The importance of fibre in fattening calves

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.

Figure 1. Ruminal metabolism of fibre components (After France and Dijkstra)

Physically effective fibre

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).

Figure 2. Penn State Particle Separator

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%.

• Roughage inclusion of 8 to 10% is common in American feedlots (Samuelson et al., 2016).
• In Spanish feedlots, when concentrate and straw are provided ad libitum in separate feeders, straw ingestion is about 4-6 % in young animals during the first week after arrival (Devant et al., 2000; Iraira et al., 2012). It later increases, to reach 8-12% on average when total feedlot consumption is measured (Devant et al., 2001; Robles et al., 2007; González et al., 2008 a, b, c; Faleiro et al., 2011).

Considering those proportions and the NDF contents of straw and concentrate, it appears that:

• 30-35% of the fibre consumed comes from straw, falling mainly into the “physically effective” category.
• 65-70% comes from the concentrate. However, due to the small particle size of a meal, it would not be considered as effective fibre.

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).

Level of forage inclusion

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.

Conclusions

• In the future, animal production should consider the commitment to productive results, in terms of feed efficiency, profitability and animal welfare, in order to produce high-quality meat.
• Therefore, it is necessary to make changes in our farms, in accordance with the earlier observation, which will allow achieving this compromise without harming their viability.

This article was originally published in nutriNews Spain, by the title “Importancia de la fibra en el cebo de terneros”