Dairy cows grazing (c) Tim Scrivener

The cold spring has left grass growth two to three weeks behind where it should be for the time of year according to Ian Matts, one of Yara’s technical agronomists.

Because of this, Mr Matts fully expects grazed and cut grass yields to be low for the season and says grassland management decisions over the month will be critical to maximising cuts of silage.

“Growth stages and tillering are behind where they should be which will impact both silage quantity and quality,” he suggests. “Physiologically day length will have induced floral development, so the heading date will hinge on growth rates from now on,” he says. “Unless cool conditions prevail, heading dates are likely to be within seven days of normal.”

The early nitrogen rates already applied will have been made based on an assumed number of days of uptake and growth which are very unlikely to have happened.

For example, applications applied based on 50 days of growth requiring 2.5kg N/day = 125 kg/ha of N, if growth has been limited to 35 days, then uptake will have been delayed and only 87.5 kg/ha of N will have been used leaving a surplus carry over of 37.5 kg/ha of N.

“If you assume that each kg/ha of N delivers around 20 kg/ha of DM, then a yield penalty from 37.5kg/ha of N multiplied by 20 is equal to a production loss of 0.75t/ha of DM, or a 25% drop in yield.”

With grass growth having been slow, there is likely to be a surplus of nitrates left in plants. “These will cause a problem in the silage clamp, leading to high pH levels and poor fermentation which will allow unwanted bacteria to survive and result in silage spoilage.”

“The best way to avoid poor silage is to have grass checked by a laboratory for nitrate content prior to cutting,” he says. “Ideally grass should contain below 500mg/kg of N fresh weight and sugar content of around 2-3%.”

Nitrogen applications will need careful planning. “If early application top dressings were split then I’d advise reducing the rate of the second split to avoid excess nitrates in the first cut. If the full rate was applied then attempt to calculate when growth started versus your original estimates and take account of the expected carryover when you calculate your second cut application.”

“With the limited tillering that we are seeing, there will be minimal reserves for regrowth so the reliance on applied NPK and S from applied fertilizer will be high. The timing too will be important, so ensure there is no delay after cutting. A week late and a further 6% yield loss is very likely, thus compounding the issues this season.”

“Application rates needs to be carefully calculated. The rule of thumb is that N demand for second cut will be based on the number of days of growth multiplied by 2.5 kg N/ha/day for example 50 days equals 125 kg/ha of N, which has been the optimal rate in our trials over recent years. You then need to deduct any expected carry over if first cut has been lower yielding than expected. For many farmers, a split application approach may be a way of managing risk in this rather unpredictable season, so I’d be suggesting applying a first 60% split with the rest applied 25 days later.”

The table below will help with rate calculations from the previous silage cutting date and highlights the yield penalties from delaying second cut nitrogen application

Nitrogen application rate calculations     
Time of N application  DM yield t/acre  % loss  % N in DM
1 day after defoliation 4.07 - 2.8
8 days after defoliation 3.85 5.9 2.9
15 days after defoliation 3.53 13.3 3.2
22 days after defoliation 3.45 15.2 3.2

More on this topic

The fertiliser management course running in the FWi Academy contains a useful module on managing grassland nutrition. Successfully completing the accompanying academy test will earn you two BASIS points, two FACTS points, two DairyPro points and one Farmers Weekly Academy Scheme (FADS) point.