fixation and phosphorus solubilisation, making it easier for plants to access these nutrients in the soil. Again, the concept is that the process is natural and will therefore be better.
Unlike for biopesticides, there has been no concerted approach for evaluation in New Zealand, but plenty has been done overseas by the companies that have developed individual products and by universities examining mode of action or comparing products.
The results overall showed a positive response in comparison with no fertiliser but tended to decrease in impact as fertiliser increased. At maximum yield, no effect of biofertiliser was apparent.
Researchers at North Dakota State University reported in 2023 that 59 of 61 site years of N rate trials with and without the use of biological N fixing products in maize, spring wheat, sugar beet and canola in 10 states within the North Central Region of the U.S. had no yield increase with use of the product over N rate alone. They suggested that “growers should be sceptical” and that the wise grower “needs to test products of interest on their own farm in a replicated manner and search for unbiased data on product performance before using them on whole fields”.
They explained that the energy required by asymbiotic organisms to fix N can come from various sources, such as soil organic matter, and intermediary compounds during the process of plant/animal decay, or the decay or other microorganisms. A rich source of energy also comes from root exudates of growing plants. If N-fixing organisms are active inside living plant tissue, the energy for N-fixation has to come from the plant itself.
This is the main lesson – the energy has to come from somewhere – and it is far easier for the plant to take up fertiliser nitrogen from the soil solution than to supply carbohydrate to bacteria to do the work. The free-living bacteria are already in the soil fixing N – adding more microorganisms will increase competition for the same carbon source.
Professor Derrick Moot, Lincoln University, says “There is no such thing as a Free Lunch”. He has estimated that for Lucerne to fix 150 kg of N requires the equivalent of 1.7 t DM from the host. This is a 28% unseen cost/penalty in yield.
Do the maths...
Science advances all the time, and it may be that a breakthrough is made in biological products in future. In the meantime, ALT suggests the questions to ask:
What is the active ingredient? If the biopesticide is described simply as a crude preparation (plant extract or mix of microbes) ask for more details about the active ingredient. If the biopesticide is based on a single micro-organism or pure compound, ask for specific identification. If the answers are vague – beware. Ask how the product works.
ALT emphasises that if something sounds too good to be true – it usually is. Biopesticides are not stand-alone solutions. They work best when incorporated into an IPM programme utilising a range of management measures.
The parallel statement about biofertilisers might be that they might have a place on low nutrient status soils in sensitive catchments where nutrient caps are in place… but that might not be where crops should be grown.
In all cases, the North Dakota State University advice is worth remembering – be sceptical.
Dr Jacqueline Rowarth, Adjunct Professor Lincoln University, is a farmerelected director of DairyNZ and Ravensdown. She is also a member of the Scientific Council of the World Farmers’ Organisation. This email address is being protected from spambots. You need JavaScript enabled to view it.
