Fibre has a modulating effect on the ruminal pH in animals such as fattening calves, which ingest high levels of concentrate. This article reviews the concepts of effective fibre an physical effective fibre, and the impact of fibre on productivity and feed intake.
Ruminants have a highly developed and specialized mode of digestion that allows them better access to energy in the form of fibrous feed (Van Soest, 1994). Also, ruminants need roughage in their diet to maximize production and maintain health through the support of a stable environment in the rumen (Allen, 1997). For fattening calves, it is recommended to add a low percentage of roughage to maximize energy intake and prevent digestive disorders (Galyean and Defoor, 2003).
Dietary fibres consist of components of the plant cell walls. These are basically cellulose, hemicellulose and lignin, which are determined using the Van Soest et al. (1991) method: NDF, ADF and ADL.
All ingredients of vegetal origin fed to calves contain fibres to a greater or lesser extent.
Depending on the degree of lignification of the cell wall, the partly-available cellulose and hemicellulose will be degraded in the rumen by microbial enzymes. Under anaerobic conditions, such enzymes will break down dietary polysaccharides into volatile fatty acids, which will provide energy to the animal.
Mastication, especially during rumination, promotes salivation and helps to avoid pH dropping below physiological thresholds. To account for the extent to what fibre stimulates mastication, especially in dairy cows, the concept of effective fibre was introduced, followed by the one of physically effective fibre. Physically effective fibre refers to the fraction of fibres that stimulate chewing and forms the floating layer of larger particles inside the rumen (Martens, 1997), The main characteristic related to physically effective fibre is particle size. The physically effective fibre could explain 71% of the rumen pH variation (Mertens, 1997).
The separation of the diet by particle size using a separator (such as the Penn State Particle Separator) allows quantifying what proportion of physically effective fibre is given to the animal. Fox and Tedeschi (2002) proposed the requirement of physically effective fibre to be between 7% and 10% (on a DM basis) in diets fed to fattening calves. This estimation was based on calculations using the Cornell Net Carbohydrate and Protein System (CNCPS) simulation. FEDNA (2008) recommends a minimum NDF content of 15-20% and effective fibre of 5-6% (both expressed as fresh matter) in diets fed to calves, also assuming that the consumption of cereal straw will be 10%.
Considering those proportions and the NDF contents of straw and concentrate, it appears that:
The effect of consuming a ration with deficient physically effective fibre is, among others, a reduced rumination and a higher risk for ruminal acidosis, which affects animal welfare (Faleiro et al., 2011).
One of the ways to increase fibre consumption in feedlots is to use unifeed mixtures. The use of Unifeed system increased forage consumption in 50-100%, and the rumination time in a 30% (Iraira et al., 2012, 2015).
Figure 3. Comparison between the effect of Unifeed feeding system and separated administration of concentrate and roughage, on the ingestion of straw (%) and time dedicated to rumination (min) (After Iraira et al., 2015).
It has been said that it is necessary to incorporate a certain level of forage to diets fed to fattening calves, in order to maximize energy ingestion. In this respect, some studies have reported that it is not advisable to go beyond levels between 10 and 15%, due to the reduction of dry matter ingestion (Hales et al., 2013; Swanson et al., 2017).
Madruga et al. (2018) studied the effect of increasing levels of alfalfa (up to 19%) in a Unifeed mixture for fattening hifers. They demonstrated that heifers increased ingestion of dry matter, fibre and physically effective fibre. A year later, the same researchers reported an increase in average daily gain, compared to the use of 10% straw, although feed efficiency was not affected (Madruga et al., 2019). Also, rumination time increased while the number of hours in which the rumen was below a critical or non-physiological pH was reduced.
When the effects of incorporating 19% of alfalfa hay to the diet on carcass and meat quality were evaluated, it was demonstrated that this forage level was enough to cause relevant changes in quality parameters, compared to the use of 10% of straw. The calves that consumed more forage in the form of alfalfa hay showed a higher proportion of linolenic acid in the fatty acid profile of intramuscular fat, however the effects disappeared when this fatty acid was quantified in 100 g of intramuscular fat. Then, it is possible to use higher forage levels than the currently used (when it is of better quality than straw) without compromising the productive results or even improving them, and without affecting carcass and meat quality.
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