Effect of cattle age, forage level, and corn processing on diet digestibility and feedlot performance

M. A. Gorocica-Buenfil and S. C. Loerch
Department of Animal Sciences, The Ohio State University, Wooster 44691

ABSTRACT

Three experiments were conducted to determine the effects of cattle age and dietary forage level on the utilization of corn fed whole or ground to feedlot cattle. In Exp. 1, 16 steers were used to investigate the effects of cattle age and corn processing on diet digestibility. Two cattle age categories were evaluated (weanling [254 ± 20 kg BW] and yearling [477 ± 29 kg BW]; eight steers per group), and corn was fed either ground or whole to each cattle age category. Cattle age and corn processing did not affect (P > 0.10) diet digestibility of DM, OM, starch, CP, NDF or ADF, and no interactions (P > 0.10) between these two factors were detected. In Exp. 2, the effects of forage level and corn processing on feedlot performance and carcass characteristics were evaluated. One hundred eighty steers (310 ± 40 kg BW) were allotted to 24 pens, and were fed one of the following diets: high-forage (18.2% corn silage) cracked corn (HFCC); high-forage shifting corn (whole corn for the first half of the trial, then cracked corn until harvest; HFSC); high-forage whole corn (HFWC); low-forage (5.2% corn silage) cracked corn (LFCC); low-forage shifting corn (LFSC); and low-forage whole corn (LFWC). For the high-forage diets, steers fed cracked corn had 7% greater DMI than those fed whole corn, whereas for the low-forage diets, grain processing did not affect DMI (interaction; P = 0.02). No interactions (P > 0.10) between forage level and corn processing were found for ADG and G:F. Total trial ADG and G:F, and percentage of carcasses grading USDA Choice, and carcass yield grade were not affected (P > 0.10) by corn processing. Cattle with fewer days on feed grew faster and more efficiently when cracked corn was fed, whereas cattle with longer days on feed had greater ADG and G:F when corn was fed whole (interaction; P < 0.10). In Exp. 3, the effects of forage level and corn processing on diet digestibility were evaluated. The high-forage cracked corn, high-forage whole corn, low-forage cracked corn, and low-forage whole corn diets used in Exp. 2 were fed to 16 steers (350 ± 27 kg BW) in a digestion trial. No interactions (P > 0.10) between forage level and corn processing were detected for starch digestibility. Forage level and corn processing (grinding) did not affect (P > 0.10) diet DM, OM, starch, CP, and NDF digestibility. Processing corn did not provide additional benefits to feedlot cattle performance under these experimental conditions.

Introduction

The merits of grain processing have been debated by feedlot nutritionists for years, and contradictory data reported in the literature and historical dogma may be limiting the use of whole corn in feedlot diets. Corn processing has been reported to increase starch digestibility (Galyean et al., 1979; Turgeon et al., 1983) and feedlot performance (Cole et al., 1976a; Zinn et al., 2002), although results have not been consistent. In extensive reviews, Owens et al. (1986, 1997) reported that corn processing (grinding) did not improve starch digestibility or feedlot performance. Several factors affect corn digestibility; however, their importance is not completely understood (Huntington, 1997).

Feedlot performance trials with weanling steers have failed to prove advantages for rolled vs. whole high-moisture corn (Loerch and Fluharty, 1998), perhaps because chewing capacity is greater in younger cattle (Nicholson et al., 1971). To our knowledge, there is no information available about the possible interaction between cattle age and the digestibility of whole corn.

There are limited and conflicting data in the literature (Cole et al., 1976b; Loerch and Fluharty, 1998) regarding the effects of forage level on the digestibility of whole corn. High forage levels in the diet can decrease metabolic disorders, but they could also increase passage rate (Cole et al., 1976b) and may lower the extent of whole corn digestion (Teeter et al., 1981).

The effects of cattle age and forage level on corn digestibility have not been adequately determined. Three experiments were conducted to evaluate the effects of cattle age, forage level, and corn processing on diet digestibility and feedlot performance.

Materials and Methods

In all our experiments, animal care followed guidelines recommended in the Guide for the Care and Use of Agricultural Animals in Agriculture Research and Teaching (FASS, 1999).

Experiment 1

A digestibility trial was conducted with 16 crossbred steers in a completely randomized experimental design with a 2 × 2 factorial arrangement of treatments to determine the effects of animal age and corn processing on diet digestibility. Two animal age categories were used in this experiment. Eight weaned steers (254 ± 20 kg of average BW) were classified as weanlings, and eight steers (477 ± 29 kg BW) were classified as yearlings.

Steers were purchased at a feeder calf sale and their exact age and background were unknown; they were classified as weanlings or yearlings based on BW. Upon arrival at The Ohio State University feedlot in Wooster, steers were vaccinated for protection against infectious bovine rhinotracheitis, parainfluenza-3, Haemophilus somnus, Pasteurella and Clostridia (Quadraplex, Somnugen 2P, and Dybelon, respectively; Bio-ceutic, St. Joseph, MO), and dewormed with Ivomec pour-on (Merck, Rahway, NJ). Steers were revaccinated 14 d later.

The animals were placed in individual pens in a totally enclosed feedlot barn, with slatted concrete floors and metal gates. Pens were 2.6 m × 1.5 m, giving each steer 3.9 m² of floor space. Steers were fed once daily at 0800, and refusals were weighed, recorded, and discarded daily before feeding. A 30-d period was allowed for adaptation to the feedlot and to gradually increase grain content of the diet to 80% corn.

After the 30-d receiving period, animals were fed a diet comprising (DM basis) 80% corn, 8% forage, and 12% supplement (Table 1). For both animal age categories, corn grain was fed either ground or whole. Thus, four treatment groups were tested: weanling, whole corn; weanling, ground corn; yearling, whole corn; and yearling, ground corn, with four animals in each treatment group.

For ground corn treatments, corn was dry rolled with a Roskamp roller mill (single duty and shortened single duty Series 9, Roskamp Champion, Waterloo, IA). Corn was rolled with the "fine-roll" mill presetting. Ninety-two percent of ground corn particles passed through a screen with 2-mm openings. Corn silage averaged 34% DM at time of feeding. Feed rolls on the corn chopper were set for a theoretical cut of 0.95 cm. The corn silage in this and subsequent experiments was not processed with a kernel processor.

Table 1. Diet composition (Exp. 1)

	Corn processing method
Item	Ground	Whole shelled
Ingredient	%, DM basis
Ground corn	80.00	---
Whole shelled corn	---	80.0
Soybean meal, 44%	9.47	9.47
Orchard grass hay, chopped	8.00	8.00
Limestone	1.08	1.08
Urea	0.46	0.46
Animal-vegetable fat	0.46	0.46
Trace mineral salt¹	0.45	0.45
Selenium, 201 mg/kg	0.05	0.05
Rumensin, 176 g/kg²	0.01	0.01
Vitamin A, 30,000 IU/kg	0.01	0.01
Vitamin D, 3,000 IU/kg	0.01	0.01
Nutrient composition
DM, %	81.1	84.3
OM, %	95.7	95.8
Starch, %	54.3	56.9
CP, %	13.5	14.1
NDF, %	14.3	14.8
ADF, %	4.6	4.9
NEm, Mcal/kg³	2.10	2.05
NEg, Mcal/kg³	1.43	1.39

¹Contained >93% NaCl, 0.35% Zn, 0.28% Mn, 0.175% Fe, 0.035% Cu, 0.007% I, and 0.007% Co.
²Elanco Animal Health, Greenfield, IN.
³Calculated using NRC (1996) values.

The experimental period consisted of 13 d, with 12 d for adjustment to the experimental diets, followed by a 24-h total collection of feces. Feces were recovered using fecal collection bags. Bags were held in position by straps attached to a harness located on the animal’s back. At the end of the 24-h collection, weight of total fecal output was recorded and fecal samples were taken and frozen for future analysis. Feed and refusal samples were taken on d 12 and 13, respectively, to determine feed intake. These samples also were frozen for future analysis.

The number of whole corn kernels fed, refused, and those that appeared in feces of cattle fed whole corn was counted. Samples with 10% of the total weight of feed, refusals, and feces were used for the counting. When feed refusals were less than 1 kg, corn kernels were counted in the total feed refusal.

Experiment 2

One hundred eighty Angus crossbred steers (310 ± 40 kg BW; 8 to 9 mo of age) were used to determine the effects of forage level and grain processing method on feedlot performance and carcass characteristics. A randomized complete block experimental design with a 2 × 3 factorial arrangement of treatments was used. Steers were blocked into four weight groups and allotted within weight groups to six pens, with seven or eight steers per pen (24 pens total). Treatments were assigned randomly to pens within a weight block. Main effects were two forage levels and three corn processing treatments.

	Diet¹
Item	HFCC	HFWC	LFCC	LFWC
Ingredient	%, DM basis
Corn, cracked	66.800	---	79.800	---
Corn, whole shelled	---	66.800	---	79.80
Corn silage	18.200	18.200	5.200	5.200
Corn, ground	2.860	2.860	3.164	3.164
Soybean meal, 44%	9.095	9.095	8.793	8.793
Limestone	1.098	1.098	1.097	1.097
Urea	0.549	0.549	0.548	0.548
Animal-vegetable fat	0.457	0.457	0.457	0.457
Trace mineral salt²	0.457	0.457	0.457	0.457
Dynamate³	0.331	0.331	0.331	0.331
Selenium, 201 mg/kg	0.046	0.046	0.046	0.046
Tylan-10†	0.046	0.046	0.046	0.046
Vitamin E, 44,000 IU/kg	0.027	0.027	0.027	0.027
Rumensin-80†	0.016	0.016	0.016	0.016
Vitamin A, 30,000 IU/kg	0.009	0.009	0.009	0.009
Vitamin D, 3,000 IU/kg	0.009	0.009	0.009	0.009
Nutrient composition
DM, %	76.4	76.9	82.0	82.7
OM, %	96.0	96.0	96.2	96.2
Starch, %	58.9	56.6	67.5	64.7
CP, %	14.4	14.5	13.8	13.9
NDF, %	16.2	15.2	13.5	12.2
ADF, %	6.1	5.9	4.3	4.1
NEm, Mcal/kg‡	2.06	2.02	2.14	2.10
NEg, Mcal/kg‡	1.41	1.38	1.48	1.44

¹HFCC = high-forage, cracked corn; HFWC = high-forage, whole shelled corn; LFCC = low-forage, cracked corn; LFWC = low-forage, whole shelled corn.
²Contained >93% NaCl, 0.35% Zn, 0.28% Mn, 0.175% Fe, 0.035% Cu, 0.007% I, and 0.007% Co.
³Magnesium sulfate and potassium sulfate, contained 22% S, 18% K, 11% Mg (Int. Minerals and Chemical Corp., Terre Haute, IN).
†Elanco Animal Health, Greenfield, IN.
‡Calculated using NRC (1996) values.

The six treatment combinations investigated were high-forage (18.2% corn silage in the diet), cracked corn (HFCC); high-forage shifting corn (steers fed whole corn for approximately the first half of the trial, then switched to cracked corn for the rest of the trial; HFSC); highforage, whole corn (HFWC); low-forage (5.2% corn silage), cracked corn (LFCC); low-forage, shifting corn (LFSC); and low-forage, whole corn (LFWC). Composition of diets is presented in Table 2.

For cracked-corn treatments, corn was dry rolled with a Roskamp roller mill. Corn was rolled with the "coarseroll" mill presetting. Median particle size was approximately 2.83 mm, with 80.3% of the particles between 1.4 and 4.0 mm. Corn silage averaged 34% DM at time of feeding. Feed rolls on the corn chopper were set for a theoretical cut of 0.95 cm.

The experiment was conducted at The Ohio State University feedlot in Wooster. Pens (5.4 m × 5.4 m) were constructed of metal gates and cable. They had concrete slatted floors, and were located in an open-sided barn. Before starting the trial, all steers received a common 65% concentrate diet for 60 d. Steers were vaccinated as described in Exp. 1 and were treated with Dectomax (Pfizer, Exton, PA) for internal and external parasites before arrival at the feedlot.

For the HFSC and LFSC treatments, the switch to cracked corn occurred on different dates for each block because the blocks had different initial weights and were predicted to have different days on feed. Animals in pens allotted to the shifting corn treatments (HFSC, LFSC) were switched from whole corn to cracked corn diets when steers within each weight block were predicted to be 84 d from their projected slaughter date.

The shift in corn processing was accomplished by switching from whole to cracked corn in one day, decreasing feed offered by 10% for 2 d, and then continuing with normal feed bunk management.

All steers were implanted with Synovex-S (20 mg of estradiol benzoate, 200 mg of progesterone; Fort Dodge Animal Health, Overland Park, KS) on d 1 of the experiment. All steers within a block were reimplanted with Synovex-S on the date corn was switched from whole to cracked for the HFSC and LFSC treatments in that block.

Feed was offered daily beginning at 0800. Approximately 60 cm of bunk space was available per animal. Feed refusals were weighed, recorded, and discarded daily. Fresh water was available at all times. Feed samples were analyzed weekly for DM to allow determination of DMI. Composite feed samples were dried in a forced-air oven at 55°C and stored for future analysis.

Initial and final BW was determined using the average BW measured on two consecutive days. Steers also were weighed at 28-d intervals during the trial. Steers were weighed before feeding at 0800 and were not withheld from feed or water.

Animals within a weight block were slaughtered when mean backfat of animals within the block was estimated to be 1.4 cm. Hot carcass weight, fat thickness, LM area, and percentage of kidney, pelvic, and heart fat were determined by Ohio State University personnel. Carcass yield grade was calculated (USDA, 1997). Quality grade and marbling score were determined by a USDA official. Carcass characteristics were measured after a 48-h chill.

Experiment 3

Sixteen crossbred steers (350 ± 27 kg) were used in a completely randomized experimental design with a 2 × 2 factorial arrangement of treatments to determine the effects of forage level and grain processing on diet digestibility.

Steers were allotted to one of the diets (HFWC, HFCC, LFWC or LFCC) used in Exp. 2 (Table 2). Corn was processed as described for Exp. 2. Median particle size was approximately 2.83 mm, with 80.3% of the particles between 1.4 and 4.0 mm. Corn silage was harvested as described in Exp. 2, and average DM at time of feeding was 35%.

Vaccination and deworming protocols, individual animal penning, and feeding procedures were as described in Exp. 1. The experimental period was 21 d in length. Days 1 through 13 were used for adjustment to the diets, and on d 14, a 24-h total collection of feces was performed. Between d 15 and 20, steers remained on the same diet, and on d 21 a second 24-h collection of total feces was done.

Feces were recovered using fecal collection bags, as described previously. At the end of each 24-h collection, weight of total feces excreted was recorded, and two fecal samples (20% of total fresh weight each) were taken and frozen for future analysis. Feed and refusal samples were taken on d 12 and 19 of the experimental period, and were frozen.

For steers fed the whole corn treatments, one of the frozen fecal samples from each collection was thawed, and the d-14 and -21 samples were composited for each animal. The number of whole corn kernels recovered in feces was counted. The number of corn kernels in feed and refusals was determined by counting the number of corn kernels in 10% of the feed offered and refused. When feed refusals were less than a kilogram, corn kernels were counted in the total feed refusal.

Feed, refusals and fecal samples were composited over sampling days for each animal, freeze-dried, and ground to pass a 1-mm screen before analysis. Corn kernels recovered from feces of each animal also were freezedried, ground, and stored for future analyses.

Analyses and Calculations

Feed samples of Exp. 2. and samples of feed, refusals, feces, and corn kernels recovered in feces of Exp. 1 and 3 were analyzed for DM, OM (AOAC, 1984), N (FP- 2000 N-analyzer, Leco Corp., St. Joseph, MI), and starch (Fleming and Reichert, 1980). For Exp. 1 and 3, feed, refusals, and fecal samples also were analyzed for NDF and ADF (Van Soest et al., 1991). Nutrient digestibility was determined by the difference between nutrient intake 24 h before fecal collection and the nutrient output in feces divided by intake.

Statistical Analyses

In Exp. 1 and 3, the DM, OM, CP, starch, NDF and ADF digestibility data were analyzed using the GLM procedures of SAS (SAS Inst., Inc., Cary, NC) for a completely randomized experimental design with a 2 × 2 factorial arrangement of treatments. For Exp. 1, the model included cattle age, corn processing method, and the cattle age × corn processing method interaction. For Exp. 3 the model included forage level, corn processing method, and the forage level × corn processing method interaction. Residual mean square was the error term and animal was the experimental unit. The starch content of feces (fecal starch as a percentage of fecalDM) and starch and DM digestibility of the diet were additionally analyzed using the CORR and REG procedures of SAS to determine the relationship between fecal starch and digestibility.

In Exp. 2, data were analyzed using GLM procedures of SAS for a randomized blocked design with a 2 × 3 factorial arrangement of treatments. The model included the effects of forage level, corn processing method, the forage level × corn processing method interaction, and block. Treatment means were compared using the PDIFF statement of SAS, protected by a significant (P < 0.05) F-value. Pen was the experimental unit for all analyses.

Results and Discussion

In Exp. 1, we hypothesized that younger cattle (weanlings) would have greater starch digestibility than older cattle when fed whole corn because of their expected better ability to fracture whole corn kernels during chewing. However, interactions between cattle age and corn processing were not detected (P > 0.10), providing no support for our hypothesis. Therefore, only main effects are presented (Table 3) and discussed. Neither cattle age nor corn processing affected diet DM, OM, starch, CP, NDF and ADF digestibility (P > 0.10).

Chewing capacity of younger cattle has been reported to be greater than that of older cattle (Nicholson et al., 1971; Morgan and Campling, 1978). When whole corn is fed, chewing capacity is important for DM and starch digestion because ruminal fermentation does not begin until the corn kernel cuticle is disrupted (Kotarski et al., 1992). This disruption can be achieved either mechanically (grain processing) or through chewing during the eating and rumination processes (Beauchemin et al., 1994; McAllister et al., 1994). The difference in cattle age (weanling vs. yearling steers) may not have been great enough to allow the expression of chewing capacity differences, or chewing capacity of both weanling and yearling steers fed whole shelled corn was sufficient to allow similar starch digestibility to that of steers fed ground corn (Hooper and Welch, 1982; Bae et al., 1983).

Starch content of feces is used in commercial feedlots to evaluate starch digestibility of the diet (Zinn et al., 2002). In this experiment, cattle age did not affect (P = 0.79) fecal starch concentration. Feeding whole corn resulted in a 44% increase (P = 0.01) in fecal starch concentration compared with feeding ground corn. Despite this, differences in starch digestibility due to corn processing were minimal (less than 2%) and not significant (P = 0.33).

Regression analysis was performed to determine the relationship between fecal starch content and starch digestibility for each dietary treatment. Slopes of the regressions did not differ among treatments (P > 0.10); therefore, the data were pooled. The resulting regression equation was: % starch digestion = 101.0 âˆ’ 0.48 (% fecal starch; r2 = 0.57; P = 0.001). Caution should be used when only starch content of feces is used to evaluate the quality of diets. To determine the digestibility of a diet, one must know not only the concentration of the nutrient in feed and feces, but also the amount of feed consumed and feces excreted (Merchen, 1988). For instance, increasing dietary fiber would increase fecal DM output, thereby decreasing fecal starch concentration without affecting starch digestibility.

Table 3. Main effects of cattle age (A) and corn type (C) on feed intake, excretion, and apparent digestibility of DM, OM, starch, CP, NDF, and ADF in steers fed corn-based diets (Exp. 1)

	Cattle age		Corn type
Item	Weanling¹	Yearling²	Ground	Whole	SEM	A	C	A x C
	Least squares means					P-value
No. of steers	8	8	8	8
Intake, kg/d
DM	5.7	6.6	5.7	6.7	0.63	0.32	0.29	0.37
OM	5.5	6.4	5.4	6.4	0.60	0.30	.027	0.39
Starch	3.2	3.8	3.1	3.9	0.39	0.23	.019	0.52
CP	0.8	0.9	0.8	1.0	0.09	0.49	.016	0.38
NDF	0.8	1.0	0.8	1.0	0.09	0.34	.026	0.26
ADF	0.3	0.3	0.3	0.3	0.03	0.56	.010	0.26
Excretion, kg/d
DM	1.3	1.4	1.1	1.6	0.14	.085	0.05	0.41
OM	1.2	1.3	1.0	1.4	0.13	0.90	0.06	0.45
Starch	0.2	0.2	0.2	0.3	0.04	0.85	0.05	0.34
CP	0.2	0.2	0.2	0.2	0.02	0.72	0.30	0.72
NDF	0.3	0.4	0.3	0.4	0.03	0.76	0.37	0.15
ADF	0.1	0.1	0.1	0.1	0.02	1.00	0.29	0.29
Apparent digestibility, %
DM	76.0	79.6	79.0	76.6	1.88	0.21	0.39	0.82
OM	76.8	80.7	80.3	77.3	1.86	0.17	0.28	0.81
Starch	93.2	94.4	94.7	92.9	1.22	0.48	0.33	0.29
CP	71.3	73.8	71.5	73.5	1.54	0.28	0.39	0.47
NDF	56.6	62.9	58.0	61.5	4.50	0.34	0.59	0.93
ADF	50.0	55.7	47.5	58.2	4.44	0.39	0.11	0.89
Fecal starch, % of fecal DM	14.7	15.3	12.3	17.7	1.69	0.79	<0.01	0.17

¹Weanling = 254 kg average BW.
²Yearling = 477 kg average BW.

Although the appearance of whole corn kernels in feces suggests that digestion is inefficient, there are limited data in the literature quantifying the importance of this phenomenon for starch utilization (Driedger and Loerch, 1999). In the present experiment, the number of corn kernels ingested and excreted did not differ (P > 0.10) between cattle age groups (Table 4). Pooled corn kernel disappearance was 92%, slightly above the 88% reported by Driedger and Loerch (1999) for nonlactating Holstein cows, which supports the concept that whole shelled corn diets can be properly digested by steers fed high-concentrate diets.

Composition of whole kernels recovered in feces did not differ (P > 0.10) between cattle age groups. Digestion of nutrients from whole corn kernels recovered in feces cannot be quantified based on the nutrient composition of these kernels; however, the concentration of starch (and to a lesser extent protein) was numerically lower in corn kernels recovered in feces vs. those in the diets. Bacterial attachment to corn kernels and penetration through the endosperm may allow some fermentation of starch to occur in kernels that seem to be intact in the feces (McAllister, 1994).

Experiment 2 was designed to evaluate the hypothesis that cattle fed whole corn would perform better with low-forage diets, whereas cattle fed high-forage diets would perform better when corn was cracked. Nonetheless, interactions between forage level and corn processing were not detected (P > 0.10) for overall feedlot performance and carcass characteristics, giving no support for our hypothesis.

Table 4. Effect of animal age on whole corn kernel excretion, disappearance, and whole corn kernel excreted composition (Exp. 1)

	Cattle age
Item	Weanling¹	Yearling²	SEM	P-value
No. of steers	4	4
Whole corn kernels
Intake, kernels/d	18,734	22,090	2,487	0.38
Excretion, kernels/d	1,770	1,770	857	1.00
Diasppearance, %	91.91	92.04	3.75	0.98
Composition of whole corn kernels recovered in feces³
DM, %	62.73	65.99	7.28	0.76
OM, % of DM	97.50	94.28	1.00	0.06
Starch, % of DM	68.19	71.54	7.24	0.76
CP, % of DM	8.57	7.98	3.01	0.21
Starch in kernels recovered in feces, % of starch in kernels fed	81.99	86.01	8.71	0.76

¹Weanling = 254 kg average BW.
²Yearling = 477 kg average BW.
³Composition of kernels fed = DM, 84.2%; OM, 98.4% of DM; starch, 83.2% of DM; CP, 8.8% of DM.

Table 5. Effect of corn processing method (PM) and two (High = 18.2% corn silage; Low = 5.2% corn silage of the diets, DM basis) forage levels (FL) on feedlot performance (Exp. 2)

	High forage			Low forage
Item	Crack	Shift¹	Whole	Crack	Shift	Whole	SEM	FL	PM	FL×PM
	Least squares means							P-value
No. of steers	30	30	29	30	30	30
No. of pens	4	4	4	4	4	4
BW, kg
Initial	310.6†	310.9†	311.8†	311.8†	310.5†	311.3†	0.4	0.69	0.12	0.07
Reimplant	447.2†	443.5†	443.5†	449.8†	449.1†	458.6†	2.8	0.01	0.27	0.10
Final	581.3	576.9	575.7	587.2	581.8	593.6	7.7	0.15	0.75	0.65
Days of feed
Phase 1²	72	72	72	72	72	72	0.0
Phase 2³	85	85	85	85	85	85	2.1	0.96	0.99	0.99
Total	157	157	157	157	157	157	2.1	0.96	0.99	0.99
Weight gained, kg
Phase 1	136.7†	132.6†	131.7†	138.0†	138.5†	147.2†	2.9	0.01	0.42	0.07
Phase 2	134.0	133.4	132.2	137.4	132.8	135.1	6.1	0.71	0.90	0.94
Total	270.7	266.0	263.9	275.4	271.3	282.3	7.8	0.16	0.81	0.62
ADG, kg/d
Phase 1	1.94†	1.88†	1.86†	1.96†	1.95†	2.07†	0.04	0.01	0.39	0.06
Phase 2	1.59	1.57	1.55	1.62	1.58	1.57	0.06	0.68	0.82	0.99
Total	1.75	1.71	1.69	1.76	1.74	1.80	0.04	0.14	0.77	0.53
DMI, kg/d
Phase 1	8.6†	8.4†	8.0†	8.4†	8.1†	8.4†	0.1	0.41	0.03	0.01
Phase 2	9.8†	9.7†	9.3†	9.8†	9.3†	9.8†	0.2	0.90	0.25	0.09
Total	9.3†	9.1†	8.7†	9.2†	8.7†	9.1†	0.1	0.68	0.06	0.02
G:F, g/kg
Phase 1	224	224	232	233	241	248	4.0	0.01	0.05	0.57
Phase 2	161	161	167	165	170	161	5.0	0.53	0.88	0.36
Total	187	187	195	193	201	197	3.0	0.02	0.21	0.25

¹Corn fed whole in Phase 1 and cracked in Phase 2.
²Phase 1 = d 0 to reimplant date.
³Phase 2 = reimplant to slaughter date.
†Within a row, means without a common superscript letter differ, P < 0.10.

An interaction occurred (P = 0.06) between forage level and grain processing treatments for ADG during Phase 1 of the trial (Table 5). For the low-forage diets, cattle fed whole corn gained 6% faster than those fed ground corn. For the high-forage diets, feeding whole corn did not increase ADG. This response was not evident during Phase 2, and total ADG for the trial was not affected (P = 0.53) by forage level or grain processing treatment. Greater ADG by cattle fed the LFWC diet was not expected and this effect may be spurious. Cattle fed LFSC also were fed whole corn during Phase 1 and their growth rate was similar to that of cattle fed cracked corn.

Corn processing has been reported to increase growth rate compared with feeding whole corn (Galyean et al., 1979; Zinn et al., 2002) because it increases the net energy content of the diet (NRC, 1996). However, in a review of published experiments, Owens et al. (1997) reported similar ADG by steers fed cracked, steamflaked, or whole corn. Improvements in ruminal starch fermentation with corn processing can be offset by excessive rates of acid production in the rumen and subclinical acidosis (Fulton et al., 1979a,b), resulting in similar feedlot performance compared with whole corn diets. Starch escaping fermentation in the rumen can be digested in the small and large intestine, and total-tract starch digestibility has been reported to be similar between cracked and whole corn (Owens et al., 1986). Furthermore, starch fermentation rate is slower with whole corn diets, decreasing the risk of digestive disorders, which is reflected in growth rates comparable to those with processed corn diets.

An interaction between forage level and grain processing treatment occurred (P = 0.02) for daily DMI during the total trial. For the high-forage diets, steers fed cracked corn had 7% greater DMI than those fed whole corn, whereas for the low-forage diets grain processing did not affect DMI. It was expected that whole corn treatments would have higher DMI to compensate for their lower energy concentration. When ruminants are fed high-energy diets and gut fill does not limit intake, they typically adjust their DMI to equalize energy intake (Conrad et al., 1964). Unexpectedly, the opposite effect was detected.

No interactions between forage level and corn processing were detected (P > 0.10) for feed efficiency. During Phase 1, feed efficiency was higher (P = 0.01) for low- than for high-forage-fed groups (240 vs. 227 g of gain/kg of feed, respectively). During Phase 2, feed efficiency was not affected (P = 0.53) by forage level, but feed efficiency for steers fed the low-forage diets remained greater (P = 0.02) than that of steers fed the high-forage diets for the total feeding period (197 vs. 190 g of gain/ kg of feed, respectively).

Higher feed efficiency in low-forage diets can be expected due to increased energy concentration with decreasing forage level. Forage is included in feedlot diets to improve ruminal health status. The inclusion of 5.2% corn silage in the diet (DM basis) likely provided adequate forage to maintain good ruminal health (Koenig et al., 2003); thus, steers were able to take advantage of the higher energy concentration of this diet. Feed efficiency was higher during Phase 1 for whole than for cracked corn diets (240 vs. 229 g of gain/kg of feed; P = 0.05). In a summary of published research, Owens et al. (1997) reported that diets containing whole corn resulted in higher feed efficiencies than those containing dry rolled corn, but were lower than those observed with diets containing steam-rolled corn. Interestingly, these authors attributed the greater efficiency reported for whole corn diets to the low forage levels typically fed when whole corn is used in feedlot diets. In this experiment, overall feed efficiency for the total trial was similar for cattle fed whole vs. those fed cracked corn.

The shifting corn treatment was intended to provide a slower fermentation rate with whole corn during the initial phase of the feeding period, decreasing the risk for acidosis when steers are more susceptible to digestive upsets, and then to provide a higher energy density with cracked corn after the initial feeding period. This strategy did not improve ADG or G:F compared with feeding corn whole or cracked throughout the feeding period.

An apparent interaction between days fed and cornprocessing method for growth rate and feed efficiency was observed. Cattle performance responses to grain processing were not consistent among the four weight blocks. Due to insufficient replication, the days fed × corn processing interaction could not be tested using GLM procedures (SAS Version 8) for a randomized block design. Thus, the interaction of days on feed (DOF) and corn processing on ADG and G:F was analyzed statistically using the Mixed procedures of SAS Version 8. Pen was the experimental unit. Because steers on the shifting corn treatments consumed both cracked and whole corn, only data from the cracked and whole corn treatments were included in the analysis. The model included forage level and corn processing as main effects, days fed as a continuous variable, and the forage level × corn processing, forage level × days fed, corn processing × days fed, forage level × corn processing × days fed interactions. Days fed × corn processing interactions for growth rate (P = 0.07) and feed efficiency (P = 0.01) were detected (Figures 1 and 2, respectively). For cattle fed cracked corn,maximum growth rate was observed in the heaviest weight block (1.87 kg/d at 129 DOF), and growth rate was progressively slower for the lighter weight blocks (1.60 kg/d at 185 DOF). Feed efficiency remained mostly unchanged among weight block groups for cattle fed cracked corn diets (191 vs. 190 g of gain/kg of feed for 129 and 190 DOF, respectively). Conversely, when whole corn was fed, growth rate remained mostly unchanged among weight blocks (1.76 vs. 1.64 kg/d at 122 and 192 DOF, respectively), but feed efficiency was greater for lighter weight blocks that were on feed for more days (188 vs. 207 g of gain/kg of feed at 122 and 192 DOF, respectively).

Figure 1. Effect of days on feed and corn processing on ADG in Exp. 2 (interaction; P = 0.07). Corn was fed either cracked (C) or whole (W).

For the heaviest weight block (shortest time on feed), cracking corn increased ADG by 4% when the low-forage diet was fed and 8.5% when the high-forage diet was fed. Cracking corn resulted in a similar improvement for cattle in the second largest weight block that were fed high forage; however, there was no advantage to cracking corn for cattle in this weight block that were fed the low-forage diet. For cattle in the medium and small weight blocks (longest time on feed), cracking corn resulted in a 5 to 7% decrease in ADG. Whereas cattle in the larger weight blocks that were fed high forage benefited from cracking corn, those in the smaller weight blocks did not benefit from cracking corn.

Figure 2. Effect of days on feed and corn processing on G:F in Exp. 2 (interaction; P = 0.01). Corn was fed either cracked (C) or whole (W)

Table 6. Effect of corn processing method (PM) and two different (High = 18.2% corn silage; Low = 5.2% corn silage of the diets, DM basis) forage levels (FL) on carcass characteristics (Exp. 2)

	High forage			Low forage
Item	Crack	Shift¹	Whole	Crack	Shift¹	Whole	SEM	FL	PM	FL×PM
	Least squares means							P-value
No. of steers	30	30	29	30	30	30
No. of pens	4	4	4	4	4	4
HCW, kg	358.7	357.6	355.7	369.6	360.9	369.9	5.1	0.04	0.62	0.55
Back fat, cm	1.42‡	1.42‡	1.36‡	1.51‡	1.38‡	1.61‡	0.06	0.05	0.29	0.07
Dressing percent	61.7‡	62.0‡	61.8‡	62.9‡	62.6‡	61.7‡	0.3	0.02	0.12	0.09
Marbling score²	386	393	339	366	406	354	13	0.81	0.01	0.39
LMA, cm²†	81.5	82.2	86.4	82.4	84.8	84.0	1.9	0.85	0.27	0.44
KPH, %	1.9	21.1	2.3	1.9	2.4	2.5	0.1	0.08	0.01	0.25
Yield grade	3.3‡	3.2‡	3.0‡	3.4‡	3.2‡	3.5‡	0.1	0.04	0.60	0.05
Quality grade³	3.6	3.6	2.9	3.2	3.6	3.2	0.2	0.70	0.01	0.15
Select, %	21.9	15.6	25.0	25.9	12.5	22.8	7.1	0.94	0.31	0.86
Choice-, %	37.1	44.2	65.2	47.3	46.0	46.9	8.8	0.78	0.28	0.28
Choice°, %	13.9	13.0	3.1	13.0	17.0	23.7	5.8	0.12	0.95	0.19
Choice+, %	17.0	20.1	6.7	10.3	14.3	6.7	4.0	0.21	0.05	0.65
Prime, %	10.3	6.7	0.0	3.6	10.3	0.0	2.4	0.60	0.01	0.12
≥Choice-, %	78.1	84.4	75.0	74.1	87.5	77.2	7.1	0.94	0.31	0.86

¹Corn fed whole in Phase 1 and cracked in Phase 2.
²Practically Devoid = 100 to 199; Slight = 200 to 299; Small = 300 to 399; Modest = 400 to 499; Moderate = 500 to 599.
†LMA = LM area.
³2 = Select; 3 = Choiceâˆ’; 4 = Choiceo; 5 = Choice+; 6 = Prime.
‡Within a row, means without a common superscript letter differ, P < 0.10.

Lack of a performance response to corn processing for cattle on feed for longer time periods may have been due to long-term, cumulative effects of increased starch fermentation rate (Huntington, 1997; Beauchemin et al., 2003). Subacute acidosis (Fulton et al., 1979a,b) and decreasing integrity of ruminal epithelial tissue (Bartle and Preston, 1992) may have contributed to the response observed. The interaction of time on feed and grain processing may explain part of the conflicting responses to grain processing previously reported in the literature. Further research specifically designed to test the interactions between corn processing and days on feed is warranted.

Forage level and corn processing effects on carcass characteristics are presented in Table 6. Low forage-fed cattle had heavier (P = 0.04) HCW, reflecting the higher energy concentration of their diets. Grain processing affected (P = 0.01) marbling score. When averaged over forage level, marbling scores were 376, 400, and 347 points for cattle fed cracked corn, shift corn, and whole corn, respectively. However, this effect on marbling score did not result in dietary treatment effects (P > 0.10) for percentage of carcasses grading Select, or percentage of carcasses grading above low Choice. In addition, processing corn did not improve yield grades. The number of cattle marketed through quality and yield grids has increased dramatically during recent years (USDA, 2003). Considering that the most common standards to receive price premiums and avoid discounts are quality grades of at least Choice, and a yield grade not greater than 4, it may be concluded that feeding whole corn does not diminish the carcass value of the animal.

In Exp. 3, the possible interaction between forage level and corn processing on diet digestibility was evaluated. It was hypothesized that whole corn diets would have increased starch digestibility when fed with low-forage diets, whereas in cracked corn diets, forage level would not influence starch digestibility. No significant interactions (P < 0.10) were detected between forage level and corn processing for starch digestibility. Therefore, forage level and corn processing main effects on nutrient digestibility are presented (Table 7). Starch digestion was not influenced by forage level or corn processing (P > 0.10). There was a tendency for an interaction (P < 0.08) between forage level and corn processing for DM digestibility. For the high-forage diets, when corn was cracked, diet DM digestibility was lower than when corn was fed whole; for the low-forage diets, feeding cracked corn increased DM digestibility (83.3, 81.4, 80.2, and 83.1%, respectively, for HFWC, HFCC, LFWC, and LFCC). This interaction inDM digestibility may be attributed in part to the interaction (P = 0.07) between forage level and corn processing on NDF digestibility; HFWC had 12% higher NDF digestibility than LFWC, whereas this value was not different for cracked corn diets.

Whole corn may partially substitute for forage in feedlot diets, due to the physical structure of the corn grain (Bartle and Preston, 1992). The combined effects of high forage and whole corn may lead to a better ruminal environment for fiber digestion, increasing NDF and DM digestibility in HFWC but not in LFWC diets.

Starch digestibility did not differ among treatments (P > 0.59). The lack of response in starch digestion to forage level is in agreement with results of McCullogh and Matsushima (1973), but in contrast to those of Cole et al. (1976a,b), who reported lower total gastrointestinal tract starch digestibility with the inclusion of 14 to 21% dietary forage. The forage levels investigated in the present trial were 5.2 and 18.2% corn silage on a DM basis. Forage source and particle size affect ruminal pH and digesta passage rate (Teeter et al., 1981; Krause and Combs, 2003). Further research is needed to determine whether the results we observed when corn silage served as the forage source would be repeatable for other forage sources.

Table 7. Main effects of corn processing method (PM) and two different (High = 18.2% corn silage; Low = 5.2 % corn silage of the diets, DM basis) forage levels (FL) on feed intake, excretion, and apparent digestibility of DM, OM, starch, CP, NDF, and ADF of steers fed feedlot corn-based diets (Exp. 3)

	Forage level		Corn processing
Item	High	Low	Cracked	Whole	SEM	FL	PM	FL×PM
	Least squares means					P-value
Intake, kg/d
DM	8.7	8.6	8.9	8.4	0.49	0.82	0.43	0.56
OM	8.3	8.2	8.6	8.0	0.47	0.87	0.44	0.57
Starch	5.1	5.5	5.5	5.1	0.31	0.44	0.41	0.58
CP	1.2	1.2	1.2	1.2	0.07	0.82	0.73	0.62
NDF	1.5	1.3	1.4	1.4	0.08	0.05	0.76	0.48
ADF	0.6	0.4	0.5	0.5	0.03	<0.01	0.79	0.43
Excretion, kd/d
DM	1.6	1.6	1.6	1.5	0.13	0.95	0.84	0.17
OM	1.4	1.5	1.5	1.4	0.12	0.83	0.83	0.20
Starch	0.3	0.3	0.3	0.3	0.07	0.63	1.00	0.81
CP	0.3	0.3	0.3	0.3	0.02	0.52	0.73	0.07
NDF	0.5	0.4	0.5	0.4	0.03	0.15	0.15	0.06
ADF	0.2	0.2	0.2	0.2	0.02	0.13	0.43	1.00
Apparent digestibility, %
DM	82.4	81.6	82.3	81.7	0.92	0.56	0.65	0.08
OM	83.0	82.3	83.0	83.3	0.91	0.58	0.57	0.10
Starch	95.2	94.4	94.8	94.8	0.96	0.59	0.97	0.64
CP	78.5	77.2	78.0	77.7	0.79	0.27	0.77	0.02
NDF	69.9	67.1	66.2	70.8	2.24	0.40	0.17	0.07
ADF	64.4	57.2	60.0	61.6	2.50	0.07	0.66	0.29
Fecal starch, % of fecal DM	15.88	19.07	18.03	16.92	2.76	0.43	0.78	0.94

Starch content of feces was not affected (P > 0.43) by forage level or corn processing. The correlation coefficient for starch content in feces and starch digestibility in whole (r = âˆ’0.98; P < 0.0001) and cracked (r = âˆ’0.95, P < 0.001) corn diets was similar to that reported by Zinn et al. (2000) for steam flaked corn (r = âˆ’0.95; P < 0.01).

The number of corn kernels ingested and excreted and corn kernel disappearance was not affected by forage level in the diet (Table 8). In both Exp. 1 and 3, less than 10% of the corn kernels fed were counted in feces. Additionally, the starch digestibility of whole corn diets was similar to that of processed corn diets. Thus, it can be concluded that the appearance of corn kernels in feces should not be the sole justification to process corn in feedlot diets.

Table 8. Effect of forage level on whole corn kernel excretion, digestibility, and whole corn kernel excreted composition (Exp. 3)

	Forage level¹
Item	High	Low	SEM	P-value
No. of steers	4	4
Whole corn kernels/d
Intake, kernels/d	39,555	37,729	3,353	0.71
Excretion, kernels.d	489	562	397	0.90
Disappearance, %	98.8	98.5	1.4	0.32
Composition of whole corn kernels recovered in feces ²
DM, %	49.59	48.41	1.62	0.63
OM,%	99.19	98.80	0.16	0.14
Starch, %	73.25	61.10	6.52	0.24
CP, %	7.70	7.55	0.25	0.68
Starch in kernels recovered in feces, % of starch in kernels fed	96.28	80.32	8.56	0.24

¹High = 18.2% corn silage; Low = 5.2% corn silage of the diets, DM basis.
²Composition of kernels fed = DM, 84.7%; OM, 98.8% of DM; starch; 76.1% of DM; CP, 9.6% of DM.

Implications

Conflicting reports have been published on the benefits of processing corn. Our experiments were intended to determine whether variations in cattle age and forage level used in digestion and performance trials might explain these discrepancies; however, our results do not support the involvement of these two factors. Furthermore, the lack of response in diet digestibility and cattle feedlot performance to corn processing (grinding) observed in these experiments, suggests that the additional cost of grinding corn may not be justified. Nonetheless, benefits of corn processing may depend on the length of time cattle are on feed. Further research is needed to identify the importance of other factors, such as the interaction between days on feed and corn processing that might influence starch digestibility and feedlot cattle performance.

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Contact Information

Effect of cattle age, forage level, and corn processing on diet digestibility and feedlot performance

M. A. Gorocica-Buenfil and S. C. Loerch Department of Animal Sciences, The Ohio State University, Wooster 44691

M. A. Gorocica-Buenfil and S. C. Loerch
Department of Animal Sciences, The Ohio State University, Wooster 44691