Processing of high fiber feedstuffs for swine

Introduction

Escalating costs of typical swine feed ingredients such as corn and soybean meal has created a demand for producers to consider feeding diets containing higher levels of byproducts to nursery and finishing pigs to reduce feed cost. However, these alternative feed ingredients are generally higher in fiber and lower in dietary energy compared to the cereal grains normally utilized. This decrease in energy along with increased fiber and decreased bulk density can lead to decreased performance and an increase in the time needed for hogs to reach target market weights.

In light of these circumstances, more emphasis is being placed on feed processing technologies to improve utilization of the higher fiber diets. Fine grinding and pelleting are the two primary feed processing technologies that exist that will improve feed efficiency. First, reducing particle size of individual ingredients or whole diets is commonly understood to improve their digestibility and consequently feed efficiency, but little research has explored the effects of fine-grinding high fiber alternative ingredients or the entire diet containing these high fiber ingredients. Secondly, pelleting high by-product diets will improve diet bulk density, reduce feed wastage, and potentially improve diet digestibility. When used in tandem, reducing particle size of ingredients or complete diets and then pelleting could potentially be useful strategies to improve feed efficiency and subsequently reduce the cost per pound of gain. 

Adding the necessary infrastructure to fine grind and pellet diets has a high initial cost and necessitates increased energy usage, which leads to a higher cost of production of the feed. However, these extra costs should provide more economic return through improved feed utilization and growth performance. Thus, the economics associated with the increased production costs of grinding and pelleting need to be studied and compared with pig performance benefits.

In this paper we briefly review recent studies conducted at Kansas State University investigating the effects of fine grinding high fiber ingredients such as dried distillers grains with solubles (DDGS), wheat middlings (midds), and soybean hulls as well as the interactive effects of fine grinding and pelleting on both nursery and finishing pig growth performance. 

Nursery pigs

Recently, four studies were conducted to investigate the effects of fine grinding high-fiber ingredients and/or complete diets as well as the effects of pelleting on growth performance of nursery pigs. 

Experiment 1 – Pelleting and particle size

De Jong et al. (2013a) used a total of 675 nursery pigs in a 21-d study to evaluate the effects of varying ingredient particle size and diet form on nursery pig growth performance and caloric efficiency. Pigs were fed 1 of 8 experimental diets including 3 corn-soybean meal–based diets consisting of: (1) corn fraction ground to an average of 620 μ and fed in meal form, (2) corn fraction ground to an average of 352 μ and fed in meal form, and (3) corn ground as in diet 2 but fed in pellet form. The remaining 5 diets were high by-product diets containing 20% midds and 30% DDGS. Diets 4 to 8 consisted of: (4) corn fraction ground to an average of 620 μ, midds and DDGS unground (as received from the supplier) with an average particle size of 534 μ and 701 μ, respectively, and fed in meal form; (5) diet 4 but corn fraction ground to an average of 352 μ and fed in meal form; (6) diet 5 but fed in pellet form; (7) corn, soybean meal, DDGS, and midds ground to average particle sizes of 352 μ, 421 μ, 377 μ, and 357 μ, respectively, fed in meal form; and (8) diet 7 but fed in pellet form. The diets were not balanced for dietary energy, thus energy  was lower for treatments 4 to 8 than for treatments 1 to 3.

Overall, pelleting diets improved ADG and G:F as expected, but reducing corn particle size reduced ADG as a result of decreased ADFI. Pigs fed the high-by-product diet had reduced ADG, ADFI, and final BW and poorer G:F, but caloric efficiency (defined as the amount of calories needed to produce one unit of gain) was similar to pigs fed the cornsoybean meal–based diet. Finally, grinding the by-products to a smaller particle size further reduced ADG, ADFI, and final BW and did not influence feed efficiency.

Experiment 2 – Particle size pelleting and grinding DDGS

In the second experiment De Jong et al. (2013b) used 687 nursery pigs in a 21-d study to evaluate the effects of fine grinding corn or DDGS in diets fed in meal or pellet form on nursery pig performance. The 10 experimental diets fed included 4 corn-soybean meal-based diets consisting of: (1) corn ground to ~638 μ in meal form, (2) treatment 2 in pellet form, (3) corn ground to ~325 μ in meal form, and (4) treatment 3 in pellet form. The remaining 6 diets contained 30% DDGS. Diets 5 to 10 consisted of: (5) corn and DDGS ground to ~638 and 580 μ, in meal form, (6) diet 5 in pellet form, (7) corn and DDGS ground to ~638 and 391 μ, in meal form, (8) diet 7 in pellet form, (9) corn and DDGS ground to ~325 and 391 μ, in meal form, and (10) diet 9 in pellet form.

A corn particle size (regardless of DDGS addition) × diet form interaction was observed as a result of increased ADFI when corn was ground and fed in pellet form but decreased intake when corn was finely ground and fed in meal form. Pelleting the diets decreased ADG, ADFI, and final BW and increased G:F and caloric efficiency on both an ME and NE basis. Fine grinding corn decreased ADG as a result of numerically decreased ADFI. Also, feeding 30% DDGS decreased ADG, ADFI, and NE caloric efficiency, and tended to decrease final BW.

Experiment 3 – Corn particle size, complete diet grinding and pelleting 

A third trial conducted in a commercial setting was completed to evaluate the corn particle size results found in the first two experiments. De Jong et al. (2013c) used 996 nursery pigs in a 21-d study to evaluate the effects of corn particle size, complete diet grinding, and diet form on nursery pig growth performance. The same corn-soybean meal-based diet containing 30% DDGS and 10% midds were used for all treatments. The 6 treatments were: 1) roller-mill ground corn (737 μ) fed in meal form; 2) treatment 1 fed in pellet form; 3) hammer-mill ground corn (324 μ) fed in meal form; 4) treatment 3 fed in pellet form; 5) complete mixed diet reground through a hammer-mill (541 μ fed in meal form; and 6) treatment 5 in pellet form. All diets contained 2% added fat prior to pelleting. 

Overall ADG and ADFI decreased when corn was finely ground and fed in meal form, but increased when fed in pelleted form. Pelleting diets improved ADG, G:F, ME and NE caloric efficiencies, and final BW. The interaction of diet form × corn μ and numerical decreases in intake of pigs fed fine ground complete diet, suggests that finely ground feed fed in meal form may reduce palatability of the diet; however, improved performance from fine grinding may be realized if the diet is fed in pelleted form. Thus data from this trial shows that pelleting diets significantly improved performance and fine grinding corn numerically improved performance when fed in pelleted form. No additional improvements were found when the complete mixed diet was finely ground compared to only grinding corn, which agrees with earlier nursery studies. 

Results from these three studies suggest that reducing the particle size of either corn or high fiber ingredients in complete feed when fed in meal form decreases performance in nursery pigs. While previous research has shown improvements in reduced particle size, most of this work only showed a response to approximately 500 μ and no additional benefit by grinding to a smaller particle size. The available research is consistent with our findings where fine grinding corn did not result in further improvements and in fact negatively impacted feed intake and gain when fed in meal form. Interestingly, as all major ingredients were ground in the high-by-product diet and fed in meal form, feed intake for these pigs was reduced even further, which suggests that feeding a finely ground complete diet may negatively influence palatability.

The varying response to pelleting on nursery pig performance in these three trials supports previous research where not all, but the majority of trials have shown a positive effect on growth performance. 

Medel et al. (2004) showed inconsistent results when pigs fed diets that were pelleted had improved G:F from d 22 to 42 post-weaning but had no differences in ADG. Traylor et al. (1996) conducted an experiment feeding meal and pelleted diets to nursery pigs. They also observed improved G:F when pigs were fed pelleted feed; however, there were no differences in ADG. Hansen et al. (1992) also showed improved G:F of nursery pigs when fed a pelleted diet compared to the same diet in meal form immediately following weaning with no changes in ADG. This trend held true in our current nursery experiments as there was an improvement in G:F for all three experiments, but pigs in the first experiment had improved ADG and pigs in the second and third experiments had decreased ADG. 

We believe the reduction in ADG from feeding pellets that was associated with a similar decrease in ADFI may be a result of increased pellet hardness. We observed a high pellet durability index which is an indicator of a hard pellet. Mavromichalis et al. (2002) conducted 2 experiments to determine the effects of pellet hardness on nursery pig growth performance. In the first experiment they showed that harder pellets decreased ADFI and ADG but had no effect on G:F and in a second experiment, ADFI tended to decrease leading to numerically decreased ADG. 

Experiment 4 – Grinding and pelleting soy hull diets

A fourth experiment was conducted in a commercial setting to determine the effects of fine grinding or pelleting diets containing soybean hulls. Goehring et al. (2013a) used a total of 1,100 nursery pigs in a 42-d growth trial to determine the effects of increasing soybean hulls (10 or 20%) and soybean hull particle size (unground or ground) in nursery pig diets fed in both meal and pelleted forms. The average particle size of the unground and ground soybean hulls were 617 and 398 μ, respectively.

Soybean hulls worsened G:F when fed at 20% compared with 10%; however, pelleting these diets resulted in little change in G:F compared with the 20% inclusion fed in meal form. Pelleting diets resulted in improved ADG, but also increased feed intake, which resulted in no change in feed efficiency compared with pigs fed a meal diet. The increased feed intake could be the result of providing a more dense feed, because soybean hulls in a meal diets reduced diet bulk density. For fine grinding soybean hulls, ADG and ADFI were reduced which resulted in no change in feed efficiency. 

Nursery summary

Pelleting and fine grinding had few positive and even some negative effects on nursery pig performance. Possible reasons include decreased palatability of finely ground ingredients, limited or no benefit to grinding grain finer than 600 μ for nursery pigs, or limited biological benefits of fine grinding other ingredients for nursery pigs. We speculate that the lack of improvement to pelleting in the second experiment may have been a result of increased pellet hardness. It is clear; however, that more research needs to be conducted to determine the optimum particle size of cereal grains and complete diets when fed to nursery pigs as well as the effects of pellet hardness on nursery pig performance.

Finishing pigs

Two studies were conducted to investigate the effects of feed processing technologies on high fiber diets for finishing pigs. 

Experiment 1 – Processing techniques for diets with high fiber ingredients

The first trial was conducted in a commercial setting investigating the effects of corn particle size, complete diet grinding, and diet form of high fiber diets on finishing pig growth performance. De Jong et al. (2013d) used a total of 855 finishing pigs in a 111-d trial. All pigs were fed the same corn-soybean meal–based diet containing 30% DDGS and 20% midds. Different processing techniques were used to achieve the 5 dietary treatments: (1) roller-mill ground corn to approximately 650 μ with the diet fed in meal form; (2) hammer-mill ground corn to approximately 320 μ with the diet fed in meal form; (3) treatment 2 pelleted; (4) corn initially roller-mill ground to approximately 650 μ, then the complete mixed diet reground through a hammer-mill to approximately 360 μ with the diet fed in meal form; and (5) treatment 4 fed in pellet form.

Overall diet form × portion ground interactions were observed for ADG, ADFI, final BW, percentage of pigs removed per pen, and HCW. These interactions occurred because fine grinding the complete diet reduced each variable when fed in meal form, whereas pigs fed that same diet in pellet form had increased responses for each of the measurements. Reducing particle size of the corn from 650 to 320 μ did not affect ADG or ADFI but improved G:F and caloric efficiency. Pelleting the diet improved ADG, G:F, caloric efficiency, final BW, HCW, and loin depth but tended to increase BF. 

Particle size reduction improved G:F and caloric efficiency; however, fine grinding the complete diet was actually detrimental to performance when fed in meal form. Particle size reduction of corn has been consistently shown to improve performance in finishing pigs. Hedde et al. (1985) observed that pigs fed finer particle corn compared to simple cracked corn had improved ADG and G:F. Wondra et al. (1995 a,b) found that reducing particle size from 1,000 to 400 or 800 to 400 μ improved G:F and apparent digestibility of diets.

It is commonly understood that fine grinding will influence feed mill production parameters. Wu et al. (1985) found that decreasing particle size increased the total particle surface area but worsened mill production rate and grinding efficiency. Thus, feed mill operators and producers must balance decreased mill efficiency with growth benefits. 

We hypothesized that grinding the corn, complete diet, and pelleting would have allowed for more calories to be available to the pig, thus improving caloric efficiency. However, only grinding the corn portion and pelleting the diets improved caloric efficiency. Interestingly, grinding the entire diet post-mix did not improve caloric efficiency over that of the diet containing fine ground corn. This means that fine grinding the other major ingredients (soybean meal, DDGS and midds) did not create additional dietary energy for the pig regardless of feed form. In addition, this study showed that ADFI decreased when the complete diet was finely ground. The decrease in ADFI may have been a result of decreased palatability of the diet when finely ground. Work by Mavromichalis et al. (2000) and Wondra et al. (1995a) both showed that finely ground ingredients in diets decreased ADFI. 

Ulceration of the pig’s esophageal region of the stomach is a concern when feeding finely ground ingredients or diets. There were increased removals off test for pigs fed the diet that was post-mix ground and pelleted. However, no clear link was found between the removals and feed processing in this study. Most removals reported during the study appeared to be caused by reasons other than experimental treatment effects. More research needs to be conducted to evaluate whether this effect was diet- related.

Fine grinding ingredients and complete diets can also potentially lead to feed handling issues. In the current experiment, angle of repose was used as an indicator of the flow ability of the diet. Angle of repose is the measure of the internal angle created by feed allowed to flow freely onto a horizontal surface. As the angle increases (becomes steeper) the feedstuff is associated with having decreased flow ability. Fine grinding only the corn fraction of the diet didn’t affect angle of repose when compared to the control and this was most likely caused by the limited concentration of corn (30-40%) in the diet. However, when the complete diet was reground there was an increase in the angle of repose when compared to the control or the meal diet with finely ground corn. 

In conclusion, fine grinding corn and pelleting the diet improved performance and carcass characteristics, as well as caloric efficiency of finishing pigs. However, grinding the complete diet and feeding it in meal form had detrimental effects on all criteria measured which may be due to reduced palatability of a finely ground diet presented in meal form. Finally, post-grinding a complete diet did not provide any advantage in growth performance to that of a diet with only finely ground corn. This indicates that fine grinding DDGS, midds and soybean meal does not provide an additional benefit.

Experiment 2 – Particle size and inclusion rate of soy hulls

In the second experiment Goehring et al. (2013b) used 1,235 finishing pigs in a 118 d growth trial to determine the effects of 7.5 and 15% ground or unground soybean hulls on growth performance and carcass characteristics of pigs raised in a commercial environment. Treatments were arranged in a 2 × 2 factorial with a positive control. Main effects were soybean hull particle size (unground or ground, 787 and 370 μ, respectively) and amount of soybean hulls (7.5 or 15%) in corn-soybean meal–based diets. The fifth treatment was a positive control, cornsoybean meal–based diet with no added soy hulls. Diets were fed in meal form and were formulated to a constant SID lysine level. 

Overall, increasing dietary soybean hulls did not impact ADG, ADFI, or final live BW; however, G:F decreased. Caloric efficiency improved on an ME and NE basis as soybean hulls were added. Feeding pigs diets with the ground soybean hulls did not influence ADG or ADFI, but resulted in poorer G:F and caloric efficiency on an ME and NE basis compared to pigs fed the diets with unground hulls.

A study in South America conducted by Moreira et al. (2009) observed an improvement in DE and ME when soybean hulls were ground through a 2.5 mm screen as compared to a screen size up to 3.5 mm. However, soybeans are processed differently in South America than in
the United States. In South America the soybean hulls are separated before roasting and trypsin inhibitors may still be present in the hull. The improvement in digestibility observed by Moreira et al. (2009) could be the result of reducing the negative effects of trypsin inhibitors and not improving digestibility of soybean hulls. In our study grinding soybean hulls resulted in no difference in ADG or ADFI but feed efficiency and caloric efficiency were worsened with soybean hull grinding. These results imply that grinding soybean hulls did not improve pig performance by means of improving digestibility and in fact, the opposite may have occurred. It has been observed that increasing the amounts of fiber in the diet will increase the rate of passage of digesta (Ehle et al., 1982; Stanogias and Pearce, 1985). It could be possible that an increased rate of passage caused by smaller particles of fiber occurred which resulted in decreased digestibility of the feed and consequently poorer caloric efficiency. 

In summary, increasing soybean hulls reduced G:F as expected due to lowering diet energy density. The hypothesis of reducing the particle size of soybean hulls to improve their energy value was proven false. Grinding soybean hulls reduced ADG and ADFI, but pelleting improved ADG and ADFI regardless of particle size. It appears that fine grinding soybean hulls when fed in meal form is detrimental to performance but performance can be restored if the diets are pelleted. 

Overall summary

• Grinding ingredients other than corn or complete diets resulted in little or no improvements in feed efficiency (Table 2).

• Fine grinding ingredients to resulted in no improvements and even negative effects on growth performance in nursery pigs.

• In nursery pigs some variability exists, but our data combined with that available in the data base suggests a positive response to pelleting (Table 1).

• In finishing pigs, pelleting appears to elicit a more consistent positive feed efficiency response. 

• Pelleting diets for finishing pigs appears to mitigate any negative effects of the finely ground ingredients.
 

References

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