Is a hammer or roller mill better for pig feed structure?

Understanding what milling processes can achieve optimal particle size of diets for pigs was the objective of a recent review by Serbian and Spanish researchers.

Their research, published in Animal Feed Science and Technology​​​,​ was conducted under the auspices of an EU project​ ​looking at the factors affecting the gastro-intestinal microbial balance and the impact on the health status of pigs (PiGutNet).

“If pig feed is used in mash form, the most convenient method for particle size reduction, considering targeted particle size and specific energy consumption of the grinding process, is to combine a hammer mill and roller mill.​

“However, if pig feed is used in pelleted form, the best option is to apply coarse grinding on [a] roller mill before pelleting.​

“In that manner, specific energy consumption of the grinding process will be lower, the share of coarse (>1600 μm) and fine particles (<400 μm) in the pellets will be lower, and also the quality of pellets will be better, compared to processes using a hammer mill in the grinding stage,”​ concluded the team.

Influence of feed structure and feed form in pigs​

Assessment of pig feed processing technology formed only one part of their review. The Serbian and Spanish researchers evaluated the available literature to determine the impact of feed structure and feed form on pig nutrition.

They found that the research results generally suggest that the reduction of feed particle size improves pigs’ performance, and that fine grinding could be recommended in the production of pig feed.  However, they concluded that fine particle size of the diet - both mash and pelleted feed - negatively affects the health of a pig's GIT by increasing the incidence of pre-ulcerous lesions and ulcerations.

They found that feeding pigs with coarsely ground mash feed reduces the incidence of Salmonella​ and other pathogen bacteria compared to pigs fed finely ground mash feed or pigs fed pelleted feed. "This is a consequence of lower pH in the stomach and small intestine content of pigs fed coarse mash diets."​

According to available data, optimal particle size of diets for pigs is in the range between 500 and 1600 μm, while particles smaller than 400 μm are considered as undesirable with high ulcerogenic capacity, said the reviewers.

Pigs today tend to be fed mainly pelleted feed due to numerous advantages of pelleted over mash feed, they said.

When pig feed is pelleted, the particle size distribution (PSD) of the diet is substantially changed, with intensive grinding of particles followed by multiple increase of the share of fine particles. This could positively affect digestibility of nutrients but, on the other hand, this will have a negative impact on gut health, said the authors.

Some efforts have been made to optimize PSD in pellets by manipulating the parameters of the pelleting process but only minor improvements were achieved, they said.

Pros and cons of hammer and roller mills

The authors noted advantages and disadvantages to be considered when choosing the optimal mill.

“Hammer mills have greater capacity of grinding and it is easier to switch from one grain to another by changing the sieve. However, hammer mills will produce higher quantity of fine particles and dust, and will require more energy per ton of material compared to roller mills (Ziggers, 2001).”​

Hammer status in the hammer mill may seriously affect particle size distribution (PSD) and the homogeneity of the PSD as function of time use, wear and quality of raw ingredients milled, they found. “Therefore, PSD could be a good indicator of need for replacement of hammers before affecting pig performance (Solà-Oriol et al., 2015).”​

The advantage of roller mills is in creating a uniform particle size distribution than hammer mills, and creating much a lower quantity of fine particles and dust, which is very important in pig nutrition concerning the negative influence of fine particles on the health status of a pig’s gastrointestinal tract (GIT), they added.

Furthermore, roller mills’ specific energy consumption is lower. Thacker (2006) found no effect of mill type - hammer vs roller - on performance of pigs but that study found roller mills had lower energy consumption, more accurate control of particle size, and quieter operation.

“Vukmirović et al. (2016b) determined that specific energy consumption of a roller mill was significantly lower compared to a hammer mill for similar geometric mean diameter of particles.”​

However, the authors said the investment and maintenance costs are higher for roller mills citing research from Barneveld and Hewitt (2003).

For this reason, combined use of both types of mills could be optimal, they said.

Grinding in barley based feed production  

In research by Lucht (2011), three grinding machines were tested during the preparation of a barley-based pig diet:

• Roller mill treatment, where a roller mill with two pairs of corrugated rollers was used without intermediate sieving.
• Stage grinding with two hammer mills and with pre-, intermediate-, and post-sieving.
• Combination of hammer mill and roller mill (one pair of rollers) with pre- and intermediate sieving.

“The aim was to obtain the highest possible quantity of medium-sized particles, and to have the lowest possible quantity of fine particles that should not exceed 25%. ​

“Additionally, coarse and very coarse fractions were undesirable due to their low digestibility rates in GIT of pigs.​

“The best structure resulted from the combination of the hammer mill and the roller mill, with the quantity of medium size particles [at] 60%, low quantity of fine particles (22.5%), and also low quantities of both coarse fractions.”​

They said the study indicated energy consumption was 50% lower when roller mill treatment was compared to stage grinding with two hammer mills, while the combination of the roller mill and the hammer mill had 30% lower energy consumption compared to the stage grinding with two hammer mills.

The reviewers cited research from Wondra et al. (1995b) investigating the effect of different uniformity of particle size, resulting from the use of different mill types, on growth performance, apparent nutrient digestibility, and stomach morphology in finishing pigs.

“In this research, corn was ground to mean particle size of 850 μm both on hammer mill and roller mill. Grinding with different mills resulted in differences in uniformity of the particle size, with more uniform particle size obtained at the roller mill. Average daily gain and average daily feed intake were not affected by uniformity of particle size. On the other hand, increased particle size uniformity by using [a] roller mill improved apparent nutrient digestibility and reduced undesirable changes in stomach morphology.”​

The authors also cited a study by Ziggers (2001) that indicated particles produced in a roller mill have more surface area for the action of digestive enzymes in the GIT of pigs, and could possibly improve feed efficiency.

Optimizing the pelleting process

They said that pelleting causes strong additional grinding of feed particles and the PSD obtained within the grinding process is dramatically changed during that process.

However, the possibilities to decrease the intensity of grinding of particles during pelleting by variation of parameters of the pelleting process are very limited, they added.

“Modified extrusion process (i.e. processing using expander) followed by [a] shaping element, is suggested in the literature as an alternative for pelleting in order to obtain agglomerated pig feed with preserved PSD, but this process is not extensively studied so far.”​

Source:​ Animal Feed Science and Technology​

Authors:​ Đ Vukmirović et al.