How does cereal milling affect performance and health of pigs?

It is not enough to completely understand each processing method and their effects. It is also necessary to understand how feed processing affects:

• feed management
• production costs
• nutrient and energy digestibility
• animal performance.

Different types of mills 

Grains are the main ingredients of pig feed that need to be milled. Other ingredients are previously processed and reach the feed mill in acceptable conditions to be mixed.

There are two types of mills usually used to reduce the particle size of ingredients: roller mills and hammer mills.

Roller mills

Roller mills (Fig. 1A) reduce particle size through crushing and triturating the ingredient with  acompressing force. This process produces a small proportion of fine material, resulting in a relatively uniform particle size.

Hammer mills

Hammer mills (Fig. 1B) reduce particle size of ingredients through impact milling (Pfost, 1976), rendering particles of a more spherical shape, as well as a higher proportion of fine and pulverized particles. The final product has less uniform particle size (Koch, 2002). Previous studies have demonstrated that the lower uniformity in particle size rendered by hammer mills results in a higher angle of fall. This entails a lower fluidity of the product (Groesbeck et al., 2006).

Figure 1. Different types of mills.

Costs associated with milling

Milling grains increases the cost associated with the production of feed. These costs include:

The initial cost of the equipment may appear high. However, depending on the type of mill, the yearly energy costs can exceed the cost of a new milling equipment. Understanding the variety of factors affecting the efficiency and production rate of milling can be beneficial to feed manufacturers.

Some data has shown that the reduction of grain particle size leads to higher energy requirements and a reduction in the production rate (Gebhardt et al., 2018)

The velocity at which these factors change can be affected by:

• type of grain
• type of mill
• milling parameters
• mill maintenance.

Considering the negative effects of particle size reduction on milling efficiency, it is important to continue investigating whether these loses are justified by an improvement in animal performance.

The effect of cereal milling on pig performance 

The idea of reducing grain particle size before feeding pigs emerged in the 1930s

• Grains have an external protective layer that impairs the access of enzymes to all the available nutrients.
• The reduction of grain particle size exposes the less protected part to enzymatic action, allowing the pig to digest a higher proportion of the nutrients.
• Fraps (1932) used milled sorghum to demonstrate that the reduction in particle size improved the digestibility of nutrients.
• Aubel (1945, 1955) continued working on this idea, demonstrating the improvement in feed efficiency when pigs were fed milled maize and sorghum grains,

These experiments laid foundations for future studies looking at optimising the application of milling in the production of pig feed.

• Owlsey et al. (1981) observed an improvement in apparent illeal digestibility (AID) and apparent total tract digestibility (ATTD) of dry matter (DM), nitrogen (N), and gross energy (GE) of sorghum in finishing pigs, after a reduction in particle size from 1262 μm to 471 μm.

Previous studies had already demonstrated the improvement in nutrient digestibility thanks to a reduction in maize particle size (Lawrence, 1967; Giesemann et al., 1990). However, recent studies have shown differences in digestibility of particles of different size, depending on the type of mill used to grind the maize.

• When using a hammer mill to mill maize, the authors did not observe improvement in energy or nutrient digestibility after reducing the particle size form 700 μm a 300 μm. However, when using a roller mill, improvements in digestibility of energy and other nutrients were observed (Patience, 2015).
• Instead, Rojas y Stein (2015) described a linear decrease in the metabolisable energy (ME) of maize as the particle size was reduced, with a hammer mill, from 865 μm a 339 μm.
• However, Bertol et al. (2017) developed a model that determined that, from a particle size of 523-524μm, there was not an improvement of on maize apparent metabolisable energy corrected for nitrogen (AMEn).
• Based on studies performed in 2012, it was concluded that there was an improvement of 1.0 to 1.3% in the growth performance of pigs for each 100 μm reduction in particle size of maize or sorghum, when going from 1000 μm a 400 μm (Cabrera, 1995; Wondra, 1995a; Paulk, 2011; De Jong 2012).
• Bertol et al. (2017) observed a 6% improvement in feeding efficiency in gilts receiving diets with milled maize, when the particle size went from 904 μm a 483 μm. However, this reduction in size did not affect feeding efficiency in farrows.
• In contrast with a previous study, Gebhardt et al. (2018) did not observe improvements in feeding efficiency of finishing pigs when offering them ground maize (processed with roller mill) with reduced particle size (from 581 μm to 285 μm).
• As mentioned before, Bertol et al. (2017)and Rojas et al. (2015) registered an increment in maize ME when maize particle size was reduced. When formulating diets based on this increment of ME, there were no differences in feed efficiency in finishing pigs.

The improvement in feed efficiency in pigs when reducing feed particle size has been well documented in pigs. However previous studies revealed a higher variability regarding optimum particle size.

The growth rate in response to the reduction in particle size is more variable in weaner piglets. At that stage, the weight gain was lower when the maize fraction of the diet was finely ground (approx. 325 μm).

• This phenomenon was associated with a reduction in feed intake, without effect in feed conversion efficiency (De Jong et al., 2013a; De Jong et al., 2013b).
• However, Bokelman et al. (2014) did not observe a significant effect of particle size on body weight or on feed intake. Instead, they verified an improvement in feed conversion efficiency when reducing the particle size from 700 μm a 400 μm in meal form diet.
• The texture and palatability of the maize fraction in diets finely milled reduced feed intake in weaner pigs (Sola-Oriol et al., 2009).
• It was demonstrated that, when given a choice, pigs consume 67% or 8% of the average daily intake when given diets with a fraction of maize ground at 525 μm or 700 μm, and 33% or 20% in the case of maize ground at 267 μm or 400 μm, respectively (Bokelman et al., 2014; Gebhardt et al., 2018).

 Weaned piglets are sensitive to diet palatability and a predictable reduction of feed intake was frequently observed in cases when particle size was lower than 500 μm.

The effect of cereal milling on pig health 

Although the reduction of grain particle size can improve performance of pigs, it can have negative effects on gastric integrity. Certain situations could lead to an increment of ulceration in the pars oesophagea (Fig. 2) of the pig stomach (see our article The effect of feeding on the development of gastric ulcers in pigs).

               Figure 2. Diagram of the regions of the pig stomach.

• Usual practices in the production of pig feeds, such as fine milling, have shown to increase the incidence of ulceration in the pars oesophagea of the stomach in fattening pigs (Mahan et al., 1966; Maxwell et al., 1970; Cabrera et al., 1995; Wondra et al., 1995a; and Ayles et al., 1996). Similar effect was also observed in lactating sows ((Wondra et al.,1995b). The ulceration has been attributed to the fact that smaller particle size will result in an increased fluidity of the gastic content, leading to higher mixing of those contents between the glandular region and the pars oesophagea.
• The mixing of the fluid content allows the continuous exposure of the unprotected mucosa of the pars oesophagea to pepsin and gastric acids (Reimann et al., 1968; Maxwell et al., 1970).
• The relationship between severity of the ulcers and its effect on growth has not yet been properly described. Hedde et al. (1985) and Ayles et al. (1996) established that the increase in severity of ulcers was associated to a drop in weight gain.
• However, other studies indicated that gastric ulcers do not affect growth rate (Backstrom et al., 1988; Guise et al., 1997; Dirkzwager et al., 1998). Apart from the effects on performance, severe gastric ulcers could result on sudden death.

This article has been previously published on nutriNews Spain, with the title “Cómo la molienda de cereales afecta la eficiencia y la salud en cerdos”