The objective of grain processing is to improve feeding value and acceptability without detrimentally affecting digestive function  . Steam flaking corn can maximize its feeding value, largely due to enhancement of both ruminal and post-ruminal starch digestion    . Grain extrusion is a process by which ground grain is preconditioned with steam before being forced through perforated die and cut to size. Like steam flaking, extruding corn disrupts the starch matrix. Matsushima  proposed that extruded grains are comparable to steam-flaked grains. However, the effects of extrusion are generally more intense, resulting in a greater degree of chemical and structural transformations, including protein denaturation and starch gelatinization  , greatly enhancing starch enzymatic reactivity   . In a direct comparison of dry rolled vs extruded corn (79% of diet DM), Gaebe et al.  observed that extruded corn markedly depressed DMI and ADG of feedlot steers. Depressed intake was attributed to protracted low ruminal pH of steers fed extruded corn. In a companion digestion trial, they observed that whereas extrusion markedly increased (13%) starch digestion, diet DE was not improved. This was attributed to low ruminal pH and negative associative effects on digestion of other dietary ingredients. Nevertheless, ruminal in situ comparisons demonstrated that rate of starch disappearance over time was more gradual for extruded corn vs ground corn  . The provision of fermentable substrate over a more prolonged period of time could have positive implications on ruminal microbial efficiency. The objective of the present study was to evaluate potential positive associative effects of extruded corn, when fed as only a partial replacement for steam flaked corn, on growth performance and characteristics of digestion in feedlot cattle.
2. Materials and Methods
All procedures involving animal care and management were in accordance with and approved by the University of California, Davis, Animal Use and Care Committee.
2.1. Experiment 1, Growth Performance and Dietary Energetics
Sixty-four Holstein steers with an average weight of 247 ± 4 kg were used in a 70-d experiment to evaluate the effects of partial replacement of steam flaked corn (SFC) with extruded corn meal (EC; Matrix Nutrition, Chandler, AZ) in a finishing diet on growth performance and dietary net energy. Steers were blocked by initial weight into four weight groupings, and randomly assigned within weight groupings to 16 pens (4 steers per pen).Pens were 43 m2 with 22 m2 of overhead shade, automatic waters, and 2.4 m fence-line feed bunks. Treatments consisted of a steam flaked corn-based finishing diet containing 0%, 10%, 20%, or 30% EC, where EC replaced 0%, 15%, 30%, or 45% of SFC (DM basis). The SFC was prepared as follows. A chest situated directly above the rollers (46 × 61 cm corrugated) was filled with corn and then brought to a constant temperature at atmospheric pressure of 102˚C using steam. Corn was steamed for approximately 20 min before starting the rollers. The first approximately 441 kg of steam-flaked grain was allowed to pass from the rollers before material was collected for use in the trial. This preliminary period served for warming the rolls and for adjusting the tension on the rolls to provide a flake density of 0.31 kg/L (24 lbs/bushel). The SFC was then allowed to air-dry for 3 days before feeding. Ingredient and nutrient composition of experimental diets are shown in Table 1. Diets were prepared at weekly intervals and stored in plywood boxes located in front of each pen. Steers were allowed ad libitum access to their experimental diets. Fresh feed twice daily at 06:00 and 14:00 h, allowing for approximately 5% residual. All steers were provided ad libitum access to water. Individual steers were weighed upon initiation and completion of the trial. In the calculation of steer performance live weights were reduce 4% to adjust for digestive tract fill. Estimates of steer performance were based on pen means.
Energy gain (EG) was calculated by the equation: EG = ADG1.097×0.0557 W0.75, where EG is the daily energy deposited (Mcal/d), W is the mean shrunk body weight (kg;  ). Maintenance energy (EM) was calculated by the equation: EM = 0.084 W0.75  . Dry matter intake (DMI) is related to EM and dietary NEm according to the equation: DMI = EM/NEm + EG/(0.877 NEm − 0.41), and can be resolved for estimation of dietary NEm by means of the quadratic formula: x = [−b ± (b2 − 4ac)0.5]/2a , where x = NEm, a = −0.877 DMI, b = 0.877 EM + 0.41 DMI + EG, and c = −0.42 EM  . Dietary net energy for gain (NEg) was derived from NEm by the equation: NEg = 0.877 NEm − 0.41  .
Pens were used as experimental units. The experimental data were analyzed as a randomized complete block design experiment according to the following statistical model:
Yij = μ + Bi + Tj + Eij
where μ is the common experimental effect, Bi represents initial weight group effect (df = 3), Tj represents dietary treatment effect (df = 3), and Eij represents the residual error (df = 9). Treatments effects were tested using the linear, quadratic and cubic polynomials (Statistix 10, Analytical Software, Tallahassee, FL).
2.2. Experiment 2, Digestive Function of Steers
Four Holstein steers (210 ± 7 kg) with cannulas in the rumen (3.8 cm internal
Table 1. Ingredients and composition of experimental diets fed to steers.
1DMB = Dry matter basis; 2Analyzed chemical composition for crude protein, NDF, starch and soluble (amyloglucosidase reactivity) starch were 8.4%, 10.9% and 72.4%, and 8.6% (DM basis), respectively; 3Analyzed chemical composition for crude protein, NDF, starch and soluble (amyloglucosidase reactivity) starch were 8.7%, 10.3%, 69.8% and 30.9% (DM basis), respectively; 4Trace mineral salt contained: CoSO4, 0.068%; CuSO4,1.04%; FeSO4, 3.57%; ZnO, 1.24%; MnSO4, 1.07%; KI, 0.052%; and NaCl, 92.96%; 5Based on tabular values for individual feed ingredients  .
diameter) and proximal duodenum  were used in a 4 × 4 Latin square experiment to evaluate treatment effects on characteristics of ruminal and total tract digestion. Dietary treatments were the same as indicated for Trial 1 (Table 1) with the inclusion of chromic oxide (2.5 g/kg) as a digesta marker. Steers were maintained in individual pens (5.6 m2) with automatic waters. Diets were fed at 0800 and 2000 h daily. In order to avoid the complications of feed refusals, DMI was restricted to 4.5 kg/d. Experimental periods were 14 d, with 10 d for dietary treatment adjustment and 4 d for collection. Between each experimental period there was a 7-d time-frame to recovery during which all steers were fed the control diet (0% extruded corn meal diet). During collection, duodenal and fecal samples were taken twice daily as follows: day 1, 0750 and 1350 h; day 2, 0900 and 1500 h; day 3, 1050 and 1650 h, and day 4, 1200 and 1800 h. Individual samples consisted of approximately 700 mL of duodenal chyme and 200 g (wet basis) of fecal material. Samples from each steer within each collection period were composited for analysis. During the final day of each collection period, ruminal samples were obtained from each steer via ruminal cannula 4 h after feeding. Ruminal fluid pH was determined on fresh samples. Samples were strained through 4 layers of cheesecloth. Two milliliters of freshly prepared (25 g/100 mL) meta-phosphoric acid was added to 8 mL of strained ruminal fluid. Samples were then centrifuged (17,000 × g for 10 min), and supernatant fluid was stored at −20˚C for VFA analysis. Upon completion of the experiment, ruminal fluid was obtained via the ruminal cannula from all steers and composited for isolation of ruminal bacteria by differential centrifugation  .
Corn, feed and fecal samples were subjected all or part of the following analysis: DM (oven drying at 105˚C until no further weight loss); ash (method 942.05, AOAC  ), Kjeldahl N (method 984.13, AOAC  ); aNDFom (Van Soest et al.  corrected for NDF-ash, incorporating heat stable α-amylase (Ankom FAA, Ankom Technology, Macedon, NY) at 1 mL per 100 mL of NDF solution); chromic oxide ; starch content and changes in starch solubility as a result of corn processing were determined using amyloglucosidase  . Duodenal samples were subjected the following analysis: DM (oven drying at 105˚C until no further weight loss); ash (method 942.05, AOAC  ), Kjeldahl N (method 984.13, AOAC  ), ammonia N (method 941.04, AOAC  ); aNDFom (Van Soest et al. , corrected for NDF-ash, incorporating heat stable α-amylase (Ankom FAA, Ankom Technology, Macedon, NY) at 1 mL per 100 mL of NDF solution); purines ; chromic oxide ; and starch  . Concentrations of VFA in ruminal fluid were determined by gas chromatography using a DB-FFAP Megabore column (30 m × 0.530 mm, J & W Scientific, Folsom, CA). Column, inlet and detector temperatures were maintained at 130˚, 195˚, and 200˚C, respectively, with carrier gas (N) flow rate at 20 mL/min.
Total DM flow to the duodenum and fecal excretion were estimated based on marker ratio: the relationship of Cr intake (g) vs. concentration of Cr in duodenal and fecal samples. Microbial organic matter (MOM) and N (MN) leaving the abomasum was calculated using purines as a microbial marker  . Organic matter fermented in the rumen was considered equal to OM intake minus the difference between the amount of total OM reaching the duodenum and MOM reaching the duodenum. Feed N escape to the small intestine was considered equal to total N entering the duodenum minus ammonia-N, MN and endogenous N (0.195 × BW0.75, Ørskov et al.  ). Methane production (mol/mol glucose equivalent fermented) was estimated based on the theoretical fermentation balance for observed molar distribution of VFA  .
The effects of extruded corn meal level in diet (0, 10, 20, or 30%) on characteristics of digestion in cattle were analyzed as a balanced 4 × 4 Latin square design experiment:
Yijk = µ + Si + Pj + Tk + Eijk,
where: Yijk is the response variable, µ is the common experimental effect, Si is the steer effect, Pj is the period effect, Tk is the treatment effect and Eijk is the residual error. Treatments effects were tested by means of linear and quadratic polynomials (Statistix 10, Analytical Software, Tallahassee, FL).
3. Results and Discussion
As expected, CP, NDF and starch content of EC were similar to that of SFC (104%, 94% and 96%, respectively). In contrast, soluble starch (a measure of disruption of the starch matrix) was markedly (359%) greater for EC (Table 1). Both steam flaking and extrusion disrupt the starch granule matrix. However, with extrusion the thermal energy transfer and shear stress are more intense, causinga greater degree of chemical and structural transformations, including protein denaturation and starch gelatinization     . With respect to steam flaking, degree of processing (flake density, steaming time, tempering) effects changes in starch enzymatic reactivity and characteristics of digestion    . However, the relationship between degree of starch gelatinization/solubility brought on by the extrusion process on characteristics of ruminal and total tract digestion has not been assessed  .
Treatment effects on growth-performance of feedlot steers and estimated net energy values of dietary treatments are shown in Table 2. The primary goal of grain processing is to enhance feeding value and diet acceptability in a manner that will improve growth-performance with minimal detrimental effects on digestive function  . Partial replacement of SFC with increasing levels of EC did not affect (P > 0.10) ADG, DMI, gain efficiency, and dietary net energy. When SFC replaces dry processed corn in finishing diets, feedlot cattle ADG, gain efficiency and dietary NE are appreciably enhanced  . In contrast, substitution of dry rolled corn with EC (79% of diet DM) markedly depressed feedlot cattle ADG and gain efficiency  . Depressed intake was attributed to protracted low ruminal pH of steers fed EC. Nevertheless, when EC was fed as only a partial replacement for SFC we did not observe marked negative effects on growth-performance.
Observed dietary NE values for the basal SFC-based diet were in good (97%) agreement with expected based on tabular feed values  . When EC replaced 15% to 30% of SFC observed dietary NE was 99% of expected. Whereas, when EC replaced 45% of SFC observed dietary NE was 94% of expected. These numerical differences in observed dietary NE were not statistically appreciable (P = 0.18).
Treatment effects on characteristics of digestion are shown in Table 3. Partial
Table 2. Influence of partial replacement of steam flaked corn with extruded corn on growth-performance of feedlot steers and net energy value of the diet.
1Initial and final BW reduced 4% to account for digestive tract fill.
Table 3. Influence of partial replacement of steam flaked corn with extruded corn on characteristics of ruminal and total tract digestion.
1DMI was restricted to 2.15% of BW daily; 2Microbial N, g/kg OM fermented; 3Nonammonia N flow to the small intestine as a fraction of N intake.
replacement of SFC with EC resulted in a numerically small, but appreciable decrease (linear effect, P < 0.01) in flow of ammonia N to the small intestine. However, there were no treatment effects (P > 0.10) on flow of non-ammonia N, undegraded feed N and microbial N to the small intestine, and ruminal microbial N efficiency (g microbial N/kg OM fermented). Likewise, there were no treatment effects (P > 0.10) on ruminal digestion of OM, NDF, starch and feed N.
As expected, total tract digestion of starch for all treatments exceeded 99%. Notwithstanding, replacing SFC with EC decreased total tract digestion of DM and OM (linear effect, P = 0.02), and dietary DE (linear effect, P = 0.03). Consistent with Trial 1, the effect was more pronounced where EC replaced 45% of SFC.
Substituting SFC with EC did not affect (P > 0.10) ruminal pH, total VFA concentration, molar proportions of acetate, propionate, butyrate and isovalerate, and estimated methane production (Table 4). Ruminal pH (4 hours postprandial) averaged 5.68 ± 0.03. Generally, ruminal proteolytic activity does not decrease as pH decrease in cattle fed high-concentrate diets  . However, microbial efficiency is affected as greater energy is expended to maintain intracellular pH  . Partial replacement of SFC with EC increased (linear effect, P <
Table 4. Influence of partial replacement of steam flaked corn with extruded corn on ruminal pH, VFA molar proportions, and estimated methane production1.
1DMI was restricted to 2.15% of BW; 2Methane, mol/mol glucose equivalent fermented.
0.01) ruminal molar proportions of valerate. The basis for this is not certain. However, it may reflect the increased denaturation and solubilization of corn protein during the extrusion process, as changes in valerate concentrations reflect ruminal protein degradation. When proline undergoes reductive ring cleavage and deamination it yields valerate  .
Partial replacement of steam flaked corn with extruded corn does not have a beneficial effect on ruminal microbial efficiency and digestive function. The feeding value of extruded corn for feedlot cattle is comparable to that of steam flaked corn provided the inclusion rate does not exceed 20% of diet dry matter.
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