Potential of High-Sugar Corn as a Fall and Winter Forage Resource for Grazing Beef Cattle

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The Professional Animal Scientist Nayigihugu et al.19 (2003):410–415

of High-Sugar Corn as a Potential Fall and Winter Forage Resource for Grazing Beef Cattle V. NAYIGIHUGU*, B. W. HESS*,1, PAS, L. BROKAW†, PAS, D. W. KOCH‡, and J. W. FLAKE‡ *Department of Animal Science, University of Wyoming, Laramie 82071; †Quality Liquid Feed, Dodgeville, WI 53533; and ‡Department of Plant Science, University of Wyoming, Laramie 82071

Abstract

animals select the greatest quality plant components (Fernandez-Rivera Three grazing experiments were and Klopfenstein, 1989a). Therefore, conducted to determine the potential of the relative amount of plant parts as dryland, high sugar (Cargill HS 60A; well as their quality could affect Cargill, Minneapolis, MN) corn forage performance of grazing animals. for fall and winter grazing. For Trials 1 Allowing livestock to consume and 2, two ruminally cannulated steers annual forage left in windrows has continuously grazing a 1-ha high-sugar become a common method to reduce corn pasture during fall and winter were costs associated with harvesting, used to collect masticate samples. storing, and feeding forage. Also, Masticate N declined (P=0.03) from mid earlier cutting and windrowing can September to mid October, but was not sometimes preserve nutritional affected (P=0.19) by sampling date in quality that is otherwise lost if the the winter. Masticate fiber steadily forage remains standing. We are not increased (P=0.04) from mid November aware of any data on the quality of to mid December. Rate of in vitro OM high-sugar corn or cattle response to digestion (IVOMD) was least (P=0.04) windrowed or standing corn forage. by the study’s end in both trials; howTherefore, the objectives of these (Key Words: Cattle, Grazing, Corn ever, extent of IVOMD did not change experiments were to determine 1) the forage, Nutritional Value.) (P=0.26) in the fall. For Trial 3, 34 nutritional quality of high-sugar corn pregnant beef cows were allowed to graze forage throughout a fall and winter either windrowed or standing corn forage grazing period, 2) the nutrient Although corn is typically grown during the fall. Leaves and upper and composition of high-sugar corn plant for grain or silage (Kercher et al., lower stalks were separated from whole components, 3) the changes in 1986), the high production of nutricorn plant collected from each forage nutritive quality of windrowed vs ents from corn warrants investigation standing high-sugar corn forage, and type for chemical and nutrient determiinto its use as stockpiled forage. nation before and during the grazing 4) the effect of feeding windrowed or High-sugar corn (Cargill HS 60A; period. Lower and upper stalks had less stockpiled forage on beef cow perforCargill, Minneapolis, MN) is a male(P=0.009) fiber content and greater mance. (P=0.002) in vitro DM digestibility than sterile corn variety that concentrates non-structural carbohydrate in stalks and leaves. Nutritive quality analysis Experimental Site. Experimental performed on the whole corn residue 1To whom correspondence should be ad- or a mixture of several corn residue high-sugar corn fields were planted dressed: [email protected] parts has limited use because grazing on May 21, 17, and 16 of 1998, 1999, leaves. Although harvesting method did not affect (P=0.10 to 0.88) nutrient variables, cows grazing standing corn forage had greater (P=0.03) ADG than cows grazing windrowed corn forage. Despite a decline in nutritional quality of high-sugar corn forage as the grazing season progressed, high-sugar corn forage provided a sufficient nutritional base to support BW gain of pregnant beef cows. Grazing windrowed highsugar corn forage would not be recommended because an undetermined quantity of forage was lost to a windstorm, which might have contributed to a lower ADG for cows grazing the windrowed corn forage.

Introduction

Materials and Methods

High-Sugar Corn for Grazing Beef Cattle

and 2001, respectively, at a rate of 85,000 seeds/ha in 75-cm rows. Forage was grown under dryland conditions at the University of Wyoming Archer Research and Extension Center in Cheyenne, WY. During yr 2, the high-sugar corn field was fertilized with 22.40 kg N/ha. In yr 3, a 8.4-ha field was fertilized with 25.85 kg N/ha when the forage was 30.48-cm tall, and a 3.2-ha field was fertilized with 45.36 kg N/ha at preemergence. In all 3 yr, random counts were made in a 30.5-m row section to estimate average plant population after emergence. At the beginning of September in each year, up to 12 random plants were measured for height, and plants were then cut off at the soil line. Forage was dried in a forced-air oven at 50°C, and leaves were separated from stalks to determine average plant height, total DM production, and leaf DM percentage. Trial 1 (Fall) and Trial 2 (Winter). After a 7-d adaptation to a separate corn forage pasture, two ruminally cannulated Gelbvieh × Angus rotationally crossbred steers (initial BW = 475 and 450 kg for 1998 and 1999, respectively) continuously grazed a 1-ha pasture of dormant high-sugar corn forage for 35 d. The grazing period began on September 8 in yr 1 and began on November 11 in yr 2. Surgical procedures and animal care were approved by the University of Wyoming Animal Care and Use Committee. Steers had ad libitum access to water and trace-mineralized salt (Champion’s Choice; Akzo Salt, Inc., Georgetown, SC; guaranteed analysis [percentage of DM]: 95 to 99% NaCl and <1.0% Co, Cu, I, Mn, Zn, and Fe) throughout each 35-d grazing period. Sample Collection and Laboratory Analyses. At 1300 h on d 0, 7, 14, 21, 28, and 35 in the fall of yr 1, and d 0, 7, 21, and 35 in the winter of yr 2, steers were completely evacuated of their ruminal contents, and the ruminal lining was washed. After grazing for an additional 1 h, fresh masticate was removed, and original ruminal contents were replaced

(Lesperance et al., 1960). Masticate samples from each steer were rinsed with distilled water to remove salivary contaminants. Masticate samples were then spread thinly (1 cm deep) on 600-cm2 trays and dried in a 50°C forced-air oven (Broesder et al., 1992). Dried ruminal masticate samples were ground in a Wiley mill (Arthur H. Thomas Co., Philadelphia, PA) to pass a 1-mm screen and analyzed for DM, ash, and Kjeldahl N (AOAC, 1990). Dried ruminal masticate samples were also analyzed for NDF and ADF by nonsequential methods of Goering and Van Soest (1970) with modifications described by Brokaw et al. (2001). Following a 21-d adaptation to bromegrass hay, approximately 500 mL of ruminal fluid were collected from each steer to inoculate ground high-sugar corn forage masticate samples collected during the previous 35-d grazing periods. Ruminal fluid was strained through four layers of cheesecloth and prepared within 1 h of collection for determination of in vitro OM disappearance (IVOMD) (Judkins et al., 1990). Extent of IVOMD was determined on masticate samples incubated in ruminal fluid at 0, 3, 6, 9, 12, 24, and 48 h (Whitney et al., 2000). Rate of OM disappearance was calculated using nonlinear regression as described by Mertens and Loften (1980). Statistical Analyses. Data for rate of IVOMD, masticate composition, and extent of IVOMD within each collection time were analyzed as a randomized complete block design with animal as the block and sampling date as the main effect using GLM procedure of SAS (1998). After a significant F test, differences among means were tested by Fisher’s protected LSD (Steel and Torrie, 1980). Trial 3 (Fall). The two nonirrigated, high-sugar corn pastures (3.2 and 8.4 ha) were divided into two halves. Half of each pasture was left standing, and the other half was windrowed using a 4.6-m wide swather head. Forage Sampling and Laboratory Analyses. Just before the grazing

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period began, two sampling points were randomly chosen on each half, and forage cages (1.5 m2) were placed over the windrowed and the standing corn forage to monitor changes in forage quality throughout the upcoming grazing period. On September 19, October 1, and November 5, standing corn plants were cut off at the soil line, and windrowed corn was also collected from a 0.25-m2 quadrant inside each cage, dried as previously described, and weighed to estimate total DM production. After drying, whole corn plants were separated into leaves and lower and upper portions of stalks. Corn forage plant parts were then ground and analyzed for DM and ash as described in Trials 1 and 2. Corn forage plants were further analyzed for N (LECO FP-528; LECO, St. Joseph, MI), NDF and ADF (ANKOM200 analyzer; ANKOM, Fairport, NY), and IVDMD (DAISYII digestion system; ANKOM). Beef Cows. Thirty-four pregnant Gelbvieh × Angus rotationally crossed primiparous cows (initial BW = 481 ± 12 kg) were weighed on two consecutive days before being randomly allotted to graze either windrowed or standing high-sugar corn from October 2 to November 2, 2001. Thirteen cows were allotted to each treatment on the 8.4-ha pasture, and four cows were allotted to each treatment on the 3.2-ha pasture. Cattle had free choice access to water and mineral supplement (Cu #1 A&C Mineral ML; Purina Mills, Inc., St. Louis, MO; guaranteed analysis [percentage of DM]: 8 to 10% Ca, ≥15% P, 8 to 10% NaCl, and ≥0.3% Mg; ≥2500 ppm of Cu; and ≥2.2 × 105 IU/kg of vitamin A) throughout the experiment. Cattle were weighed on two consecutive days at the conclusion of the experiment. The two BW were averaged each time to determine the initial and the final BW. Statistical Analyses. Forage data were analyzed as a split-split block in a randomized complete block design using the GLM procedure of SAS (1998). The block effect was pasture (8.4 and 3.2 ha), and the treatment

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effect was harvesting method (windrowed and standing corn). The pasture × treatment interaction (Error a) was used to test treatment effects. The pasture × treatment × plant part interaction (Error b) was used to test the plant part effect and treatment × plant part interaction. Sampling date effect and respective interactions associated with sampling date were tested using residual error. Cattle performance data were analyzed as a randomized complete block with pasture as the block and harvesting method as treatment.

Results and Discussion Production and Forage Quality During the Fall and Winter. Dryland production of high-sugar corn is presented in Table 1. Although plant height was variable (60 to 185 cm) in the fall, the high sugar variety showed greater drought tolerance and appeared to be more palatable than the grain variety planted at the Archer Research Station (Koch, personal communication). Moreover, Koch et al. (1999) reported that plant population of the unirrigated, highsugar corn in the fall was similar to that high-sugar corn grown under irrigation, but plant height and total DM production was one-half to onethird less than irrigated, high-sugar corn. Conversely, leaf DM proportions of high-sugar corn grown under dryland conditions in the fall and winter was approximately double that reported by Koch et al. (1999) for irrigated, high-sugar corn. As with most corn varieties, high-sugar corn forage has high plant biomass, and as the plant ages the proportion of leaves decreases. Total DM and leaf yield reported in the fall and winter were similar to that produced by grain variety grown under dryland conditions in Nebraska (FernandezRivera and Klopfenstein, 1989b). Chemical and nutrient analyses of masticates collected from steers grazing corn forage during the fall and the winter are presented in Table 2. During yr 1, masticate OM was not affected (P=0.10) by sampling date.

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In yr 2, masticate OM (P=0.06) tended to decrease on d 21 and then increase on d 35. Although masticate N was greater (P=0.03) at the beginning (except on d 0) than at the end of the grazing season during yr 1, masticate N was not affected by sampling date in yr 2. In yr 1, masticate NDF and ADF were least (P=0.02 and 0.003) on d 28 and d 7, respectively, and then increased on d 35. In yr 2, masticate ADF and NDF were greater (P=0.003 and 0.02) on d 35 compared with that on d 0. Fernandez-Rivera and Klopfenstein (1989a) reported similar trends in fiber and N content of masticate samples collected from steers consuming dryland grain corn forage. Masticate NDF content reported in yr 1 and 2 for sterile, high-sugar corn in our studies was similar to leaf, husk, stem, and cob portions of dryland grain corn forage (Fernandez-Rivera and Klopfenstein, 1989b). Similarly, our values for masticate ADF during both yr 1 and 2 were within ranges reported for husks, leaf blades, stems, and cobs of grain corn forage harvested from late October to late December (Gutierrez-Ornelas and Klopfenstein, 1991). Rate and extent of IVOMD for masticate collected from steers grazing Cargill HS 60A during yr 1 and 2 is presented in Table 3. In yr 1, rate of IVOMD was greater (P=0.04)

on d 7 than on d 35. The differences in rate of IVOMD coincide with changes in the extent of IVOMD at the 6-, 9-, and 12-h incubation times. During yr 2, rate and extent of IVOMD were greater (P=0.05) on d 0 than on d 35. Masticate IVOMD in our trials were greater than leaf blades, stems, and cobs, but less than husks of grain corn forage harvested from late October to late December (Gutierrez-Ornelas and Klopfenstein, 1991). The lesser extent of forage digestibility combined with a slower rate of digestion suggest a lesser nutritive quality of high-sugar corn that has been stockpiled longer. Trial 3 (Fall 2001). Nutrient composition and performance of cattle grazing corn forage during yr 3 is presented in Table 4. A three-way interaction was noted for ash only (P=0.05). Upper stalks had less ash (P=0.04) than did leaves and lower stalks. The least squares means of ash were the same (P=0.07) from October 2 to November 2. Furthermore, ash contents of the two harvesting methods were comparable (P=0.78). Forage ADF (P=0.40), NDF (P=0.10), CP (P=0.88), and IVDMD (P=0.57) were similar for standing and windrowed corn forage. Forage NDF increased (P=0.0005) and IVDMD decreased (P=0.02) in November when compared with September. Forage ADF was not affected (P=0.10)

TABLE 1. Dryland production of Cargill HS 60Aa high-sugar corn. Collection dates 2001

Item Plant population, plants/ha Average plant height, cm Total DM production, kg/ha Leaf DM, % of total aCargill, bSD

1998

1999

Block 1 (3.2 ha)

68,170 71,703 62,383 110 124 104 5200 7240 3443 54 19 40

Block 2 (8.4 ha) 65,825 145 4464 31

Minneapolis, MN. = Standard deviation for all replicates over the three trials.

SDb 8959.6 22.6 1068.7 6.6

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High-Sugar Corn for Grazing Beef Cattle

TABLE 2. Chemical and nutrient analysis of masticates collected from steers grazing Cargill HS 60Aa during fall and winter (Trials 1 and 2, respectively). Sampling date 1998b Item

d0

d7

OM, % of DM N, % of OM NDF, % of OM ADF, % of OM

93.3 93.7 1.5ef 2.0d 81.8de 80.5de 44.5d 38.1e

d 14

d 21

1999c d 28

d 35

94.1 94.9 94.3 94.3 1.8de 1.5ef 1.6ef 1.2f 79.9ef 81.6de 77.9f 82.0d 38.4f 40.9e 40.5e 43.4d

SEM P 0.3 0.1 0.5 0.6

d0

0.10 95.2 0.03 2.2 0.02 61.1f 0.003 23.5d

d7

d 21

d 35

SEM

P

93.5 92.9 2.0 2.1 81.6de 78.1e 35.4d 37.3d

95.4 1.6 86.2d 44.2d

0.4 0.1 1.4 2.1

0.06 0.19 0.003 0.02

aCargill,

Minneapolis, MN. 0, 7, 14, 21, 28, and 35 refer to September 8, September 15, September 22, September 29, October 6, and October 13 (yr 1) sampling dates. cd 0, 7, 21, and 35 refer November 11, November 18, December 2, and December 17 (yr 2) sampling dates. d,e,fMeans within the same row lacking common superscripts are different at the designated P value. bd

by sampling date. Fernandez-Rivera and Klopfenstein (1989a) reported similar trends in fiber content and IVOMD of masticate samples collected from steers consuming dryland corn forage. Results reported in yr 3

corroborate with data reported in yr 2, which demonstrated an increase in forage NDF and ADF and a decrease in IVDMD at the completion of the grazing period. Greater NDF for harvested forage in yr 3 might have

occurred because of leaching of soluble nutrients into the soil caused by weathering. Also, regardless of the harvesting method, data in yr 3 indicated that as grazing season advances, cell wall content of plants

TABLE 3. In vitro rate and extent of forage OM disappearance (IVOMD) for masticates collected from steers grazing Cargill HS 60Aa during fall and winter (Trials 1 and 2, respectively). Sampling date 1998b Item

1999c

d0

d7

d 14

d 21

d 28

d 35

15.1 16.9f 22.0ef 28.1e 39.4 53.8 5.9de

17.5 21.6d 26.6d 32.2d 45.6 61.2 6.5d

15.1 20.1de 26.7d 32.0d 47.7 62.6 5.9de

12.7 17.7ef 24.1dec 28.4e 46.1 60.9 5.0ef

15.4 18.4ef 25.6de 30.0de 48.6 59.9 5.9de

12.2 13.9g 19.4f 23.6f 39.6 53.9 4.4f

aCargill,

d0

d7

d 21

d 35

SEM P

35.0d 45.5d 53.3d 58.8d 70.7d 83.1d 13.1d

17.8ef 26.1ef 44.9ef 50.4ef 63.2ef 74.3ef 9.5e

21.6e 32.6e 41.7e 49.5e 61.5e 71.7e 9.9e

13.2f 21.3f 30.1f 40.5f 56.3e 69.9e 6.4f

1.6 2.3 2.4 2.0 1.7 2.0 0.5

IVOMD (%)

Incubation time 3h 6h 9h 12 h 24 h 48 h Rate, %/h

SEM P

1.0 0.8 1.1 0.9 2.6 2.9 0.3

0.10 0.01 0.03 0.01 0.17 0.26 0.04

0.01 0.01 0.02 0.03 0.03 0.05 0.004

Minneapolis, MN. 0, 7, 14, 21, 28, and 35 refer to September 8, September 15, September 22, September 29, October 6, and October 13 (yr 1) sampling dates. cd 0, 7, 21, and 35 refer November 11, November 18, December 2, and December 17 (yr 2) sampling dates. d,e,f,gMeans within the same row lacking common superscripts are different at the designated P value. bd

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TABLE 4. Nutrient composition of high-sugar corn (Cargill HS 60Aa) and performance of beef cattle grazing Cargill HS 60A during fall (yr 3) Harvesting method Plant part

Sampling date

Wind- StandItem

rowed ing corn corn

SEM P

Lower Upper Leaves stalks stalks SEM P

Sep 19

Oct 1

Nov 5

SEM P

Ash, % of DM NDF, % of DM ADF, % of DM CP, % of DM IVDMD, % ADG, kg/d

9.7 52.2 29.4 15.5 69.1 0.4c

1.0 0.4 1.3 2.2 2.3 0.1

10.2 59.7c 34.3c 14.2 60.6c —

10.9 43.0b 24.4 18.7 75.3c —

9.9 45.1b 26.5 15.1 73.3c —

8.4 51.1c 27.1 14.7 70.2b —

0.8 0.9 0.8 1.0 1.5 —

aCargill,

9.2 48.1 26.8 14.9 71.8 0.6b

0.78 0.10 0.40 0.88 0.57 0.002

10.6 44.9b 26.6b 16.0 73.5b —

7.6 45.9a 23.4a 15.5 77.1a —

0.8 0.9 1.3 0.8 1.4 —

0.09 0.0005 0.009 0.33 0.002 —

0.16 0.0005 0.10 0.34 0.02 —

Minneapolis, MN. within the same row lacking common superscripts are different at the designated P value.

b,cMeans

increased and cell content or cell solubility decreased. Less cell solubility, in turn, likely results in decreased digestibility. In a demonstration research trial conducted on the same site by Koch (1998, 1999; unpublished data), NDF and ADF for leaves vs stalks of standing corn plants were 72.4% vs 50.7%, 35.5% vs 21.5%, and 69.3% vs 54.1%, 31.8% vs 24.0% during 1998 and 1999, respectively. During yr 3 in our study, the NDF and ADF of leaves vs stalks of corn plants were 59.7% vs 45.4% and 34.3% vs 25.0%, respectively. Fernandez-Rivera and Klopfenstein (1989b) reported that NDF, IVDMD, and CP contents of leaves plus husks vs stems of corn forage grown under dryland condition were 81.1% vs 80.9%, 49.7% vs 47.8%, and 6.4% vs 5.9%. In yr 3, plant part affected NDF (P=0.0005), ADF (P=0.009), and IVDMD (P=0.002) values. Leaves had greater NDF and ADF than upper and lower stalks. Consequently, IVDMD of leaves was less than the upper and lower portions of the stalks, suggesting greater concentration of nonstructural carbohydrate in corn stalks. Klopfenstein et al. (1987) attributed the greater IVDMD of corn stalks sometimes noted by researchers

to greater content of sugar when the plant is less mature. This effect might have occurred with the high-sugar corn used in our study because IVDMD was consistently least at the end of the grazing season when very little leaf material remained in the grazed pastures. Alternatively, we would have expected to observe a greater change in digestibility as well as significant plant part × sampling date interaction for IVDMD if the non-structural content of the stalks declined significantly as the plant progressively matured. The greater IVDMD for the stalks noted herein could be related to the high-sugar corn concentrating more nonstructural carbohydrate in the stalks than in the leaves. In contrast to results from yr 1, forage CP in yr 3 was not affected (P=0.34) by sampling date. Hitz and Russell (1998) also noted a slight change in CP of corn crop residues in yr 1 of their experiment, but this effect was not observed in the following yr. Harvesting methods (P=0.88) and plant part (P=0.33) did not affect forage CP during yr 3. Similarly, Fernandez-Rivera and Klopfenstein (1989b) did not observe a difference in CP among plant parts of dryland corn residues.

The pregnant beef cows used in our study had an ADG of 0.5 ± 0.1 kg/d over the 36-d trial. In a summarization of several experiments conducted over 10 yr in Nebraska, Klopfenstein et al. (1987) reported that calves maintained 0.51 kg/d ADG. Furthermore, Klopfenstein et al. (1987) suggested that cow BW gain may be excessive during the first few weeks of grazing corn stalks primarily because of the consumption of grain. Feeding hay to pregnant beef cows grazing corn crop residues during the winter was necessary to maintain an average of 0.24 kg/d ADG (57-d grazing periods) in a 3-yr study (Hitz and Russell, 1998). The grazing period in our study was only 36 d, but grain was not available to the cows because the high-sugar corn variety is malesterile. The high-sugar corn forage used in our study had greater digestibility and CP as well as less fiber content when compared with corn forage residue used in the previous studies (Fernandez-Rivera and Klopfenstein, 1989a,b; GutierrezOrnelas and Klopfenstein, 1991; Hitz and Russell, 1998; Klopfenstein et al., 1987). Therefore, high-sugar corn forage would be expected to support reasonable cattle ADG without

High-Sugar Corn for Grazing Beef Cattle

supplementary feeding. Cows grazing standing corn forage had greater (P=0.03) ADG than cows grazing windrowed corn forage. Although this result would not be expected based on forage quality indices, an undetermined quantity of forage left in windrows was lost to a strong windstorm, which decreased the amount of forage available for cows grazing windrowed corn forage.

Implications Digestibility of stockpiled highsugar corn forage decreased by the conclusion of the grazing periods, whereas CP content remained fairly constant over the course of the grazing seasons. Despite detection of differences indicative of declining quality, high-sugar corn supported reasonable cattle BW gains throughout the fall grazing period. Windrowed high-sugar corn forage had comparable nutritional quality indices to that of stockpiled highsugar corn; however, cattle gains were reduced when corn forage was grazed in windrows. Stockpiled highsugar corn forage is an acceptable alternative forage crop for grazing beef cattle, but the forage is not suitable for windrow grazing.

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Hitz, A. C., and J. R. Russell. 1998. Potential of stockpiled perennial forages in winter grazing systems for pregnant beef cows. J. Anim. Sci. 76:404.

Literature Cited AOAC. 1990. Official Methods of Analysis. (15th Ed.). Association of Official Analytical Chemists, Arlington, VA. Broesder, J. T., S. A. Gunter, L. J. Krysl, and M. B. Judkins. 1992. Digestion of ruminal masticate dried by three different methods. Anim. Feed Sci. Technol. 37:323. Brokaw, L., B. W. Hess, and D. C. Rule. 2001. Supplemental soybean oil or corn for beef heifers grazing summer pasture: Effects on forage intake, ruminal fermentation, and site and extent of digestion. J. Anim. Sci. 79:2704. Fernandez-Rivera, S., and T. J. Klopfenstein. 1989a. Diet composition and daily gain of growing cattle grazing dryland and irrigated cornstalks at several stocking rates. J. Anim. Sci. 67:590. Fernandez-Rivera, S., and T. J. Klopfenstein. 1989b. Yield and quality components of corn crop residues and utilization of these residues by grazing cattle. J. Anim. Sci. 67:597. Goering, H. K., and P. J. VanSoest. 1970. Forage fiber analyses (apparatus, reagents, procedures, and some applications). Agric. Handbook No. 379. ARS, USDA, Washington, DC. Gutierrez-Ornelas, E., and T. J. Klopfenstein. 1991. Changes in availability and nutritive value of different corn residue parts as affected by early and late grazing seasons. J. Anim. Sci. 69:1741.

Judkins, M. B., L. J. Krysl, and R. K. Barton. 1990. Estimating diet digestibility: A comparison of 11 techniques across six different diets fed to rams. J. Anim. Sci. 68:1405. Kercher, C. J., J. Lauer, and R. Jones. 1986. Barley, sugar corn and regular corn silage for growing beef steers. Univ. Wyoming Anim. Sci. Roundup Rep., Laramie, WY. Koch, D. W., B. W. Hess, L. M. Yun, and J. W. Flake. 1999. Sugar corn for winter grazing. Univ. Wyoming Agric. Exp. Stn. Rep., Laramie, WY. Klopfenstein, T., J., L. Roth, S. FernandezRivera, and M. Lewis. 1987. Corn residues in beef production systems. J. Anim. Sci. 65:1139. Lesperance, A. L., V. R. Bohman, and D. W. Marble. 1960. Development of techniques for evaluating grazed forage. J. Dairy Sci. 43:682. Mertens, D. R., and J. R. Loften. 1980. The effect of starch on forage fiber digestion kinetics in vitro. J. Dairy Sci. 63:1437. SAS. 1998. SAS Language Guide for Personal Computers (Version 4.1, Release 7.0). SAS Inst., Inc., Cary, NC. Steel, R. G. D., and J. H. Torrie. 1980. Principles and Procedures of Statistics: A Biometrical Approach. (2nd Ed.). McGrawHill Publishing Co., New York. Whitney, M. B., B. W. Hess, L. A. BurgwaldBalstad, J. L. Sayer, C. M. Tsopito, C. T. Talbott, and D. M. Hallford. 2000. Effects of supplemental soybean oil level on in vitro digestion and performance of prepubertal beef heifers. J. Anim. Sci. 78:504.