At a time when feed represents 40-60 per cent of milk production costs, every gram of nutrient counts. Forage protein content is a key nutritive element that varies greatly according to the crop species, maturity and the soil.
Poor silage-making processes can lead to the degradation of protein into ammonia and soluble nitrogen inside the silage. When produced in excess, these products are detrimental to the animal and lead to a decrease in performance and health problems.
Assessing protein content The University of Kentucky's College of Agriculture, Food and Environment extension professor Donna Amaral-Phillips describes forage nutrient content analysis as "the first key step towards developing a sound and practical nutritional program for a dairy business". In this analysis, crude protein (CP) is, with energy value, one of the most important nutrients for livestock, which can be a limiting factor for animal performance. Forage protein content varies greatly according to the crop species, maturity and the soil (use of fertiliser).
It is, however, important to keep in mind that CP, which appears on the analysis, is an indirect measure of the protein content, based on the total amount of nitrogen in the forage. This is based on the fact that proteins are the only organic molecules containing nitrogen atoms.
Thus, CP level represents protein content, but also other sources of nitrogen with no feeding value, such as soluble nitrogen and ammonia, which come from the degradation of proteins in the forage. Hence, the actual true protein level can vary for a given CP value.
Excess mineral nitrogen It is typically known that the ensiling process does not alter the CP content of forage. However, if the true protein content and not overall nitrogen content (CP value) is examined, this is different. Indeed, silage fermentation is often a source of protein degradation, known as proteolysis. This results from the action of either: Forage endogenous enzymes, which are released from the plant cells at harvest. Proteolytic micro-organisms, such as clostridia or enterobacteria, present on the plant.
Both of these have to be inhibited as quickly as possible following harvest to keep proteolysis to a minimum (see Figure 1). Thus, silage of poor quality (with clostridia and enterobacteria dominating the fermentation) will show proteolysis and high release of ammonia.
Impact on performance Excessive proteolysis in silage not only decreases the true protein content of the forage, leading to loss of nutritive value, but also produces nitrogen sources that are detrimental to the animal, its health and performance.
The excess of ammonia cannot be all used by the rumen microflora and will be absorbed through the rumen wall and transferred to the liver through the blood flow. In the liver, it will be detoxified into urea, which will be eliminated by the kidneys in urine and saliva.
However, the detoxifying capacities of the liver and the kidneys are limited. An excess of ammonia and urea in the blood and an excess of ammonia in the rumen can be observed. These high levels result in several pathologies for the animal, mainly due to the toxicity of ammonia: alkalosis (when rumen pH >7.2), but also other performance and reproduction troubles (see Table 1).
In addition, the excretion of excessive urea is an energy-consuming process -- energy that can no longer be used for milk production.
It is estimated that the energy lost due to an excess of 2 per cent in protein in the diet is 1.5 megajoules/day, this being equal to a potential of 0.5 litres milk/day.
Monitoring blood and milk urea remain interesting indicators of nitrogen utilisation from the diet. Ammonia may cross the rumen wall when above 50 milligrams/millilitres of rumen fluid. As soon as the ammonia reaches 1mg/100ml (blood alcalosis), intoxication becomes lethal. Thus, it is crucial to keep protein degradation, or proteolysis, to a minimum during the ensiling process to preserve the nutritive quality of silage (see Table 2).
Controlling proteolysis Good silage preservation aims at maintaining the silage value as close as possible to the fresh forage in the field: this includes the preservation of its protein content. Certain good silage practices can help ensure an optimal fermentation pattern and thus lower protein losses. Here are some recommendations to attain this goal:
At harvest: High dry matter (DM) inhibits proteolytic enzymatic activity. Ensiling at correct DM may minimise proteolysis. Proteolytic micro-organisms are mainly located in the soil. A cutting height above 6-7 centimetres should limit soil contamination and the presence of these micro-organisms. Ensuring a rapid and strong acidification at ensiling quickly inhibits the activity of proteases and proteolytic bacteria. The use of specific acidifying inoculants, designed and selected bacteria for forages, accelerates the acidification process, thus limiting proteolysis. Optimal packing and sealing to favour anaerobiosis: the absence of oxygen within the silo inhibits proteases and contributes to a better acidification.
After harvest: Avoid silage heating: the increase in temperature increases Maillard reaction and proteolysis. Certain forages, in particular when high in DM, are prone to heating. This can be prevented by the use at harvest of adapted silage inoculants containing Lactobacillus buchneri 40788, proven to prevent mould development after ensiling and heating.
Article provided by Lallemand Animal Nutrition, website www.lallemandanimalnutrition.com<>