Ohio State University Research/Extension Bulletin

Animal Sciences Research and Reviews

Special Circular 156

Kinetics of Hydration and Functional Specific Gravity of Fibrous Feed By-Products

S.A. Bhatti and J.L. Firkins
Department of Animal Sciences

Summary

Studies were conducted to relate the kinetics of functional specific gravity (FSG) of various feed by-products to NDF digestion in vitro. Gas produced during fermentation delayed the increase in FSG of brewers grains, beet pulp, corn gluten feed, distillers grains, orchardgrass, alfalfa, and wheat middlings but not of corn cob pellets, cottonseed hulls, and soyhulls. Averaged across hours of incubation, the FSG of by-products (except beet pulp) was either higher (P < 0.05) or tended to be higher than alfalfa and orchardgrass. The FSG of feeds may be helpful in predicting the rate of passage and ruminal digestion of by-products.

Introduction

Specific gravity (SG) is the ratio of mass of a sample to the mass of gas-free fluid that takes up an equal volume of the sample. Particles with an SG < 1.2 are likely to float above and those > 1.5 are likely to sink below the reticulomasal orifice, so 1.2 to 1.5 is optimal for passage. In contrast to work with inert plastic particles, density of forage particles is profoundly altered by exposure to ruminal conditions. Functional SG (FSG) accounts for the void spaces in a sample (from air or gas).

Various feed by-products often are economical sources of nutrients in ruminant rations. If 1) these fibrous by-products are initially denser than dried forages, 2) these by-products are smaller than the "critical size" above which particles are retained in the rumen of sheep, and 3) high SG and small particle size combine to decrease filtration of by-products through the ruminal mat, then rate of passage from the rumen should be higher for by-products than for forages. The objectives of the current study were to evaluate FSG of by-products and to relate these data to kinetics of NDF digestion in vitro.

Materials and Methods

Rate and Extent of Digestion In Vitro (Experiment 1). Duplicate samples (approximately 0.5 g) of each by-product (Table 1) were weighed into 50 ml centrifuge tubes, which were used as fermentation vessels. Ruminal fluid was taken from a ruminally cannulated cow fed grass and legume hay. Ruminal fluid inocula were dispensed under constant O₂-free CO₂ and incubated in 50 ml cen-trifuge tubes with one-way check valves at 39^oC (the body temperature of cattle) at several times from 0 to 96 hours. Both the residue and the initial feeds were analyzed for NDF at each time point.

Effect of Feed By-products on Functional Specific Gravity (Experiment 2). Four time points (0.5, 4, 8, and 27 hours) were selected so that the behavior of the curve of the FSG over time could be discerned. Feeds were incubated in pycno-meters as described by University of Wisconsin researchers.

Table 1. Chemical composition and mean particle size of forages and feed by-products.

Feed NDF ADF Lignin CP Mean particle size, mm

(% of DM)

Ground alfalfa 44.5 31.1 7.6 17.7 0.6
Ground orchardgrass 65.9 44.7 7.5 9.3 0.5
Brewers grains pellet 49.4 18.5 4.0 24.2 0.7
Brewers grains 54.2 16.3 3.3 31.9 1.6
Ground brewers grains 60.5 18.6 3.7 10.9 0.6
Ground beet pulp 43.4 22.0 2.4 24.2 0.5
Wheat middlings 44.3 12.8 3.9 19.5 0.9
Corn cob pellets 80.2 41.1 6.6 4.3 0.6
Ground cottonseed hulls 90.5 64.5 21.3 4.6 0.9
Soyhulls 67.9 48.2 2.9 11.3 0.9
Corn gluten feed 39.6 11.5 1.7 24.5 0.9
Distillers grains 42.5 19.8 . . .¹ 27.8 0.8
¹ Filtration problems prevented accurate measurement.

Results and Discussion

Rate and Extent of Digestion In Vitro (Experiment 1). Lag time of cottonseed hulls was the highest (14.7 hours), followed by distillers grains (9.7 hours) and alfalfa (8.2 hours; Table 2). The lowest lag time was observed in corn gluten feed (0.6 hours). Generally, lag times are higher for in vitro than in situ procedures. However, the higher lag time for alfalfa than by-products agreed with results from other studies.

The rate of digestion of NDF in vitro was highest for beet pulp and lowest for cottonseed hulls (Table 2). Beet pulp has highly degradable cell wall contents. The lowest rate of digestion for cottonseed hulls probably was due to their higher lignin content (Table 1) and higher cellulose crystallinity. Among by-products, the NDF digestion rates were similar for brewers grains pellets, wheat middlings, corn cob pellets, soyhulls, and corn gluten feed (0.0275 to 0.0379 hour^-1) and were slightly lower than those of two brewers grains samples and distillers grains (0.0465 to 0.0626 hour^-1). Feeds with a low rate or extent of digestion would be more likely to pass out of the rumen without being digested. By-products have a considerable variance in NDF digestion rates in vitro among studies, helping to explain the large variance among sources of by-products in digestibility in the rumen.

Effect of Feed By-products on Functional Specific Gravity (Experiment 2). The FSG of feeds interacted (P < 0.05) with hour of fermentation, so the data were analyzed by hour (Table 3). At 0.5 hour of fermentation, the FSG of feeds ranged from 1.03 for alfalfa to 1.33 for corn gluten feed. Alfalfa particles were seen floating on the surface of the fluid, resulting in its lower FSG. The highest FSG was observed for corn gluten feed (1.325), which may have been due to dense particles that sank to the bottom immediately after wetting. Soyhulls and wheat middlings had FSG of 1.08, which may have been due to more rapid fermentation of these feeds than others at this time (gasses of fermentation were more visible on the surface of liquid in pycnometers than on other feeds). Lag times for digestion were 4.6 to 5.7 hours (Table 2), but these feeds still contained cell solubles, which would ferment rapidly. High FSG at early times postfeeding may promote early settling of feed particles toward the reticulo-omasal orifice without movement into the rumen by normal reticular contraction.

Table 2. Kinetics of NDF digestion of feeds in vitro (Experiment 1).

Kinetic estimates

Feed Lag, hour Rate, hour^-1 Extent,
%

Ground alfalfa 8.2^abc 0.0547^abc 41.3^f
Ground orchardgrass 4.1^bcd 0.0328^bc 48.4^ef
Brewers grains pellet 2.8^bcd 0.0354^bc 51.1^def
Brewers grains 1.7^cd 0.0539^abc 56.6^bcdef
Ground brewers grains 6.1^bcd 0.0465^bc 56.9^bcdef
Ground beet pulp 3.9^bcd 0.0841^a 67.4^bc
Wheat middlings 4.6^bcd 0.0275^bc 46.1^ef
Corn cob pellets 3.2^bcd 0.0301^bc 65.6^bcd
Ground cottonseed hulls 14.7^a 0.0208^c 54.2^cdef
Soyhulls 5.7^bcd 0.0332^bc 82.8^a
Corn gluten feed 0.6^d 0.0379^bc 70.0^ab
Distillers grains 9.7^ab 0.0626^ab 57.2^bcde
SE 2.6 0.0127 5.5

^a,b,c,d,e,f Means within a column with different superscripts differ (P < 0.1).

The FSG of alfalfa, which was the lowest at 0.5 hour, increased to 1.13. The increased FSG of alfalfa at 4 hours probably was due to greater hydration at this time. Perhaps less gas production (which reduces the FSG) occurred, because alfalfa had a lag time of NDF digestion of 8.2 hours (Table 2). The FSG of beet pulp was lowest, probably due to the associated fermentative gasses, which were much more visible at this point than for any of the other feeds, and continued until 8 hours of fermentation. Beet pulp had a lag time of 3.9 hours and the fastest rate of NDF digestion (Table 2), corroborating this observation. Beet pulp was high in pectin, which was rapidly degraded. However, the NDF (which mostly excludes pectin) also was rapidly available in beet pulp. The FSG of cottonseed hulls was the highest and remained almost unchanged during the whole incubation period. Cottonseed hulls were the least digestible (12.6 hour lag time and low rate of NDF digestion; Table 2) and had a fast rate of hydration (data not shown). Therefore, their FSG probably was not affected greatly by fermentation, and hydration was complete at early time points. The FSG of corn gluten feed was highest (1.33) at 0.5 hour but dropped to 1.13 at 4 hours; this may have been due to greater fermentation at this time, and associated gasses may have, therefore, reduced its FSG. Corn gluten feed had the shortest lag time (0.6 hour; Table 2), also corroborating this conclusion.

At 8 hours of fermentation, the FSG of alfalfa decreased to 1.06, which may be indicative of post-lag fermentation. Beet pulp had the lowest FSG (0.99), again probably because of high amounts of fermentation gasses produced. Cottonseed hulls still had the highest FSG (1.3), and the remainder of the feeds had FSG ranging from 1.1 to 1.2. Lack of a major change in FSG from 0.5 to 8 hours in distillers grains may be related to its relatively long lag time (9.7 hours; Table 2). Wheat middlings had a slower rate of NDF digestion in vitro (0.0275 hour^-1) than all the other by-products except cottonseed hulls; this may help explain why it did not show any notable change in its FSG over time.

At 27 hours of fermentation, the FSG of cottonseed hulls was the highest. The lowest FSG was noted for soyhulls. Soyhulls did not exhibit much change in FSG until 27 hours of incubation, which may be due to its moderate rate (0.0332 hour^-1) and high extent of NDF digestion in vitro (82.8%; Table 2).

Averaged across fermentation times, the FSG of cottonseed hulls was highest (1.26), followed by distillers grains (1.19), corn gluten feed (1.18), brewers grains (1.17), corn cob pellets (1.16), orchardgrass (1.13) and wheat middlings (1.12). The lowest FSG was from beet pulp (1.06), which was similar to that of both alfalfa and soyhulls (1.08). These results indicated that, except for beet pulp (with its high rate of fermentation and therefore gas production), the FSG of by-products were generally higher than those of alfalfa and orchardgrass.

Table 3. Least square means for functional specific gravity of feeds over time of incubation in vitro (Experiment 2).

Hours

Feeds 0.5 4 8 27

Ground alfalfa 1.026^d 1.127^bc 1.056^de 1.129^bcd
Ground orchardgrass 1.135^bc 1.095^bcd 1.107^cd 1.198^abc
Brewers grains pellet 1.247^ab 1.108^bc 1.125^bcd 1.204^ab
Brewers grains 1.217^ab 1.149^bc 1.107^cd 1.214^ab
Ground brewers grains 1.235^ab 1.146^bc 1.089^cd 1.137^bcd
Ground beet pulp 1.152^bc 1.024^d 0.992^e 1.093^cd
Wheat middlings 1.079^cd 1.145^bc 1.119^bcd 1.153^bcd
Corn cob pellets 1.159^bc 1.134^bc 1.201^b 1.178^abc
Ground cottonseed hulls 1.217^ab 1.232^a 1.314^a 1.277^a
Soyhulls 1.078^cd 1.088^cd 1.135^bcd 1.052^d
Corn gluten feed 1.325^a 1.132^bc 1.157^bc 1.130^bcd
Distillers grains 1.237^ab 1.182^ab 1.197^b 1.153^bcd
SE 0.034 0.026 0.026 0.032
^a,b,c,d,e Means within a column with different superscripts differ (P < 0.05).

Particles with a density range of 1.2 to 1.5 seem to have the highest rate of passage in cattle and sheep. Forage particles are much larger than by-products and generally need to be ruminated. Many by-products have particle sizes below that associated with stimulation of rumination. Moreover, most by-products had a slower or similar rate of NDF digestion compared with forages (Table 2). From the results of the present study, it was inferred that by-products (except beet pulp and soyhulls) would have a higher rate of passage from the rumen than forages because of the higher FSG and smaller particle size of by-products. Beet pulp has a fast rate of NDF digestion, low FSG, and large particle size. Therefore, as opposed to many by-products, beet pulp should have a very high ruminal digestibility,

potentially supporting levels of ruminal microbial protein synthesis and milk production similar to corn (as observed in a study done at the University of Minnesota). However, generalization of these results under different feeding conditions would be an oversimplification, because different dietary treatments can interact with each other to affect the rate of passage. Nebraska workers reported that ruminal passage rate of soyhulls tended to decrease by increasing the proportion of coarsely chopped alfalfa hay in the diet of lactating cows.

Conclusions

Considerable variation exists in chemical and physical composition among and within types of by-products. Further research comparing the relationship between FSG and passage rate of feed by-products could potentiate development of models to predict the rate of passage of feed by-products and predict extent of ruminal digestion of various batches of by-products.

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Table 1. Chemical composition and mean particle size of forages and feed by-products.
Feed	NDF	ADF	Lignin	CP	Mean particle size, mm
	(% of DM)
Ground alfalfa	44.5	31.1	7.6	17.7	0.6
Ground orchardgrass	65.9	44.7	7.5	9.3	0.5
Brewers grains pellet	49.4	18.5	4.0	24.2	0.7
Brewers grains	54.2	16.3	3.3	31.9	1.6
Ground brewers grains	60.5	18.6	3.7	10.9	0.6
Ground beet pulp	43.4	22.0	2.4	24.2	0.5
Wheat middlings	44.3	12.8	3.9	19.5	0.9
Corn cob pellets	80.2	41.1	6.6	4.3	0.6
Ground cottonseed hulls	90.5	64.5	21.3	4.6	0.9
Soyhulls	67.9	48.2	2.9	11.3	0.9
Corn gluten feed	39.6	11.5	1.7	24.5	0.9
Distillers grains	42.5	19.8	. . .¹	27.8	0.8
¹ Filtration problems prevented accurate measurement.

Table 2. Kinetics of NDF digestion of feeds in vitro (Experiment 1).
	Kinetic estimates
Feed	Lag, hour	Rate, hour^-1	Extent, %
Ground alfalfa	8.2^abc	0.0547^abc	41.3^f
Ground orchardgrass	4.1^bcd	0.0328^bc	48.4^ef
Brewers grains pellet	2.8^bcd	0.0354^bc	51.1^def
Brewers grains	1.7^cd	0.0539^abc	56.6^bcdef
Ground brewers grains	6.1^bcd	0.0465^bc	56.9^bcdef
Ground beet pulp	3.9^bcd	0.0841^a	67.4^bc
Wheat middlings	4.6^bcd	0.0275^bc	46.1^ef
Corn cob pellets	3.2^bcd	0.0301^bc	65.6^bcd
Ground cottonseed hulls	14.7^a	0.0208^c	54.2^cdef
Soyhulls	5.7^bcd	0.0332^bc	82.8^a
Corn gluten feed	0.6^d	0.0379^bc	70.0^ab
Distillers grains	9.7^ab	0.0626^ab	57.2^bcde
SE	2.6	0.0127	5.5
^a,b,c,d,e,f Means within a column with different superscripts differ (P < 0.1).

Table 3. Least square means for functional specific gravity of feeds over time of incubation in vitro (Experiment 2).
	Hours
Feeds	0.5	4	8	27
Ground alfalfa	1.026^d	1.127^bc	1.056^de	1.129^bcd
Ground orchardgrass	1.135^bc	1.095^bcd	1.107^cd	1.198^abc
Brewers grains pellet	1.247^ab	1.108^bc	1.125^bcd	1.204^ab
Brewers grains	1.217^ab	1.149^bc	1.107^cd	1.214^ab
Ground brewers grains	1.235^ab	1.146^bc	1.089^cd	1.137^bcd
Ground beet pulp	1.152^bc	1.024^d	0.992^e	1.093^cd
Wheat middlings	1.079^cd	1.145^bc	1.119^bcd	1.153^bcd
Corn cob pellets	1.159^bc	1.134^bc	1.201^b	1.178^abc
Ground cottonseed hulls	1.217^ab	1.232^a	1.314^a	1.277^a
Soyhulls	1.078^cd	1.088^cd	1.135^bcd	1.052^d
Corn gluten feed	1.325^a	1.132^bc	1.157^bc	1.130^bcd
Distillers grains	1.237^ab	1.182^ab	1.197^b	1.153^bcd
SE	0.034	0.026	0.026	0.032
^a,b,c,d,e Means within a column with different superscripts differ (P < 0.05).