Criteria That Determine Forage Quality in Ruminant Livestock
When it comes to feeding dairy cows and beef cattle, the proportion of forage needed varies significantly according to the diet. An accurate analysis of forage quality is essential to ensure the proper functionality of animals’ rumen and their digestion processes, regardless of the type of production considered. How can the value of forages be properly assessed? What criteria should be considered to this end?
Thanks to the size of their particles and fibres, forages are at the core of rumination and of ruminant digestion. Forage quality relates to the characteristics affecting consumption, nutritive value and resulting animal health and performance. Submitting forage to NIR (near-infrared spectroscopy) analysis provides an accurate estimation of its quality. In addition to traditional criteria such as moisture, or protein fibers content, other parameters must be used to determine the optimal utilisation of a forage and therefore contribute to the profitability of livestock farming.
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What risks does a poor estimation of forage quality pose?
Accurate knowledge of the amount and quality of forages used is key to a proper ruminant diet. An accurate estimation is essential for determining the amount and characteristics of the additional share of feed needed to meet production targets. Feed concentrates and/or supplements are designed to compensate for the nutritive deficiencies of forages. For example, these concentrates and/or supplements must be rich in proteins if these are lacking in the forage, so that these nutrients are adequately supplied by the whole diet.
Failure to properly consider voluntary intake and forage quality often leads to errors in diet composition. These may result in an improper utilisation of the ration, low levels of performance (i.e below the targets set) and metabolic disorders (acidosis, ketosis, etc…). These inaccuracies can end up having a long-term impact on economic performance.
Moreover, different forage of the same crop species can have very different values depending on crop management, harvesting conditions and conservation.
A ruminant diet based on forage values based on reference tables can therefore lead to unsuitable recommendations. As a result, relying on these tables may negatively impact the profitability of livestock farming.
What criteria can be used for properly estimating forage quality?
A better characterisation of a forage must go beyond the traditional criteria (percentage of dry matter, protein, fiber, starch, minerals), and allow for an accurate estimation of the way ruminants are likely to optimise the forage. In addition to the net energy calculations and the amount of digestible protein in the intestine, the following criteria must be considered:
Knowledge of the voluntary intake of forage is essential as it directly impacts nutritional intakes. One extra kg of DM ingested corresponds to an additional supply of 1.55Mcal NEL / animal/day kg DM. This is more or less equal to the amount of energy necessary for producing 2 kg of milk.
Starch degradability in the rumen (soluble, slowly degradable or bypass): When it comes to predicting the extent of acidogenic risks raised by forage ingestion, starch content is often the main benchmark. However, practical observations have clearly shown significant differences in acidosis risks for forages of similar starch content. Since acidogenic risks are linked to the soluble starch content of forages, these discrepancies mainly relate to the way starch is degraded in the rumen.
DM rapidly degraded in the rumen (within four hours): This criterion allows one to evaluate the extent of likely SARA (sub-acute ruminal acidosis) risk in forage. Consideration of this parameter when designing a ration will lead to striking the right balance between the amount of fermentable energy required for production ends and of SARA risk prevention.
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Fibre digestibility: Ruminants are able to easily digest the fibres contained in their forages. Fibres largely contribute to the net energy intake. The ruminant’s digestion process highly depends on the composition of these fibres.
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Fractions of fat content (digestible in the rumen and bypass): Fat is a very concentrated source of energy. Fat is also useful for increasing the total energy intake of animals endowed with strong genetic merit (strong production potential). However, special attention ought to be paid to fat fractions that are too promptly digested in the rumen. These may have a significant impact on the rumen ecosystem and therefore,on the efficiency of fermentation.