A broad approach to environment
OPINION: As an on-farm judge for the Ballance Farm Environment Awards for many years, I’ve witnessed first-hand how dramatically New Zealand agriculture and horticulture has transformed over the past three decades.
Across New Zealand, we apply more than two million tonnes of nutrients each year, vital to enable our food and fibre exports. It's simply not a cost line farmers and growers can afford to get wrong.
One question we don't always ask is, is it landing in the best way to maximise return? When it comes to nutrients, that could be the difference between investing well and potentially leaving money sitting in the paddock. If the bout width is too wide for the product and spreader combination, you create stripes, where some areas are underfed and others overfed. The paddock might look green overall, but the performance underneath tells a different story.
Variation can lead to poor yield
Research shows that as spreading variability increases, yield losses climb rapidly. One of the most comprehensive multi-year studies published about this topic was in 1999 by the Grassland Association and it showed at a coefficient of variation (CV) of 30-40%, nitrogren applied to ryegrass seed crops resulted in significant production losses.
In pasture systems, these effects compound over time. With phosphate and sulphur, uneven spreading repeated year after year led to mounting economic losses by year three, particularly in dairy and sheep and beef systems. In other words, stripes might not hurt much in year one, but they'll soon add up.
Let's put that into farm terms
At a CV of 10-15%, losses are small. Push that out to 30-40% and the lost dry matter and the revenue attached to it grows sharply. For the Waikato dairy pastures featured in the study, uneven P and S applications at a 40% CV resulted in more than $110/ha in annual loss by year five, which would be a lot higher today if we account for inflation and milk prices.
That shows bout width, product quality and calibration need to all interact together to work, and if one of those elements are off, accuracy and profitability suffer.
Why bout width is the lever you control
A fertiliser granule placed into a spreader behaves like a projectile. If you’re using larger particle size or a higher bulk density, it throws further. If you’re using finer less dense particles, they won’t throw as far. Well granulated compounds typically achieve wider effective bout widths compared with dusty or blended products.
Think of it like using a golf club to hit a golf ball versus a ping pong ball. The club can have the same swing, but vastly different outcomes of where the ball lands because of the density of the ball.
A blend won’t behave the same as a compound, or a lime compared with a granulated fertiliser. Even changes in dustiness or granule hardness affect spread pattern.
Yet too often bout widths stay fixed out of habit.
Pasture responses to nitrogen are relatively linear, meaning some unevenness can self-compensate in total dry matter. You might see stripes, but total yield loss may appear modest in the short term.
However, phosphorus and sulphur are different. When soil fertility is marginal, underfed strips underperform year after year. Overfed strips can’t compensate fully because plants can only use so much at once. And the result? Cumulative losses.
Calibration an investment, not a cost
Spreader calibration costs are modest, often measured in cents per hectare, and correct calibration can even increase effective bout width, reduce paddock time and improve spreading efficiency.
We should also acknowledge the skill of spreaders who work with different products, landscapes and conditions every day, developing a practical understanding of how fertiliser behaves.
Warwick Catto, is science strategy manager Ballance Agri-Nutrients
