A challenging spring may pose increased risks for high residual nitrates in silage

14 May 2018


Is the nitrogen out of your grass?

Late nitrogen (N) fertilizer applications on first cut silage ground, coupled with initially below average grass growth this spring, (as a result of the disastrous weather conditions), could potentially result in high nitrate levels in silage if the cutting date is not delayed accordingly. High nitrates in grass at cutting can lead to low grass sugars, poor silage fermentation, reduced silage palatability and hence reduced silage intakes. However grass heading date will be largely unchanged from previous years, therefore delaying cutting date too far beyond the usual cutting date will cost the farmer in reduced silage quality. Typically after the middle of May with every week delay in cutting to the middle of June, grass will lose three units of dry matter digestibility and delay second cut by two weeks. Therefore getting first cut right this year could be a playoff between high residual nitrogen fertilizer in grass and grass heading date and let’s not forget the weather. 

Luxury uptake of nitrates by grass

Grass typically takes up nitrate from the soil at a quicker rate than it is converted into proteins within the plant. This excess nitrate taken up, also known as luxury uptake, accumulates in the stem until it is ready to be formed into plant proteins. Residual nitrate will be present at the time of cutting if:

  • There is not enough time between fertilizer application and cutting e.g. due to the delayed fertilizer applications this spring.

  • Grass growth rates are poor, due to colder temperatures or low light levels, slowing the conversion of nitrates in plants into plant protein e.g. growth rates at the start of April struggled to reach 20kgDM/ha/day.

  • A period of dry weather persists following N fertilizer application, when grass can’t take up nitrates, with this then followed by a period of wet weather, resulting in luxury uptake of nitrate – However I strongly doubt a dry spell after application will be a risk factor this season!

Any of the 3 points above will mean that there is insufficient time to convertall the accumulated nitrate in grass into plant proteins.

Effects of residual nitrates in silage at cutting on silage quality

High nitrates in grass at ensiling are associated with a low crop sugar content and a slower fermentation process.  This will cause more nutrients in the silage to be lost and allow more undesirable bacteria to multiple before a stable pH is achieved. A slower and poor fermentation can result in a less palatable silage for livestock. High nitrates in silage can affect silage fermentation in a number of ways:

  1. By producing grass with low sugar content. Sugars provide the energy needed to support increased rate of grass growth and conversion of nitrates in grass into plant protein. Without sufficient sugars there will be less lactic acid produced, leading to a poorer fermentation.

  2. Nitrates in ensiled silage are converted to ammonia which increase pH, counteracting the rapid pH drop needed for a good fermentation.

  3. If slurry or soil contamination harbors undesirable bacteria (e.g. enterobacteria and clostridia) in the presence of excess nitrate, weaker acids (acetic and butyric acid) are produced as well as ammonia, increasing the buffering capacity of the silage, leading to a poor fermentation. These undesirable bacteria can degrade nutrients, such as protein in silage, reducing the feeding value of the forage.

Poorly preserved silage, with a high ammonia nitrogen content and a high pH, will be less stable and at risk of secondary fermentation, causing spoilage on the silage face shortly after exposed to air. Hence, good silo face management and getting across the silage face as quick as possible will be essential in these circumstances.

Calculating nitrogen usage

Calculating nitrogen usage by grass is important to ensure grass is not cut with excessive nitrates remaining in the crop. The commonly used rule-of-thumb is that grass will use 2 units of N per acre per day of growth (or 2.5kgN/ha/day). Note that this usage rate is when grass is actively growing. Now that grass growth rates are high, it is easy to forget that when many farmers applied their N this spring (slurry or artificial N) that grass was only marginally growing if growing at all (0-15kgDM/ha/day) for a period of time after application. Grass needs temperatures of over 6°C for significant growth. Therefore to allow for the period of little to no grass growth, allow 5-10 days extra (safety margin) when calculating the date by which fertilizer should be out of the crop.

It is very difficult to predict nitrogen contribution from early spring applications of slurry due to significant variation in slurry nutrient content and plant usage rate. However slurry typically provides from 6 to 10 units of N per 1000 gallons.

Pre-cut testing for free nitrates and sugars

This year analysis fresh grass for free nitrates and sugars before cutting could be particularly useful to prevent cutting grass with excessive ‘free nitrates’ and to ensure a good fermentation. Results are typically received within 24-36hrs.

It is recommended not to cut grass for ensiling until the ‘free nitrate’ level is below 1000mg/kg (0.1%) in the freshweight. ‘Free nitrates’ represent the N fertiliser which has not been converted into protein. When the ‘free nitrate’ level is between 1,500mg/kg (0.15%) and 2,500mg/kg (0.25%) in the fresh weight, silage fermentation may be affected, although the extent of the effect on fermentation may only be marginal if the optimum dry matter (28-32%) and sugar levels are achieved. A minimum sugar content of 3% in fresh weight or 10% in the dry matter should be achieved before cutting to ensure a good fermentation. Cutting should be delayed if ‘free nitrate’ level is above 2,500mg/kg (0.25%) in the freshweight. If this occurs retest again in 2-4 days.

Additional tips for when residual nitrates in grass may be a risk at the time of cutting

  • Cut grass at a minimum of 20% DM and wilt to 28-32% DM. This will concentrate the sugars and ensure a good fermentation.

  • Aim for a quick and short wilt (no longer than 24hrs), as this will reduce loss of sugars. A quick wilt can be achieved by tedding the crop within a couple of hours after cutting as the pores on the leaves only remain open for 2 hours after cutting, during which time water loss from the grass is 5 times higher than when the pores are closed.

  • Postpone mowing until the afternoon if possible as grass sugars will be higher then.

  • Silage additives can play a role under these circumstances. Seek advice from your local agronomist on the best additive for use.