Applying fertiliser to paddocks

Follow cows with the fertiliser spreader to avoid luxury uptake of nitrogen in the grass plant being grazed. Blanket spreading on the grazing platform risks cows eating very high levels of nitrates, warns grazing consultant Andre van Barneveld.

Andre explains that as nitrate nitrogen travels via water, plant nutrient uptake leads to a nitrogen spike in the grass leaf 4–7 days post application. “This has an animal impact as it costs the cow energy to process and urinate this excess nitrogen. It also has an environmental impact because the concentration of the discharged nitrate in the urine patches contributes to pollution. And, if that nitrogen is in the cow, you don’t get a response in the plant, so it’s uneconomic.”

For greatest nitrogen efficiency, therefore, Andre says fertiliser should be spread post grazing – ideally as cows leave a paddock and walk to the parlour. However, being practical, the basic rule of thumb is to stay 14 days ahead of the cows. “This means on a 21-day rotation, you spread one-third of the farm every week, if you have a large enough area. This gives the grass chance to use up the nitrogen and grow more leaf before cows graze the paddock again.”

He also reminds grazing managers that the goal is to balance grass growth with demand and predict shortages, applying fertiliser to boost growth rates. “When grass is in its vegetative stage, use 1.0 kgN/ha/day, increasing this slightly to 1.5 kgN/ha/day when in reproductive phase.

“Don’t be tempted to skip a round of fertiliser in June, if it starts to dry up, because a plant that is used to chemical fertiliser gets stressed when in the aggressive reproductive phase and tries even harder to go to seed. If there is also moisture stress, the whole farm could go to seed.”

Find out how to determine the nutrient requirements of grass and forage crops