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Journal of Animal Science - Animal Nutrition

In vivo ruminal degradation characteristics and apparent digestibility of low-quality prairie hay for steers consuming monensin and Optimase1

 

This article in JAS

  1. Vol. 93 No. 8, p. 3941-3949
     
    Received: Dec 01, 2014
    Accepted: May 08, 2015
    Published: July 2, 2015


    2 Corresponding author(s): david.lalman@okstate.edu
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doi:10.2527/jas.2014-8772
  1. S. K. Linneen*,
  2. A. R. Harding*,
  3. M. T. Smallwood*,
  4. G. W. Horn*,
  5. J. S. Jennings,
  6. C. L. Goad and
  7. D. L. Lalman 2*
  1. * Department of Animal Science, Oklahoma State University, Stillwater 74078
     Agrilife Research and Extension Center, Texas A&M, Amarillo 79106
     Department of Statistics, Oklahoma State University, Stillwater 74078

Abstract

Seven ruminally cannulated crossbred steers (BW = 720 ± 62 kg) were used in a randomized crossover design (4 periods, each 18 d) to evaluate in vivo rumen characteristics and apparent digestibility of steers consuming low-quality prairie hay and 1 of 4 isonitrogenous protein supplements. Treatments included 1) 40% CP (DM basis) cottonseed meal and wheat middlings–based supplement (Control), 2) a cottonseed meal and wheat middlings–based supplement with slow-release urea and a fibrolytic feed enzyme (Optimase; Alltech, Inc., Nicholasville, KY) designed to replace 30% of plant-based CP provided in the Control (OPT), 3) the Control plus 0.40 mg∙kg–1 BW∙d–1 monensin (Rumensin 90; Elanco Animal Health, Greenfield, IN; MON), and 4) the OPT plus 0.40 mg∙kg–1 BW∙d–1 monensin (COMBO). Steers were allowed ad libitum access to prairie hay (5.0% CP and 76% NDF) and were provided each respective supplement at 0800 h daily at a rate of 1.0 g/kg of BW. Steers were adapted to diets for 10 d before sample collection. Beginning on d 11, DMI was measured and samples were collected to determine apparent digestibility. On d 15 of the 18-d period, rumen fluid was collected 10 times over a 24-h period. Forage DMI was greater (P ≤ 0.02) for steers consuming the OPT compared with steers consuming the MON or COMBO, although forage DMI was not different (P = 0.10) among steers consuming the Control compared with steers consuming the OPT, MON, or COMBO. Steers fed the MON and COMBO had lower (P ≤ 0.05) passage rate compared with steers fed the Control and the OPT. The MON-fed steers had lower (P = 0.01) ruminal pH and increased (P = 0.03) propionate as a percentage of total VFA production. A time × treatment (P = 0.01) interaction was observed for ruminal NH3–N due to a rapid (0 to 1 h after feeding) increase followed by a quick (1 to 4 h after feeding) decline in NH3–N by steers consuming the OPT and COMBO that was not observed for steers consuming all other treatments. Apparent digestibility of DM (P = 0.01) and NDF (P = 0.03) were improved for steers fed the COMBO supplement compared with steers consuming all other experimental supplements. This work suggests that the OPT may be an effective replacement for a portion of supplemental degradable intake protein in low-quality forage. Further research is necessary to determine if the combination of monensin and the Optimase consistently improves low-quality forage utilization.



INTRODUCTION

In the southern Great Plains, supplements for beef cattle grazing dormant winter forage typically contain degradable intake protein (DIP) sources such as cottonseed meal or soybean meal. Nonprotein nitrogen is a readily degradable source of DIP and is frequently a less expensive source of CP than cottonseed meal and soybean meal per unit N. Optimase (Alltech, Inc., Nicholasville, KY) combines a slow-release NPN source with a fibrolytic feed enzyme (FFE) with a N release rate similar to that of soybean meal (Kononoff et al., 2006; Akay et al., 2004). Fibrolytic feed enzymes have improved fiber digestion and increase particle passage rate (Murillo et al., 2000); however, others report no effect on forage digestibility (Piños-Rodríguez et al., 2002; Avellaneda et al., 2009).

Monensin is an ionophore commonly fed to feedlot cattle to improve efficiency due to reducing DMI or improving ADG (Bretschneider et al., 2008). Monensin consistently alters VFA production by increasing the concentration of propionate and reducing the concentration of acetate (McGuffey et al., 2001; Ellis et al., 2012). Turner et al. (1988) reported that cows maintained 12% more BW when they consumed 90% of the protein requirement plus monensin compared with cows fed 100% of the protein requirement with no monensin. Consequently, including monensin in supplements for grazing animals has the potential to reduce DMI and improve energy availability per unit of forage consumed. The objective of this study was to evaluate the effects of monensin or combining monensin and Optimase on forage intake, in vivo rumen fermentation characteristics, and apparent digestibility when beef steers consumed low-quality forage. We hypothesized that the combination of the potential advantages of monensin with a slow release urea plus FFE may allow for replacement of a portion of the oilseed meal in a low-quality forage supplement with little to no reduction in forage intake or digestibility.


MATERIALS AND METHODS

This experiment was conducted in accordance with an approved Oklahoma State University Animal Care and Use Committee protocol.

Animals and Experimental Diets.

This experiment was conducted at the Range Cow Research Center, North Range Unit, located approximately 16 km west of Stillwater, OK. Seven ruminally cannulated crossbred steers (BW = 720 ± 62 kg) were used in a randomized crossover design (4 periods, each 18 d). This study was designed to evaluate change in ruminal VFA concentration and pH over a 24-h time period and apparent digestibility of low-quality prairie hay. All steers consumed each supplement once. In each of the 4 sequences, 2 steers were randomly assigned by BW. One sequence had only 1 steer assigned due to an uneven number of animals. Sufficient adaptation periods between active experimental periods were determined from previous literature (Chabot et al., 2008; Winterholler et al., 2009) to be 10 d before initiating sample collection in each period. Although adaptation periods were scheduled and believed to be sufficient, sequences of treatments were chosen so that the design was balanced for carryover (residual) effects (Kuehl, 2000).

Steers were allowed ad libitum access to prairie hay (5.0% CP, 52% TDN, 76% NDF, and 52% ADF, DM basis) and provided 1 of 4 isonitrogenous treatment supplements (40% CP, DM basis) daily. Daily supplement feeding rate was 1.0 g/kg of BW and was selected by adjusting the feeding rate of the control treatment until a slightly negative DIP balance was achieved (–91 g/d DIP and + 202 g/d MP; model level 1; NRC, 1996). Our goal was to avoid excessive DIP in the control supplement because abundant DIP would mask potential differences in forage DMI and digestion due to experimental treatment effects. Model inputs included 1.8% of BW forage DMI, 78% of supplement CP as DIP in the control supplement (Winterholler et al., 2009), 62% forage DIP (% of CP; Krizsan et al., 2012), 54% total diet TDN, and 9.8% microbial efficiency (% of TDN intake; Karges et al., 1992; Hollingsworth-Jenkins et al., 1996; Köster et al., 1996). Treatments were balanced for Ca and P to meet NRC (1996) requirements. Experimental treatments (Table 1) included 1) 40% CP (DM basis) cottonseed meal and wheat middlings–based supplement (Control), 2) a cottonseed meal and wheat middlings–based supplement with slow-release urea and a FFE (Optimase) designed to replace 30% of plant-based CP provided in the Control (OPT), 3) the Control plus 0.40 mg∙kg–1 BW∙d–1 monensin (Rumensin 90; Elanco Animal Health, Greenfield, IN; MON), and 4) the OPT plus 0.40 mg∙kg–1 BW∙d–1 monensin (COMBO). Minimal (≤10%, DM basis) soybean hulls and corn were included in the OPT and COMBO supplements to balance energy and N. Steers were fed once daily at 0800 h. The BW used to determine feeding amount was recorded at the onset of each period after a 12-h withdrawal from feed and water. Steers were fed supplement at the rate of 1.0 g/kg BW for the duration of the study. The monensin dose was intended to approximate the maximum approved dose for reproducing beef cows, which is 200 mg/d or 0.40 mg∙kg–1 BW∙d–1 for a 499-kg beef cow.


View Full Table | Close Full ViewTable 1.

Supplement composition and amount of nutrients supplied daily to steers

 
Treatment1
Control
OPT
MON
COMBO
Item % of DM
Cottonseed meal 86.3 42.5 86.3 42.5
Wheat middlings 8.09 31.8 7.90 31.7
Cane molasses 5.50 5.50 5.50 5.50
Soybean hulls 10.0 10.0
Corn 5.00 5.00
Optimase 5.00 5.00
Rumensin 90 0.19 0.19
Vitamin A, 30,000 IU/g2 0.11 0.11 0.11 0.11
Vitamin E acetate 50% 0.05 0.05 0.05 0.05
Nutrient supplied, kg/d3
    DM 0.71 0.75 0.76 0.77
    CP 0.33 0.34 0.34 0.35
    TDN 0.63 0.62 0.65 0.63
    Crude fat 0.01 0.02 0.01 0.02
Chemical composition, %
    DM 90.0 89.7 90.0 89.7
    CP 40.1 40.8 40.1 40.8
    TDN 76.2 74.0 76.0 74.0
1Control = 40% CP (DM basis) cottonseed meal and wheat middlings–based supplement; OPT = a cottonseed meal and wheat middlings–based supplement with slow-release urea and a fibrolytic feed enzyme (Optimase; Alltech, Inc., Nicholasville, KY) designed to replace 30% of plant-based CP provided in the Control; MON = the Control plus 0.40 mg∙kg–1 BW∙d–1 monensin (Rumensin 90; Elanco Animal Health, Greenfield, IN); COMBO = the OPT plus 0.40 mg∙kg–1 BW∙d–1 monensin.
2Provided 32,960 IU of vitamin A per kilogram of supplement DM.
3Calculated based on average steer daily feed allotment per treatment for all periods.\

Apparent Digestibility.

Individual steer hay intake was recorded from d 11 to 15 of the 18-d collection period. Fecal grab samples were collected twice daily at 0800 and 1700 h per steer to estimate fecal output from acid detergent insoluble ash (ADIA) concentration. Subsamples of the supplements, hay, and orts were dried at 100°C for 48 h to determine DM. Supplement, hay, orts, and feces were dried at 50°C and ground in a Wiley mill (model 4; Thomas Scientific, Swedesboro, NJ) through at least a 1-mm screen before analysis. After grinding, samples were composited by steer within period. Composited samples were analyzed for CP, NDF, ADF, and ADIA. Neutral detergent fiber and ADF content were determined using an Ankom Fiber Analyzer (Ankom Technology, Macedon, NY) according to the manufacturer’s instructions. Samples were analyzed for N content using a Leco CN-628 N Analyzer (Leco Corp., St. Joseph, MI) to determine CP, and ADIA was determined as that remaining after complete combustion of the residue (Van Soest et al., 1991).

Rumen Fluid Collection.

Beginning on d 15 of the 18-d period, rumen fluid was collected by hand with a beaker from the ventral sac of each steer at 10 time points after feeding across 24 h. Feeding time was moved 1 h earlier on this day to accommodate all the rumen fluid collections. Collection times after feeding included 30 min (0730 h), 1 h (0800 h), 2 h (0900 h), 4 h (1100 h), 6 h (1300 h), 7.5 h (1430 h), 11.5 h (1830 h), 16.5 h (2330 h), 20.5 h (0330 h), and 24.5 h (0730 h). Steers were fed the following day (d 16) at 0700 h, 30 min before the final rumen fluid collection. Rumen fluid was strained through 4 layers of cheese cloth until approximately 100 mL of fluid was collected at each sampling. Approximately 2 mL of the sample was placed into a disposable-beaker to determine pH using a pH electrode (Oakton pH 6+; Oakton Instruments, Vernon Hills, IL). The pH reading was read in duplicate per sample per steer per collection and reported as an average. The remaining sample was placed into duplicate (2 tubes/steer) sterile 50-mL falcon tubes (Becton, Dickinson and Company, Franklin Lakes, NJ) containing 5 mL of HCl per 50-mL tube to terminate microbial growth. Samples were placed on ice and later stored at –20°C until analysis for VFA and ruminal NH3–N.

Samples were analyzed for VFA and NH3–N at the Langston University E (Kika) de la Garza American Institute for Goat Research Analytical Laboratory (Langston, OK). Concentrations of VFA were measured using gas chromatography (Hewlett-Packard 6890 gas chromatography, Hewlett-Packard, Mississauga, ON, Canada; 183 × 0.635 cm column, Supelco SP. 1200 packing, N2 carrier at 30 mL/min, and flame ionization detector at 250°C). Ruminal NH3–N was determined by automated analysis (Bran Luebbe AutoAnalyzer 3; SEAL Analytical, Mequon, WI).

Passage Rate.

On d 18 of the period, passage rate was determined by procedures described by Coblentz et al. (1999). Manual evacuation of rumen contents was conducted for all 7 steers before feeding (0 h) and 4 h after feeding. At each evacuation time, total rumen contents were weighed, mixed, and subsampled in triplicate and then returned to the rumen. The samples were subsequently dried for 96 h at 50°C in a forced-air oven before grinding through a 2-mm screen with a Wiley mill. Concentration of ADF and ADIA (ashed ADF residue) were determined for the concentrate, hay, orts, and rumen contents per laboratory assays as previously described. The fractional passage rate of ADIA (Kp) was determined by dividing the mean ADIA intake (g/h) by the mean (from the 0- and 4-h ruminal evacuations) ruminal mass of ADIA (Waldo et al., 1972). The hourly ADIA intake for each steer was calculated by dividing total daily intake of ADIA by 24 h. The reported Kp values (Table 2) represent the mean passage rate among steers consuming each respective treatment.


View Full Table | Close Full ViewTable 2.

Effects of feeding monensin and Optimase on DMI, rumen fill, and passage rate

 
Treatment1
Item Control OPT MON COMBO SE2 Probability, P <
No. of observations 7 7 7 7
Average BW, kg 711 715 727 728 15.9 0.32
Forage DMI, % of BW 1.08ab 1.22a 1.04b 1.02b 0.05 0.07
Rumen contents
    Fill, % of BW 1.79 1.75 1.53 1.53 0.32 0.88
    ADIA,2 % 3.20 3.37 3.38 3.39 0.11 0.66
    Passage rate (Kp3), %/h 1.41a 1.46a 0.83b 1.01b 0.07 0.05
a,bMeans within a row with different superscripts differ (P < 0.05).
1Control = 40% CP (DM basis) cottonseed meal and wheat middlings–based supplement; OPT = a cottonseed meal and wheat middlings–based supplement with slow-release urea and a fibrolytic feed enzyme (Optimase; Alltech, Inc., Nicholasville, KY) designed to replace 30% of plant-based CP provided in the Control; MON = the Control plus 0.40 mg∙kg–1 BW∙d–1 monensin (Rumensin 90; Elanco Animal Health, Greenfield, IN); COMBO = the OPT plus 0.40 mg∙kg–1 BW∙d–1 monensin.
2ADIA = acid detergent insoluble ash.
3Kp = fractional passage rate of ADIA.

Statistical Analysis.

Steer DMI, degradation characteristics, and apparent digestibility were analyzed using the MIXED procedure of SAS (SAS Inst. Inc., Cary, NC) and the Satterthwaite approximation for degrees of freedom. For steer DMI and average BW, the model included supplement treatment, steer, sequence, and carryover as independent variables. Treatment effects and carryover effects were tested using methods by Ratkowsky et al. (1993). Ruminal fluid characteristics (NH3–N, VFA, and pH) were analyzed using mixed models methods for repeated measures analysis using the GLIMMIX procedure of SAS. Fixed effects included supplement treatment, sequence, period, time, and time by treatment interaction. Random subject effects were period by steer within sequence. For all analyses, when the P-value for the F-statistic was ≤ 0.10, least squares means were separated and reported using α ≤ 0.05.


RESULTS

Forage Intake and Rumen Contents.

Forage DMI was greater (P ≤ 0.02) for steers consuming the OPT compared with steers consuming the MON or COMBO, although steers consuming the Control treatment had DMI that was not different than forage DMI of steers consuming the MON (P = 0.53), OPT (P = 0.10), and COMBO (P = 0.42; Table 2) treatments. There were no differences in rumen fill or rumen content ADIA concentration (P ≥ 0.48) due to dietary treatment. Passage rate was reduced (P = 0.05) for steers fed the MON and COMBO compared with steers fed the Control and OPT.

Apparent Digestibility.

Dry matter (P = 0.01) and NDF (P = 0.03) apparent digestibilities were improved when steers consumed the COMBO compared with steers consuming the Control, OPT, and MON (Table 3). The COMBO treatment improved (P ≤ 0.01) ADF digestibility compared with the OPT and MON treatments, although the ADF digestibility of steers consuming the COMBO treatment was not different (P = 0.25) from steers consuming the Control treatment. Digestibility of ADF was reduced (P ≤ 0.01) when steers consumed the OPT compared with steers consuming the Control, OPT, and MON. There was no treatment effect on CP apparent digestibility (P = 0.47).


View Full Table | Close Full ViewTable 3.

Effects of feeding monensin and Optimase on apparent digestibility of DM, NDF, ADF, and CP

 
Treatment1
Item Control OPT MON COMBO SE2 Probability, P <
Apparent digestibility, %
    DM 63.0b 60.5b 62.0b 68.2a 1.48 0.01
    NDF 64.5b 60.9b 63.3b 69.8a 1.87 0.03
    ADF 57.5ab 46.2c 53.4b 60.5a 1.79 <0.01
    CP 52.9 53.1 51.3 58.9 3.72 0.47
a–cMeans within a row with different superscripts differ (P < 0.05).
1Control = 40% CP (DM basis) cottonseed meal and wheat middlings–based supplement; OPT = a cottonseed meal and wheat middlings–based supplement with slow-release urea and a fibrolytic feed enzyme (Optimase; Alltech, Inc., Nicholasville, KY) designed to replace 30% of plant-based CP provided in the Control; MON = the Control plus 0.40 mg∙kg–1 BW∙d–1 monensin (Rumensin 90; Elanco Animal Health, Greenfield, IN); COMBO = the OPT plus 0.40 mg∙kg–1 BW∙d–1 monensin.
2Represents the average SE.

Rumen Fluid Characteristics.

There was a treatment × time interaction (P = 0.02; data not shown) for pH because pH declined over time after feeding for all treatments although at different magnitudes. Overall treatment means for ruminal pH indicate inclusion of monensin in the supplement decreased (P < 0.01; Table 4) mean pH over the 24-h collection.


View Full Table | Close Full ViewTable 4.

Effects of feeding monensin and Optimase on ruminal pH and VFA

 
Treatment1
Item Control OPT MON COMBO SE2 Probability, P <
Ruminal pH 6.87a 6.88a 6.75b 6.96a 0.04 <0.01
VFA, % of total
    Acetate 74.4a 74.2a 72.4b 73.3ab 0.58 0.07
    Propionate 15.8b 16.0b 18.6a 18.0ab 0.79 0.10
    Butyrate 9.6 9.8 9.2 9.3 0.24 0.11
    Acetate:propionate 4.85a 4.86a 3.95b 4.07b 0.24 0.05
    Total VFA, mg/dL 70.4 78.5 66.9 71.3 3.09 0.23
a,bMeans within a row with different superscripts differ (P < 0.05).
1Control = 40% CP (DM basis) cottonseed meal and wheat middlings–based supplement; OPT = a cottonseed meal and wheat middlings–based supplement with slow-release urea and a fibrolytic feed enzyme (Optimase; Alltech, Inc., Nicholasville, KY) designed to replace 30% of plant-based CP provided in the Control; MON = the Control plus 0.40 mg∙kg–1 BW∙d–1 monensin (Rumensin 90; Elanco Animal Health, Greenfield, IN); COMBO = the OPT plus 0.40 mg∙kg–1 BW∙d–1 monensin.
2Represents the average SE.

As a percent of total VFA, acetate concentration was less and propionate concentration was greater (P ≤ 0.05) in MON-fed steers compared with Control- and OPT-fed steers and concentration of acetate and propionate was intermediate for steers fed the COMBO. Butyrate concentrations were not affected (P = 0.11) by treatment. The acetate:propionate ratio was decreased (P = 0.05) for steers consuming the MON and COMBO compared with steers fed the Control and OPT. Total VFA concentration was unaffected by treatment (P = 0.23).

A treatment × time interaction (P = 0.01; Fig. 1) for ruminal NH3–N was present. Ruminal NH3–N concentrations were greatest (P = 0.01) for steers consuming the COMBO treatment compared with steers consuming all other treatments until 6 h after feeding. From the 6- through the 24-h sampling times, NH3–N levels were not different among the Control, OPT, MON, or COMBO. Ruminal NH3–N concentrations for steers consuming the OPT were greater than for steers consuming the MON and Control and less than for steers consuming the OPT (P ≤ 0.05) at the 1- and 2-h sampling times. Finally, the NH3–N level among steers consuming either the Control or MON was not different (P > 0.17) for the duration of the 24-h collection.

Figure 1.
Figure 1.

Effect of feeding monensin and Optimase (Alltech, Inc., Nicholasville, KY) on ruminal fluid ammonia (NH3–N) concentration of steers over a 24-h period. Treatments included 40% CP (DM basis) cottonseed meal and wheat middlings–based supplement (Control), a cottonseed meal and wheat middlings–based supplement with slow-release urea and a fibrolytic feed enzyme (Optimase) designed to replace 30% of plant-based CP provided in the Control (OPT), the Control plus 0.40 mg∙kg–1 BW∙d–1 monensin (Rumensin 90; Elanco Animal Health, Greenfield, IN; MON), and the OPT plus 0.40 mg∙kg–1 BW∙d–1 monensin (COMBO). Largest SEM (range = 0.71–0.84). a–cMeans within time with different superscripts differ (P < 0.05).

 


DISCUSSION

The OPT and the COMBO experimental treatments used Optimase, which combines a FFE and a form of slow-release urea. Therefore, discussion of the effects of the OPT and COMBO treatments is restricted to the effects of the commercial product Optimase because the potential effects of the FFE and/or the slow release urea product cannot be separated.

Voluntary forage DMI was less than anticipated, with a mean of 1.1% BW. In previous work at our laboratory, steers fed prairie hay (47% ADF; McMurphy et al., 2014) or bermudagrass hay (49% ADF; Banta et al., 2011) and a supplement, similar to the Control, consumed 1.5 and 1.64% of BW of forage, respectively. Forage consumption less than 2% of BW in the current experiment could be due, in part, to heat stress as the experiment was conducted (June through September), lower than expected DIP and MP balances, or exceptionally low forage nutritive value (55% forage ADF) and palatability. Because forage DMI was lower than expected, DIP and MP balance was recalculated using model level 1 of the NRC (1996) with observed forage DMI and previously described model inputs. This calculation resulted in a predicted +64 g/d DIP balance and –82 g/d MP balance for the Control treatment.

Steers fed the MON and COMBO consumed less hay than steers fed the OPT, whereas forage DMI was intermediate and not significantly different for steers fed the Control. Reported effects of monensin supplementation on DMI in forage-fed beef cattle have been inconsistent. Both Ellis et al. (1984) and Bretschneider et al. (2008) suggested that ruminal fill regulating DMI may limit the response of monensin for cattle consuming forages. Furthermore, Bretschneider et al. (2008) summarized 13 experiments in a review of feeding monensin to grazing cattle and concluded that monensin did not increase DMI. Conversely, Lemenager et al. (1978a) reported a reduction in forage DMI during grass dormancy among cows consuming monensin. In the current experiment, low-quality forage DMI was not significantly different among mature steers fed the Control and MON experimental treatments. Measuring unprocessed (long-stemmed) forage DMI in mature beef cattle is difficult, and relatively little research has been published related to monensin supplementation in mature beef cattle consuming low-quality forage. Further research is necessary to determine if monensin consistently reduces voluntary forage consumption in mature beef cattle consuming low-quality forage.

When steers were supplemented with the MON and COMBO, particulate passage rate was reduced by approximately 42 and 30%, respectively, compared with the average of the Control and OPT passage rates. Passage rate of the OPT did not differ from that of the Control and passage rate of the MON did not differ from that of the COMBO, suggesting that the MON was the primary supplement ingredient influencing passage rate. This is likely due, in part, to reduced DMI among steers receiving the MON and COMBO. Lemenager et al. (1978b), Pond and Ellis (1979), and Pond et al. (1980) similarly found that particulate passage rate was decreased in monensin-supplemented cattle fed forage diets at ad libitum intakes. In fact, Lemenager et al. (1978b) reported that monensin reduced particulate passage rate by 44% in mature steers fed low-quality hay similar to the hay provided in this experiment.

Steers consuming the COMBO had greater apparent digestibility of DM and NDF than steers fed the Control, OPT, or MON. We are unaware of other data reporting the effects of Optimase or the effects of in combination with monensin on low-quality forage digestibility in beef cattle. A limited number of experiments have reported the effects of combining monensin and urea in a high-fiber diet (Davis and Erhart, 1976; Lemenager et al., 1978c; Poos et al., 1979; Vagnoni et al., 1995). Unfortunately, apparent digestibility was not measured in those studies. Considering monensin, slow-release urea, and FFE research separately, the effect of monensin on fiber digestion in previous literature is inconsistent. Monensin has been shown to improve (Faulkner et al., 1985; Sexten et al., 2011), reduce (Poos et al., 1979), or not affect (Dinius et al., 1976; Lemenager et al., 1978b) fiber digestibility. Certainly, monensin supplementation alone did not impact total tract DM, NDF, or ADF digestibility in this experiment when compared with Control-fed steers.

Previous research would indicate that feeding urea increases digestibility of OM and fiber when DIP is limited (Owens et al., 1980; Lee et al., 1987; Currier et al., 2004) although to a lesser extent compared with feeding a similar amount of N in the form of an oilseed meal (Farmer et al., 2004). In vitro studies reported an increase in DM digestibility using Optigen (Alltech, Inc.; Harrison et al., 2007, 2008), although additional in vivo studies reporting effects of Optimase or Optigen on fiber digestibility are unavailable.

Rode et al. (1999) and Hristov et al. (2008) verified improvement in fiber digestion when FFE in the form of xylanase was fed; however, multiple other studies have found no impact of FFE on digestibility (Piños-Rodríguez et al., 2002; Avellaneda et al., 2009; Giraldo et al., 2008). Fibrolytic feed enzymes are substrate specific, resulting in limited enzymatic activity if the forage substrate does not match the enzyme (White et al., 1993) and causing inconsistent research results. Additionally, Adesogan et al. (2014) identified multiple challenges to maximizing enzyme effectiveness, including ruminal pH. Xylanase activity is most optimal at a pH of 5 (Adesogan et al., 2014), which would suggest that the ruminal pH (μ = 6.8) in this study may have limited xylanase activity.

Overall treatment means for ruminal pH are reported even though there is a significant interaction, because the modest differences in pH response to treatments over time are thought to be of little biological significance (Rumsey et al., 1970). Monensin reduced ruminal pH in MON-fed steers compared with Control-fed steers, although this shift is considered to be modest and, again, of little biological significance. Other work suggests that pH will mirror the response of the acetate:propionate ratio in forage-fed cattle (Lana and Russell, 1997; Lana et al., 1998); therefore, this may reflect the increase in propionate concentration among steers consuming monensin.

Feeding the OPT or COMBO treatments did not significantly affect ruminal pH compared with steers fed the Control treatment. Wahrmund and Hersom (2007) reported that slow-release urea in the form of Optigen reduced mean pH among cows consuming bahiagrass hay. Other studies indicated that ruminal pH was unaffected when urea and high-fiber diets were fed (Köster et al., 2002; Farmer et al., 2004; Wahrmund and Hersom, 2007). Similarly, Giraldo et al. (2008) and Piños-Rodríguez et al. (2002) found that FFE did not affect pH in forage-fed sheep.

Previous research with urea (Lemenager et al., 1978c; Farmer et al., 2004) and slow-release NPN products (Taylor-Edwards et al., 2009) suggests that Optimase supplementation may not influence VFA concentrations. Monensin increases propionate at the cost of acetate and butyrate in both concentrate and forage diets (Lemenager et al., 1978b; Lana and Russell, 1997; Ellis et al., 2012). These results indicate then when steers consume low-quality forage diets with a modest supply of DIP, monensin shifts VFA production in favor of propionate. The response in acetate:propionate ratio for steers consuming the COMBO treatment was not different from the response observed when steers consumed the MON treatments, indicating that this response can be attributed to monensin.

During the first 2 h after feeding, ruminal NH3–N concentration was relatively low for steers consuming the MON and Control treatments and lower than expected for all treatment groups from h 4 through 24. According to Satter and Slyter (1974), the level in which maximum microbial protein synthesis occurs is 5 mg NH3–N/100 mL, with 2 mg NH3–N/100 mL limiting microbial protein synthesis. The average ruminal NH3–N concentration in this study for Control-fed steers was 1.06 mg NH3–N/100 mL. The concentration of NH3–N among steers in all treatments in the current study returned to a baseline level around 6 h.

Altogether, the relatively low concentration of NH3–N for the Control- and MON-treated steers and the low concentration of NH3–N for all other treatment groups after 4 h after feeding combined with low forage DMI and low passage rate indicates that DIP supply may have been underestimated using model level 1 (NRC, 1996) under these conditions. The negative MP balance previously mentioned and estimated using observed forage DMI suggests little blood urea nitrogen would be available to recycle N to the rumen to offset a potential negative DIP balance (Sletmoen-Olson et al., 2000; Reed et al., 2007).

Steers consuming the COMBO treatment had substantially greater NH3–N concentrations for the first 4 h after feeding compared with steers receiving the Control, OPT, or MON treatments. Similarly, NH3–N concentration for OPT-fed steers was lower than that for COMBO-fed steers although greater than that for MON- and Control-fed steers during h 1 and 2 after feeding. These results indicate more rapid ruminal N availability after feeding when cattle are fed a proportion of DIP from the OPT compared with cottonseed meal. However, the additional NH3–N after feeding in steers receiving the OPT may not have been efficiently utilized as DMI, passage rate, and DM or NDF digestibilities were not positively influenced compared with Control-fed steers.

The results of this study aligned with other literature showing that ruminal NH3–N concentration was not affected by monensin (Walker et al., 1980; Faulkner et al., 1985; Lana and Russell, 1997). In contrast to our results, Lemenager et al. (1978c) and Vagnoni et al. (1995) found that monensin lowered ruminal NH3–N concentrations in diets containing urea. Considering protein-sparing characteristics of monensin, it was hypothesized that combining monensin with urea would reduce ruminal NH3–N levels to a more optimum release rate, causing less ammonia to be wasted and, presumably, more microbial protein production. The cause of greater ruminal NH3–N concentration for steers fed the COMBO treatment compared with steers consuming the OPT treatment remains unclear. Nevertheless, these results suggest that inclusion of monensin with Optimase may extend the time period that ruminal NH3–N remains above a critical threshold in low-quality forage diets. Whether increased NH3–N availability after feeding played a role in observed improvement in DM digestibility is uncertain, although it seems unlikely because a similar response was observed in the OPT treatment with no improvement in DM digestibility.

Including monensin in a protein supplement for low-quality forage diets reduced particulate passage rate and caused a reduction in the ruminal acetate to propionate ratio. However, monensin alone did not improve DM or fiber digestibility. Replacing a portion of the oilseed meal N with Optimase did not reduce DMI or DM or NDF digestibility, although it did result in a reduction in ADF digestibility. When Optimase and monensin were provided together in the protein supplement, DM and fiber digestibility were improved.

 

References

Footnotes


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