J. Anim. Sci. 2004. 82:3526-3536
© 2004 American Society of Animal Science
Effects of wet corn gluten feed and intake level on diet digestibility and ruminal passage rate in steers1
S. P. Montgomery*,
J. S. Drouillard*,2,
E. C. Titgemeyer*,
J. J. Sindt*,
T. B. Farran*,
J. N. Pike*,
C. M. Coetzer*,
A. M. Trater* and
J. J. Higgins
* Department of Animal Sciences and Industry and
and
Department of Statistics, Kansas State University, Manhattan 66506-1600
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Abstract
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Twelve ruminally cannulated Jersey steers (BW = 534 kg) were used in an incomplete Latin square design experiment with a 2 x 2 factorial arrangement of treatments to determine the effects of wet corn gluten feed (WCGF) and total DMI level on diet digestibility and ruminal passage rate. Treatments consisted of diets formulated to contain (DM basis) steam-flaked corn, 20% coarsely ground alfalfa hay, and either 0 or 40% WCGF offered once daily for ad libitum consumption or limited to 1.6% of BW (DM basis). Two consecutive 24-d periods were used, each consisting of 18 d for adaptation, 4 d for collection, and a 2-d in situ period. Rumens of all steers were evacuated once daily at 0, 4, 8, and 12 h after feeding. Chromic oxide (10 g/[steer•d]) was fed as a digestibility marker, and steers were pulse-dosed with Yb-labeled alfalfa hay to measure ruminal particulate passage rate. Dacron bags containing 5 g of steam-flaked corn, WCGF, or ground (2-mm screen) alfalfa hay were placed into the rumens of all steers and removed after 3, 6, 12, or 48 h. Wet corn gluten feed increased percent apparent total-tract digestion of OM (P < 0.01), NDF (P < 0.01), and starch (P < 0.03), decreased (P < 0.01) ruminal total VFA concentration, increased (P < 0.01) ruminal NH3 concentration, and increased (P < 0.01) ruminal pH. Wet corn gluten feed also increased (P < 0.01) ruminal passage rate of Yb. Limit feeding decreased (P < 0.01) percent apparent total-tract digestion of both OM and NDF, ruminal total VFA concentration (P < 0.01), and ruminal fill (P < 0.01), but increased (P < 0.01) ruminal NH3 concentration. Apparent total-tract digestion of starch was not affected (P = 0.70) by level of DMI. A DMI level x hour interaction (P < 0.01) occurred for ruminal pH. Limit feeding increased ruminal pH before and 12 h after feeding, but decreased ruminal pH 4 h after feeding compared with diets offered ad libitum. A diet x DMI level interaction (P < 0.02) occurred for in situ degradation of alfalfa hay, with dietary addition of WCGF increasing (P < 0.02) the extent of in situ alfalfa hay degradation in steers fed for ad libitum consumption. This study suggests that WCGF increases OM and NDF digestion, and that limit feeding diets once daily might depress OM and NDF digestion, possibly due to decreased stability of the ruminal environment.
Key Words: Cattle • Digestibility • Intake • Limit Feeding • Maize Gluten • Passage Rate
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Introduction
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Wet corn gluten feed (WCGF) is a by-product of the corn wet milling industry and consists mainly of corn bran and corn steep liquor. Although the effects of WCGF on diet digestibility and passage rate in ruminants offered diets for ad libitum consumption have previously been reported (Firkins et al., 1985
), data are limited regarding the effects of WCGF on diet digestibility and passage rate in cattle that are limit-fed.
Limit feeding has been shown to be an effective method of growing cattle (Lake, 1987; Loerch, 1990; Sip and Pritchard, 1991). However, limit-fed cattle often exhibit meal-eating behavior (i.e., rapidly consuming large amounts of a diet; Löest et al., 2001; Montgomery et al., 2003a,b). Rapid ingestion of highly fermentable diets as a result of meal-eating behavior might cause feed-related metabolic disorders such as acidosis, founder, and bloat. Soto-Navarro et al. (2000) reported that increasing feeding frequency of limit-fed steers consuming high-concentrate diets resulted in a more stable ruminal environment by decreasing the amount of feed ingested during each feeding. Because WCGF supplies dietary energy in the form of fermentable fiber, and not starch, and has been reported to decrease the rate of DMI by cattle limit-fed growing diets (Montgomery et al., 2003b), WCGF might decrease negative effects of meal-eating on ruminal fermentation as well. The objective of this study was to determine the effects of WCGF on diet digestibility, passage rate, and ruminal fermentation in diets either offered for ad libitum consumption or limit-fed once daily. Our hypothesis was that meal-eating behavior in limit-fed cattle might provide for unstable ruminal conditions, and that replacing steam-flaked corn with a fermentable fiber source such as WCGF might decrease negative effects of meal-eating behavior on the ruminal environment.
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Materials and Methods
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Twelve ruminally cannulated Jersey steers weighing 534 ± 52 kg were used in an incomplete Latin square design experiment with a 2 x 2 factorial arrangement of treatments. Treatments were assigned randomly to steers and consisted of diets (Table 1) formulated to contain (DM basis) steam-flaked corn, 20% coarsely ground alfalfa hay and either 0 or 40% WCGF. Diets were offered either for ad libitum consumption (2.1 and 2.6% of BW daily, respectively, for 0 and 40% WCGF diets) or restricted to 1.6% of BW (DM basis). Therefore, factors were 0 or 40% WCGF and intake level. Restricting DMI to 1.6% of BW was based on Montgomery et al. (2003b), where heifers were fed similar diets at this level of intake. Two consecutive 24-d periods were used, each consisting of 18 d for adaptation, 4 d for collection, and a 2-d in situ incubation period. At the start of the experiment, steers were individually weighed, placed into partly covered individual pens containing slatted floors, and allowed ad libitum access to water. Diets were fed at 0800 throughout the experiment. Feed refusals were collected before feeding on d 19 through 22 and weighed, and a subsample was obtained and immediately frozen at – 20°C. Representative samples of each diet were obtained before feeding on d 18 through 21 and were immediately frozen at – 20°C.
Fecal output was estimated by feeding 10 g/(steer(d) of chromic oxide, hand-mixed into the diet of each steer immediately before feeding on d 12 through 22. On d 19 through 22, fecal samples were collected from the rectum of steers three times daily and immediately frozen at – 20°C. Fecal samples were collected every 8 h, with the sampling time advanced by 2 h each day, so that a fecal sample was obtained every 2 h in a 24-h period during the 4-d collection period. Fecal samples and feed refusals for each steer in each period were thawed, composited, dried at 55°C, and ground in a Wiley mill (Thomas Scientific, Swedesboro, NJ) to pass a 1-mm screen. Chromium concentrations of feces and feed refusals were determined by atomic absorption spectrophotometry with an acetylene/air flame (Perkin Elmer 3110, Norwalk, CT) according to Williams et al. (1962).
On d 19 through 22 of each period, ruminal contents of each steer were evacuated once daily at 0, 4, 8, and 12 h after feeding. Ruminal contents were immediately weighed, sampled, and placed back into the rumen. Samples consisted of whole contents (approximately 500 g), as well as ruminal fluid (strained through four layers of cheesecloth). A portable pH meter was used to measure pH of strained ruminal fluid, after which 8 mL of strained ruminal fluid was added to 2 mL of 25% (wt/vol) m-phosphoric acid. All samples were frozen at – 20°C immediately after collection.
At the completion of the experiment, ruminal fluid samples were thawed and centrifuged at 30,000 x g for 20 min, and a portion of the supernatant fluid was analyzed for VFA concentrations using gas chromatography (Hewlett-Packard 5890A, Palo Alto, CA; 183 cm x 0.635 cm column; Supelco column packing, Bellefonte, PA), with N2 as the carrier gas, a flow rate of 80 mL/min, and a column temperature of 130°C. A portion of the supernatant fluid was analyzed for NH3 concentrations using a Technicon Autoanalyzer III (Bran and Luebbe, Elmsford, NY). Samples of whole ruminal contents were lyophilized to determine DM content.
Solid passage rates were measured for ruminal contents using Yb-labeled alfalfa hay. On d 19, immediately before feeding at 0800, 100 g of coarsely ground alfalfa hay (7.62 mm) containing 6 g of Yb was mixed into the 0 h evacuated ruminal contents of all steers. The labeling of alfalfa hay with YbCl3 was accomplished similar to Titgemeyer et al. (2004) by applying 100 mL of a YBCl3-containing solution to 100-g samples of the hay followed by drying at 39°C. Because washing procedures were not employed to remove unbound Yb from the alfalfa hay, migration of Yb to other feedstuffs might have occurred within the rumen, and data should be interpreted accordingly. Because meal-eating behavior disrupts ruminal steady state conditions and consequently invalidates marker kinetics based on samples collected within a single, disrupted period, ruminal samples were collected at 24, 48, 72, and 96 h after dosing to allow for measurement of ruminal Yb concentrations at similar times each day and subsequently for calculation of marker kinetics related to passage over a day long period. Samples were strained through four layers of cheesecloth, and the solid fraction was immediately frozen at – 20°C. Solids were dried at 55°C, ground to pass a 1-mm screen, and subsequently ashed at 450°C for 8 h. Ashed residues were then solubilized in a solution containing 3 N HCl and 3 N HNO3 according to Ellis et al. (1980), and were analyzed for Yb by using atomic absorption spectrophotometry with an acetylene/nitrous oxide flame.
Diet samples were composited by period, dried at 55°C, and then ground to pass through a 1-mm screen. Diet, orts, and fecal samples were dried at 105°C in a forced-air oven to determine DM, and ashed at 450°C for 8 h to determine OM. Determination of NDF was conducted according to Van Soest et al. (1991). Because of the high starch content of the diet and ort samples, 0.10 mL of heat-stable 
-amylase (Sigma Chemical, St. Louis, MO) was added to aid filtration. Starch content was determined according to Herrera-Saldana and Huber (1989) using a Technicon Autoanalyzer III to measure free glucose (Gochman and Schmitz, 1972).
In situ incubations were conducted similar to Sindt et al. (2003). On d 23, before steers were fed, Dacron bags (10 x 20 cm, pore size 50 µm) containing 5 g (as-is) of steam-flaked corn, WCGF, or ground (2 mm) alfalfa hay were placed into nylon mesh bags containing a weight and a blank Dacron bag. The nylon mesh bags and their contents were soaked in warm water for 20 min and then placed into the rumen of each steer. Dacron bags were removed after 0, 3, 6, 12, and 48 h, and soluble fractions were immediately determined from 0-h bags, which were soaked in warm water for 20 min, but were never placed into the rumen. At the time of removal, bags were hand-rinsed under cold tap water until rinse water was clear and then dried at 105°C in a forced-air oven. During Period 1, one steer displaced its ruminal cannula, so these in situ data were not included in the analysis.
Calculations and Statistical Analyses.
Fecal output for each steer within each period was calculated as the amount of chromic oxide consumed (g/d) divided by the chromic oxide concentration in the feces (g/g of DM). Digestibilities of OM, NDF, and starch were calculated by the following formula: [1 – (fecal output of nutrient/ intake of nutrient)] x 100.
Particulate passage rates were determined using the REG procedure of SAS (SAS Inst., Inc., Cary, NC) to regress the natural logarithms of Yb concentration against time (Grovum and Williams, 1973). The resulting slopes represented passage rates. For in situ data, sample DM was characterized as consisting of A, B, and C fractions, where A represented 100% degradable DM (determined by 0-h bags), B represented potentially degradable DM, and C represented undegradable DM (determined by 48-h bags). Rate of in situ DM disappearance of the B fraction was calculated by the following equation:
where DM = dry matter disappearance at time (t) and k = rate of degradation of the B fraction (Vanzant et al., 1996). The percentage of B fraction remaining was transformed to its natural logarithm, and values were then regressed against time using the REG procedure of SAS to determine degradation rates (slopes) of B fractions.
All digestibility, particulate passage rate, and in situ data were analyzed using the mixed procedure of SAS as an incomplete Latin square with a 2 x 2 factorial treatment structure. Factors in the model included diet, intake, and diet x intake, and random effects consisted of steer and period. Ruminal fill, VFA, NH3, and pH data were analyzed as an incomplete Latin square with repeated measures over time; the model included fixed effects of diet, intake, time, and the two- and three-way interactions. Random effects consisted of steer and period. The covariance structure was compound symmetry.
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Results and Discussion
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Effect of Diet
Digestibility and passage rate data are shown in Table 2. Wet corn gluten feed increased (P < 0.02) DMI of steers allowed ad libitum access to diets, and increased apparent total-tract digestibility of OM (P < 0.01), NDF (P < 0.01), and starch (P < 0.03). Green et al. (1987) found that dry corn gluten feed linearly increased starch digestion in cattle fed corn silage-based diets and concluded that a decrease in negative associative effects might have been responsible. Also, replacing steam-flaked corn with WCGF decreased starch intake, which might have affected apparent total-tract starch digestion. Karr et al. (1966) decreased starch digestibility in the small intestine of steers by feeding diets containing increasing amounts of ground corn, and Kreikemeier et al. (1991) reported decreased starch digestion in the small intestine of steers when increasing starch was abomasally infused.
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Table 2. Effects of wet corn gluten feed and dry matter intake level on apparent total tract digestibility and ruminal passage rate of steers fed diets containing either 40% wet corn gluten feed offered ad libitum or limit-fed, or 0% wet corn gluten feed offered ad libitum or limit-fed
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A diet x DMI level interaction was detected (P < 0.04) for particulate passage rate. For steers fed WCGF, ruminal particulate passage rate was numerically higher in steers fed for ad libitum consumption, whereas it was numerically higher in steers consuming the corn diet when they were limit-fed. In addition to the diet x DMI level interaction, the diet containing WCGF increased (P < 0.01) ruminal particulate passage rate. For steers fed WCGF, the increase in NDF digestion might have contributed to the increase in particulate passage rate. A decrease in particle size facilitates passage of feed particles from the rumen. Conceivably, increasing the digestibility of an ingredient such as alfalfa hay would accelerate particle size reduction, and thereby allow for faster passage from the rumen. Because WCGF replaced a portion of steam-flaked corn in our experiment, the increase in NDF digestibility might be attributed to decreased negative associative effects of starch on NDF digestion (Firkins, 1997), to a greater digestibility of WCGF NDF compared with alfalfa hay NDF, or to both. Allen and Grant (2000) reported greater passage rates when WCGF rather than alfalfa silage replaced a portion of the dry-rolled corn in lactating dairy cow diets, and suggested that differences were due to differences in particle size and specific gravity. Allen and Grant (2000) also observed numerically greater passage of WCGF when WCGF replaced corn in diets containing 40% alfalfa silage. Because Yb-labeled alfalfa hay was not washed before it was administered in our experiment, migration of Yb might have occurred, making it unclear whether passage rates reflect movement of only alfalfa hay from the rumen.
A diet x DMI level x hour interaction (P < 0.02) was observed for liquid fill (Figure 1). Steers limit-fed the WCGF diet had the greatest liquid fill 4 h after feeding, whereas steers receiving the other three treatments followed a pattern similar to each other over time. This finding is likely a result of both increased moisture content of the diet containing WCGF and increased rate of DMI in response to limit feeding. Diet did not affect (P = 0.63) ruminal DM fill (Figure 2). Steers consuming the diet containing WCGF had a higher (P < 0.01) ruminal pH (Figure 3). The higher ruminal pH in steers consuming the diet containing WCGF in our experiment is in agreement with Sindt et al. (2002) who fed steers finishing diets containing steam-flaked corn and either 0, 30, or 60% WCGF. Grant (1994) reported decreased in vitro NDF digestion of alfalfa hay when pH was decreased from 6.8 to 5.5. Though the differences in ruminal pH were of a lesser magnitude, it is conceivable that the higher ruminal pH in steers consuming the diet containing WCGF in our experiment might have contributed to the greater NDF digestion observed. A diet x hour interaction (P < 0.01) was observed for ruminal NH3 concentrations (Figure 3), with steers fed the diet containing WCGF having greater ruminal NH3 concentrations 4 h after feeding compared with steers fed the diet without WCGF. This suggests that corn steep liquor protein is readily degraded in the rumen, and replacing steam-flaked corn with WCGF most likely decreased the amount of readily available carbohydrate, which might have decreased the ability of ruminal bacteria to utilize free NH3 for microbial protein synthesis.
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Figure 1. Ruminal liquid fill of steers fed diets containing 40% wet corn gluten feed (WCGF) offered ad libitum (WCGF Ad Lib), 40% WCGF limit-fed (WCGF Limit-Fed), 0% WCGF offered ad libitum (Corn Ad Lib), or 0% WCGF limit-fed (Corn Limit-Fed). Values are means ± SEM; n = 6. A diet x DMI level x hour interaction (P < 0.01) was observed for liquid fill. Steers limit-fed the WCGF diet had the greatest liquid fill at 4 h after feeding, whereas steers offered the corn diet had the least liquid fill at 4 h after feeding. Diet x hour (P < 0.01); DMI level x hour (P < 0.01).
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Figure 2. Effect of diet on ruminal DM fill of steers fed diets containing 40% wet corn gluten feed (WCGF) or 0% WCGF (Corn). Values are means ± SEM; n = 12. Diet did not affect (P = 0.63) ruminal DM fill.
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Figure 3. Effect of diet on ruminal pH and NH3 concentrations of steers fed diets containing 40% wet corn gluten feed (WCGF) or 0% WCGF (Corn). Values are means ± SEM; n = 12. Steers fed diets containing WCGF had a higher (P < 0.01) ruminal pH. A diet x hour interaction (P < 0.01) was observed for ruminal NH3 concentrations, with steers fed the diet containing WCGF having greater ruminal NH3 concentrations 4 h after feeding than diets without WCGF. Effect of diet on ruminal NH3 concentrations (P < 0.01).
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Feeding WCGF decreased (P < 0.01) total VFA concentration in the rumen (Figure 4) and ruminal concentration of acetate (P < 0.01; Figure 4). The decrease in total VFA concentration in steers consuming the diet containing WCGF is in agreement with the results of Krehbiel et al. (1995) and Sindt et al. (2002), and it likely contributed to the higher ruminal pH observed. A diet x hour interaction (P < 0.01) was present for ruminal propionate, valerate, and isobutyrate concentrations (Figure 4). Ruminal propionate concentrations in steers fed the diet containing WCGF were lower, increased at a slower rate, and plateaued by 4 h after feeding, whereas ruminal valerate concentrations in steers fed diets without WCGF were lower, increased at a slower rate, and plateaued by 4 h after feeding. Ruminal isobutyrate concentrations in steers fed diets without WCGF were lower at 4 h after feeding than those of steers fed WCGF, and the concentration decreased over time. Isovalerate concentrations were lower (P < 0.05) in steers fed diets containing WCGF (Figure 4). A diet x hour (P < 0.04), as well as a diet x DMI level (P < 0.05) interaction occurred for ruminal concentration of butyrate (Figure 5). Diets containing WCGF offered for ad libitum consumption provided the greatest butyrate concentrations, with butyrate concentrations increasing steadily over time. Addition of WCGF to the diet also increased (P < 0.01) the ruminal acetate:propionate ratio (2.1 and 1.6 for steers fed the diet containing WCGF or corn, respectively; SEM = 0.11), which is consistent with observations of Sindt et al. (2002).
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Figure 4. Effect of diet on ruminal VFA concentrations of steers fed diets containing 40% wet corn gluten feed (WCGF) or 0% WCGF (Corn). Values are means ± SEM; n = 12.
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Figure 5. Ruminal butyrate concentrations of steers fed diets containing 40% wet corn gluten feed (WCGF) offered ad libitum (WCGF Ad Lib.), 40% WCGF limit-fed (WCGF Limit-Fed), 0% WCGF offered ad libitum (Corn Ad Lib), or 0% WCGF limit-fed (Corn Limit-Fed). Values are means ± SEM; n = 6. A diet x hour (P < 0.04), as well as a diet x DMI level (P < 0.05) interaction occurred. Steers fed the diet containing WCGF offered for ad libitum consumption had the greatest ruminal concentrations of butyrate. Ruminal butyrate concentrations increased steadily over time in steers fed the diet containing WCGF offered for ad libitum consumption. Effect of diet (P < 0.02); effect of DMI level (P < 0.02).
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Diet did not affect the rate of in situ DM disappearance (Table 3) of WCGF (P = 0.99), steam-flaked corn (P = 0.74), or alfalfa hay (P = 0.44); however, extent of in situ DM degradation of steam-flaked corn (P < 0.03) was increased for steers fed WCGF and supports the observed increase in apparent total-tract digestibility of starch in steers fed the diet containing WCGF. Sindt et al. (2003) observed a numerical increase in the extent of in situ DM degradation of steam-flaked corn in steers fed diets containing 45% WCGF compared with 25% WCGF. Combined, these data suggest that the extent of in situ DM degradation of steam-flaked corn is increased in cattle fed diets containing WCGF, possibly because WCGF provides for a ruminal environment more conducive to in situ starch digestion. The greater Fraction A of WCGF relative to steam-flaked corn and alfalfa hay likely can be attributed to the water soluble steep liquor component of WCGF, with 43.3% of WCGF DM disappearing at 0 h. A diet x DMI level interaction (P < 0.02) was observed for extent of in situ DM degradation of alfalfa hay, with steers allowed ad libitum access to diets containing no WCGF having the lowest in situ DM degradation of alfalfa hay. Thus, either restriction of intake or feeding WCGF was able to improve the extent of in situ DM disappearance for alfalfa hay, but these responses were not additive.
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Table 3. Rate and extent of in situ dry matter disappearance for wet corn gluten feed, alfalfa hay, and steam-flaked corn in steers fed diets containing either 40% wet corn gluten feed offered ad libitum or limit-fed, or 0% wet corn gluten feed offered ad libitum or limit-fed
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Effect of Intake Level
Limit feeding decreased (P < 0.01) apparent total-tract digestion of OM and NDF. This result is in contrast to observations of Galyean et al. (1979) and Murphy et al. (1994b), who both reported increased digestibilities with limit feeding. However, Galyean et al. (1979) fed steers eight times daily with a diet containing 84% cracked corn. Multiple offerings of high-grain diets to limit-fed cattle decreases the amount of grain ingested during a meal, and this feeding strategy might decrease negative effects of meal-eating behavior by providing a more stable ruminal environment. Murphy et al. (1994b) limit-fed a diet containing 75% corn silage to steers once daily, but because of the greater fiber content of the diet, any negative effects on the ruminal environment due to meal-eating behavior might have been decreased. Soto-Navarro et al. (2000) fed steers a 90% concentrate diet at 90% of ad libitum consumption either once or twice daily and reported that feeding twice daily resulted in a more stable ruminal environment. Shabi et al. (1999) fed Holstein cows diets containing 40% corn two or four times daily and reported that feeding four times daily decreased diurnal variation of both ruminal pH and ruminal NH3 concentration, and also increased total-tract digestibility of OM. In our experiment, diets were offered for feeding once daily, regardless of intake level. When steers in our experiment were offered diets once daily for ad libitum consumption, feed intake was presumably distributed throughout a 24-h period. Such eating behavior might have resulted in more frequent meals and, it can be presumed, a more stable ruminal environment. Conversely, when the same diets were limit-fed, steers exhibited meal-eating behavior by rapidly consuming their respective diets. Although rate of DMI was not measured in our experiment, Montgomery et al. (2003b) limit-fed steers similar growing diets once daily at 1.6% of BW (DM basis) and reported that the entire ration was consumed by approximately 2 h after feeding. Rapid ingestion of diets due to meal-eating behavior also might decrease the degree of mastication and subsequent communition of feed particles, further contributing to the decrease in apparent total-tract digestibility of OM and NDF observed. Reinhardt et al. (1998) fed Holstein steers a diet based on whole corn, either offered for ad libitum consumption or limit-fed, and reported increased masticate particle size of corn with limit feeding.
Apparent total-tract digestion of starch was not affected by DMI level (P = 0.28), but was numerically greater for limit-fed steers. Zinn et al. (1995) observed that DMI level did not affect total-tract starch digestion in steers fed steam-flaked corn-based diets, and Murphy et al. (1994b) reported that DMI level did not affect total-tract digestion of starch in steers limit-fed diets based on corn silage or whole corn. In contrast, Galyean et al. (1979) reported decreased total-tract digestibility of starch with increasing feed intake.
A diet x DMI level interaction was detected (P < 0.04) for ruminal particulate passage rate because steers limit-fed the diet containing WCGF had a slower particulate passage rate than when the same diet was offered for ad libitum consumption. Particulate passage rate was slightly increased when the corn diet was limit-fed relative to being offered for ad libitum consumption. Limit feeding has been reported to decrease ruminal passage rate in cattle compared with offering diets for ad libitum consumption (Galyean et al., 1979; Murphy et al., 1994b). The decrease in passage rate observed when cattle are limit-fed has provided some explanation for the increases in digestibility reported (Galyean et al., 1979; Murphy et al., 1994b). The failure to decrease passage rate with limit feeding in our experiment is likely related to differences in intake patterns in response to limit-feeding, and this lack of change in passage rate might help to explain the difference between our experiment and others (Galyean et al., 1979; Murphy et al., 1994b) with regard to the effects of intake level on digestibility.
Limit feeding decreased (P < 0.01) ruminal fill (DM basis); however, a DMI level x hour interaction (P < 0.01) was observed (Figure 6). Dry matter fill increased at a faster rate for limit-fed steers in the first 4 h after feeding, further illustrating the rapid ingestion of DM by limit-fed steers.
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Figure 6. Effect of DMI level on ruminal DM fill of steers offered diets ad libitum (Ad Lib) or limit-fed (Limit-Fed). Values are means ± SEM; n = 12. A DMI level x hour interaction (P < 0.01) was observed. Dry matter fill increased at a faster rate for limit-fed steers in the first 4 h after feeding. Limit feeding decreased (P < 0.01) ruminal DM fill compared with offering diets for ad libitum consumption.
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A DMI level x hour interaction was detected for ruminal pH (P < 0.01; Figure 7), NH3 concentration (P < 0.01; Figure 7), and total VFA concentration (P < 0.01; Figure 8). Ruminal pH of limit-fed steers was higher before feeding, became lower 4 h after feeding, and was again higher 12 h after feeding than that of steers allowed ad libitum access to diets. This finding is in contrast with Murphy et al. (1994a), who reported that decreasing intake up to 30% did not affect ruminal pH in steers fed once daily; however, Murphy et al. (1994a) fed steers diets based on whole-shelled and dry-rolled corn. In our experiment, steam-flaked corn was fed, which is fermented more rapidly in the rumen than less processed corn (Zinn et al., 2002). The combination of steam-flaked corn and increased rate of feed intake in limit-fed steers in our experiment might have been responsible for the differences in ruminal pH observed. Ruminal NH3 concentrations of limit-fed steers were greater at 0 and 4 h after feeding, but were similar between intake levels at 8 and 12 h after feeding. Murphy et al. (1994a) reported increased ruminal NH3 concentrations 3 h after feeding in steers limit-fed once daily compared with steers offered diets for ad libitum consumption and suggested that increased supplemental nonprotein N in the limit-fed diets was partially responsible. In our experiment, the same diets were offered across intake levels and the increase in ruminal NH3 concentrations observed 4 h after feeding was most likely a result of increased rate of feed intake in limit-fed steers. Ruminal total VFA concentrations increased rapidly through the first 4 h after feeding for limit-fed steers, then decreased, whereas ruminal total VFA concentrations for steers allowed ad libitum access to diets continued to increase up to 12 h after feeding. Ruminal total VFA concentrations follow a similar pattern to that of ruminal DM fill, indicating that rate of feed intake affects total VFA concentrations in limit-fed steers. The changes in ruminal pH, NH3, and total VFA concentrations due to limit feeding reflect the rapid ingestion and subsequent ruminal fermentation of dietary substrate, providing evidence that changes in the ruminal environment caused by meal-eating behavior might have contributed to the decrease in total-tract OM and NDF digestibility observed when diets were limit fed.
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Figure 7. Effect of DMI level on ruminal pH and NH3 concentrations of steers offered diets ad libitum (Ad Lib) or limit-fed (Limit-Fed). Values are means ± SEM; n = 12. A DMI level x hour interaction (P < 0.01) was observed for ruminal pH and NH3 concentrations (P < 0.01). Ruminal pH of limit-fed steers was higher at 0 h, became lower 4 h after feeding, and was again higher 12 h after feeding compared with steers allowed ad libitum access to diets. Ruminal NH3 concentrations of limit-fed steers were greater at 0 and 4 h after feeding, but were similar between intake levels at 8 and 12 h after feeding.
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Figure 8. Effect of DMI level on ruminal VFA concentrations of steers offered diets ad libitum (Ad Lib) or limit-fed (Limit-Fed). Values are means ± SEM; n = 12.
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A DMI level x hour interaction (P < 0.01) was present for ruminal acetate concentration (Figure 8
), with ruminal acetate concentrations increasing rapidly through the first 4 h after feeding for limit-fed steers, then decreasing, whereas ruminal acetate concentrations for steers allowed ad libitum access to diets remained greater over time. Limit feeding decreased (P < 0.01) ruminal propionate concentrations (Figure 8) and increased (P < 0.02) the ruminal acetate:propionate ratio (1.7 and 2.1 for steers offered diets for ad libitum consumption or limit-fed, respectively; SEM = 0.11) and is in agreement with Murphy et al. (1994a). An increase in the acetate:propionate ratio in limit-fed cattle would conceivably be undesirable because of decreased hepatic gluconeogenesis. Intake level did not affect ruminal concentrations of valerate (P = 0.70), isovalerate (P = 0.07), or isobutyrate (P = 0.15; Figure 8).
Rates of in situ DM disappearance (Table 3) of alfalfa hay, WCGF, or steam-flaked corn were not affected by DMI level (P < 0.67). However, the diet x DMI level interaction observed for extent of in situ DM degradation of alfalfa hay agrees with observed apparent total-tract NDF digestibility measurements for the corn diets, but not for the WCGF diets.
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Implications
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Replacing steam-flaked corn with wet corn gluten feed seems to increase ruminal pH and apparent total-tract digestibility of organic matter, neutral detergent fiber, and starch in diets containing steam-flaked corn. Limit feeding diets containing steam-flaked corn once daily might decrease apparent total-tract organic matter and neutral detergent fiber digestibility because of meal-eating behavior, which provides for rapid feed intake and a decrease in the stability of the ruminal environment. Multiple feedings might be warranted to obtain maximum benefit from the limit feeding of diets containing steam-flaked corn and 20% alfalfa hay, with or without 40% wet corn gluten feed.
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Footnotes
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1 Article No. 04-289-J from the Kansas Agric. Exp. Stn. 
2 Correspondence: Call Hall, Room 133 (phone: 785-532-1204; fax: 785-532-5681; e-mail: jdrouill{at}oznet.ksu.edu).
Received for publication March 10, 2004.
Accepted for publication August 9, 2004.
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Abstract
Introduction
