J. Anim. Sci. 2005. 83:1445-1454
© 2005 American Society of Animal Science
Effect of cutting height and genetics on composition, intake, and digestibility of corn silage by beef heifers1
L. R. Kennington*,
C. W. Hunt*,2,
J. I. Szasz*,
A. V. Grove* and
W. Kezar
* Department of Animal and Veterinary Science, University of Idaho, Moscow 83844; and
and
Pioneer Hi-Bred International, Inc., Boise, ID 83704
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Abstract
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This research was conducted to determine the effect of corn genetics and cutting height on the composition and nutritive characteristics of corn silage. An in situ study involving eight commercially available corn hybrids indicated main effects and interactions (P < 0.01) of hybrid and cutting height on NDF, ADF, and starch content and on in situ DM and NDF degradablility. Four ruminally cannulated Angus heifers (initial BW = 378 ± 3 kg) were used in a 4 x 4 Latin square digestion experiment with a 2 x 2 factorial treatment arrangement. Main effects and interactions of hybrid (Pioneer Hi-Bred Int., Inc., hybrids 3335 and 3223) and cutting height (LO = 20.3 cm, and HI = 61 cm) were evaluated. Dietary treatment consisted of 40% chopped alfalfa hay and 60% corn silage. Although corn silage hybrids used were of equivalent maturity at harvest (60% milkline), 3335 treatments had 37.8% starch and 34.8% NDF, whereas 3223 treatments had 33.7% starch and 38.6% NDF. The LO treatments averaged 3.1 percentage units greater in NDF and 3.45 percentage units less in starch content than the HI treatments. Intake of DM was greater for heifers fed 3335-HI than 3335-LO; however, DMI was greater by heifers fed 3223-LO than 3223-HI (hybrid x cutting height interaction, P < 0.05). Starch intake was greater (P < 0.05) and NDF intake was less (P < 0.05) by heifers fed HI vs. LO and fed 3335 vs. 3223 dietary treatments. Digestibility of DM, starch, and NDF was greater (P < 0.05) by heifers fed 3223 than 3335 dietary treatments, but digestibility differences were not observed (P > 0.10) between cutting heights. Rate of in situ DM and starch degradability was not affected (P > 0.10) by hybrid or cutting height; however DM degradability was greater (P < 0.05) for HI than LO corn silage substrates at 8, 16, and 24 h of incubation. Rate of NDF degradability tended (P = 0.08) to be greater for 3223 than for 3335, and for LO compared with HI corn silage. Degradability of NDF was greater (P < 0.05) for 3223 compared with 3335 substrates at 24, 36, and 48 h of incubation. These data suggest fiber may not be an accurate measure of corn silage quality. Whereas cutting height affected chemical composition, we observed genetics to have a greater effect on corn silage quality.
Key Words: Beef Cattle • Corn Silage • Cutting Height • Digestibility
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Introduction
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Corn silage is commonly cultivated as a feed ingredient largely due to the high amount of digestible DM harvested per hectare. Research has revealed that plant genetics can affect the digestibility of whole-plant corn silage (Hunt et al., 1992
; Wolf et al., 1993). Consequently, there is an interest in identifying new corn hybrids with improved nutritive value (Johnson, Jr. et al., 1997; Kuehn et al., 1999).
The nutritive value of corn silage can also depend on stage of maturity, mechanical processing, and chop length (Bal et al., 1997; Doggett et al., 1998; Johnson et al., 1999; Andrae et al., 2001). Further, increasing the cutting height of corn silage may increase digestibility due to the elimination of the high-fiber lower stalk. Research involving sorghum silage showed no increase in DM digestibility when cutting height was varied from 12 to 65 cm (Hart, 1990). Recent reports evaluating corn silage quality indicate that raising the cutting height may increase the quality of corn silage; however, these effects might be diluted by differences in maturity (Neylon and Kung, 2003) and corn genetics (Bernard et al., 2004). Research evaluating the interactive effects of cutting height with other factors that affect corn silage quality is warranted. Thus, our objectives were to evaluate the effects of genetics and cutting height at harvest on composition and in situ and in vivo digestibility of whole-plant corn silage.
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Materials and Methods
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In Situ Study
A laboratory-based study was conducted to establish the contribution of genetics and cutting height to nutrient composition and in situ degradability of whole plant corn. Samples for this study were obtained from a hybrid test plot, in which hybrids were planted in a uniform, flood-irrigated field. The plots were on silt loam soil and sufficient irrigation was provided for optimal plant growth. Eight hybrids were selected for evaluation and each hybrid was represented in six rows with 72-cm spacing. Ten whole plants (WP) of each hybrid (Pioneer hybrids 3130, 3223, 32K61, 32R42, 3335, 34B23, 34W67, and 35R57; Pioneer Hi-Bred Int., Inc., Johnston, IA) were harvested by hand from three randomly selected areas of the test plot. Whole plants were harvested at 20.3-cm stubble height. At the edge of the field, each replicate of 10 WP was divided into two sets of five plants each. One set of five WP remained intact to represent the low cutting height treatment. The bottom 40.7 cm of each plant in the other WP set was removed to create the high cutting height treatment. The 40.7-cm section of lower stalk (LS) was retained to evaluate this portion of the WP. Each set of WP and LS was then ground through a garden shredder (Kemp Co., Lititz, PA) to an approximate 1.9-cm particle length. The ground samples were then weighed and dried in their entirety at 55°C for 72 h in a forced-air oven and DM content was recorded. Samples were then ground with a hammer mill (Fitzmill, W. J. Fitzpatrick Co., Chicago, IL) to pass a 5-mm screen. A subset of the samples also was ground to pass a 2-mm screen using a Udy Cyclone mill (Udy Corp., Fort Collins, CO) for analysis of DM, CP by Kjeldahl N, ash (AOAC, 1990), NDF, ADF, and ADL (Van Soest et al., 1991). Whole plant samples also were analyzed for starch (Megazyme, Wicklow, Ireland). The portion of the samples that remained ground to 5 mm was weighed in triplicate into 5- x 10-cm nylon bags (pore size = 53 µm; Ankom Corp., Fairport, NY); each bag contained approximately 5 g of sample DM. Bags were inserted into the rumens of three ruminally cannulated Angus heifers fed a 60% corn silage and 40% alfalfa hay diet (DM basis) at staggered time intervals and removed simultaneously such that bag contents were incubated for 24 and 72 h. Bags were removed and washed with cold tap water until the water ran clear and then dried in a forced-air oven at 55°C for 72 h. Residues were then analyzed to determine DM, NDF, and ADF disappearance.
Composition and in situ degradability data for WP and LS were analyzed as a completely randomized design using GLM procedures of SAS (SAS Inst., Inc., Cary, NC). Data for WP variables were analyzed as a 2 x 8 factorial arrangement (cutting height x corn hybrid) of treatments. Partial correlation values were also determined using SAS to evaluate the relationship between composition and in situ DM degradability.
Digestion Study
Dietary Treatments and Animals.
Four ruminally cannulated (10 cm i.d.) Angus heifers (initial weight 378 ± 3 kg) were used in a 4 x 4 Latin square digestion experiment with a 2 x 2 factorial treatment arrangement. Main effects and interactions of hybrid (Pioneer 3335 and 3223) and cutting height (LO = 20.3 cm, and HI = 61 cm) were evaluated. The corn hybrids were selected as two widely grown hybrids that were known to differ in grain yield (3335 has greater yields than 3223). Cutting heights used for this study were determined by lowering the cutting bar of the silage harvester to its minimum level for the LO corn silage (20.3 cm), and then cutting the HI corn silage (61 cm) at approximately three times this height. Corn silage treatments were stored in manually packed 1.0-m x 1.25-m x 1.5-m boxes lined with plastic bags and were stored sealed for 60 d before the initiation of the trial. Corn silage was harvested using a field chopper (Claas 880 Jagua; Claas of America, Columbus, IN) equipped with on-board rollers set at a 1-mm spacing. The theoretical chop length was set at 1.9 cm. Particle size distribution was determined following fermentation using the procedure outlined by Lammers et al. (1996). Dietary treatments (DM basis) were 40% chopped alfalfa hay (theoretical particle length of 3.8 cm) and 60% corn silage. To avoid large differences between animals, heifers were obtained from one source and were of the same breed composition from a single herd. Heifers were housed individually in covered pens (3.0 m x 13.7 m) and provided with rubber mats for bedding. The University of Idaho Animal Care and Use Committee approved all procedures involving experimental animals, and a licensed veterinarian at Washington State University College of Veterinary Medicine performed surgical placement of cannulas. A 2-mo postsurgery period was allowed before the initiation of the experiment.
Heifers were individually fed daily at 0700 and 1900 and had ad libitum access to a trace mineral salt supplement (97% NaCl, 0.35% Zn, 0.28% Mn, 0.175% Fe, 0.035% Cu, 0.007% I, 0.007% Co) at all times. Heifers were adjusted to silage and alfalfa hay-based diets for 28 d. Subsequently, each period of the Latin square consisted of a 10-d adaptation period followed by a 4-d sample collection period. Feed was offered such that a minimal quantity of feed refusals was present in the bunk before the 0700 feeding. Feed intake was recorded daily. Heifers were restricted to 90% of ad libitum intake 2 d before sample collection. Thereafter, feed intake was held constant throughout the collection period.
Sample Collection and Analysis.
Feed samples were obtained daily beginning 2 d before the collection period and ending on the last day of the collection period. Feed samples were immediately frozen at –20°C. Feed samples from each day for each heifer within each period were subsampled (100 g from each day), composited, and ground to pass a 2-mm screen using a Udy Cyclone mill. Subsample composites were then analyzed for DM, CP by Kjeldahl N, ash, NDF, ADF, ADL, and starch as described previously.
Ruminal fluid (200 mL) was collected before feeding and at 2, 4, 6, 8, and 10 h following the morning feeding. Ruminal fluid was strained through two layers of cheesecloth, and pH was measured immediately with a pH meter equipped with a glass electrode (Orion Research, Boston, MA). Ruminal fluid was then acidified with 1 mL of 50% (vol/vol) H2SO4/100 mL of ruminal fluid, and a 100-mL aliquot was partitioned for ammonia N analysis by the steam distillation method of Bremner and Keeney (1965). A second 100-mL aliquot of ruminal fluid was stored frozen at –20°C and later analyzed for VFA by gas chromatography (Supelco, 1998; GS model 6890, Hewlett Packard, Palo Alto, CA).
Chromic oxide (10 g/d) was used as an external digestibility marker. Chromic oxide was thoroughly mixed with the morning feeding beginning 7 d before and throughout the sample collection period. Fecal samples were collected over 4 d to represent every 4 h of a 24-h cycle. Samples were initially stored frozen at –20°C. Fecal samples were then air-dried at 55°C for 72 h, composited on an equal weight basis by animal within period, and ground to pass a 2-mm screen using a Udy Cyclone mill. Fecal samples were then analyzed for starch, NDF, ADF, and ash as described previously, and Cr was analyzed by atomic absorption (Williams et al., 1962). Digestibility was calculated using the equations of Merchen (1988).
In situ ruminal degradability of the corn silage treatments was analyzed. Corn silage samples from each treatment were obtained for each period at the beginning of the trial and stored frozen at –20°C. Samples were dried and ground with a Fitzmill hammer mill to pass a 5-mm screen as described previously. Approximately 5 g of sample DM was then placed in 5-cm x 10-cm nylon bags (pore size = 53 µm; Ankom Corp.). Bags were soaked in cold water for 15 min before incubation. Bags were inserted into the rumen at staggered time intervals and removed simultaneously such that triplicate incubations of 0, 8, 16, 24, 36, 48, and 96 h were represented. Washing and drying procedures were as described previously. Triplicate incubation bag residues were composited, ground to pass a 2-mm screen, and analyzed as previously described to determine OM, starch, NDF, and ADF disappearance. Due to large variability in NDF and ADF degradability before the 24-h incubation, only 24, 36, 48, and 96 h incubation data are reported. Rates of DM, starch, NDF, and ADF in situ disappearance were determined using least squares regression of natural logarithmic-transformed residuals with correction for undegraded residue (Nocek and English, 1986).
Statistical Analyses.
The experimental design of this study was a 4 x 4 Latin square. The experimental unit was individual animal. In vivo digestibility and in situ degradability (each h of incubation) data were analyzed with the Mixed procedure of SAS. The model included heifer, period, hybrid, cutting height, and hybrid ( cutting height interaction. Ruminal fluid measurements taken over time (VFA, pH, and ammonia N) were analyzed as repeated measures also with the Mixed procedure of SAS. Means were separated by Fisher’s LSD when a significant (P < 0.05) overall treatment F-value was observed.
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Results and Discussion
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In Situ Study
Results of WP composition and ruminal degradability measured in the in situ study are presented in Table 1. Main effects due to cutting height and corn hybrid were observed (P < 0.01) for DM, NDF, ADF, and starch content; a main effect due to cutting height also was observed (P < 0.01) for ADL content. Cutting height and hybrid main effects also were observed (P < 0.01) for in situ DM disappearance at 24 and 72 h of ruminal incubation and for in situ NDF disappearance at 24 h of incubation. Similarly, main effects due to hybrid were observed (P < 0.01) for DM, NDF, ADF, and in situ degradability of LS samples (data not shown). These data clearly establish the existence of variability in WP corn due to hybrid and cutting height for key nutritive variables.
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Table 1. The effect of hybrid (H) and cutting height (CH) on composition and ruminal in situ degradability of whole plant corn
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Cutting height x hybrid interactions also were observed (P < 0.01) for WP composition of NDF, ADF, and starch, and for in situ NDF disappearance at 24 h of incubation. Evidence of these interactions can be observed in the hybrid range columns in Table 1
. Although WP harvested at the high cutting height averaged across all hybrids had lesser (P < 0.01) NDF and ADF contents and greater (P < 0.01) starch content compared with the average WP of all hybrids harvested at the low cutting height, some hybrids harvested at the high cutting height had greater NDF and ADF and lesser starch content than hybrid samples harvested at the low cutting height. Partial correlations of all variables measured with 24-h in situ DM disappearance were weak, with only ADF content of WP being different (P < 0.05) from zero. Bernard et al. (2004) also recently reported a corn hybrid x cutting height interaction, as only one of the two hybrids they evaluated had lower ADF and greater in vitro DM disappearance at the high vs. low cutting height. These data indicate that raising the cutting height may not always achieve improved corn silage quality and that the cutting height response is greatly dependent on the corn hybrid.
Digestion Study
The two hybrids selected for evaluation in the digestion study, Pioneer 3223 and 3335, were similar in composition and in situ degradability as evaluated in the in situ study. In the in situ study, hybrid 3223 and 3335 HI samples had ADF contents of 20 and 20.4%, respectively, and LO samples had ADF contents of 24.2 and 23.9%, respectively. In situ DM disappearance at 24 h was 60.5 and 62.3% for Hybrid 3223 and 3335 HI samples, respectively, and 59.1 and 59.6% for 3223 and 3335 LO samples, respectively. These ADF and in situ disappearance values are very similar to the eight-hybrid averages presented in Table 1.
Chemical composition of corn silage and dietary treatments for the digestion study are shown in Table 2 and 3, respectively. Hybrid 3335 harvested at 20.3 cm had numerically 9% greater DM yield per hectare than when harvested at 61 cm. For Hybrid 3223, DM yield was numerically 18.7% greater when harvested at 20.3 vs. 61 cm (data not shown). Although no statistical analyses were conducted for composition, numerical values for chemical analysis conducted indicate Hybrid 3223 had 10.9% greater NDF, 10.4% greater ADF, and 11.1% lower DM content than 3335 corn silage. These numerical differences reveal an interesting hybrid effect because the two hybrids were harvested at the same starch milkline (60%). As expected due to the greater fiber content, ADL content was 5.3% greater for diets containing 3223 than 3335. Because the two corn silage hybrids were harvested at a similar milkline (60%), this would suggest Hybrid 3223 contained more fibrous stover at similar maturity than did 3335. Huber et al. (1965) reported as grain content increased with advanced maturity, crude fiber content tended to decrease due to the dilution effect of increased starch content. Therefore, it is difficult to conclude which hybrid had the more mature stover fraction at a given kernel starch milkline. As a percentage of the WP, numerical starch content was 10.8% greater for 3335 than 3223, presumably due to greater grain content for 3335. Numerous other studies have shown differences among corn hybrids in percentage of grain and fiber (Johnson, Jr. et al., 1985; Hunt et al., 1992; Roth, 1994). Hybrid 3335 had numerically 1.4% greater CP content than hybrid 3223, perhaps due to the increase in protein matrix formation within the endosperm as starch content increased. Numerical differences described here for samples collected during the digestion study were not observed when these hybrids were evaluated in the in situ study
In addition to the effects of hybrid on chemical composition of corn silage, cutting height also affected the numerical composition values of corn silage (Table 2). When corn silage was cut at 20.3 cm, numerical DM content was 1.1% less for 3335 and 5.8% less for 3223. This is due to the low DM content of the stalk included in the corn silage at this cutting height. Corn silage cut at 20.3 cm had numerically 9.2% greater NDF and 9.2% less starch than corn silage cut at 61 cm (Table 2). Greater content of starch when silage is cut higher may have important implications in digestibility. Although starch is recognized to be more digestible than fiber, when greater amounts of starch are added to diets, NDF digestibility can be depressed (Grant and Mertens, 1992; Heldt et al., 1999) due to the increased production of organic acids in the rumen.
Ruminal Fluid Characteristics
Characteristics of ruminal fluid collected are reported in Table 4. No interactions involving hybrid, cutting height, or time of sampling were observed; therefore, main effects of corn hybrid and cutting height are reported. No differences were detected between corn hybrid and cutting height for ruminal fluid ammonia N concentration. Diets were formulated to have similar levels of CP from alfalfa and corn silage; therefore, ruminal ammonia concentrations were not expected to differ. Mean ruminal fluid pH was lower (P < 0.05) for heifers fed 3335 compared with 3223. Average ruminal fluid pH was below 6.2 for heifers fed 3335 for approximately 2 h; however, average ruminal fluid pH did not fall below 6.2 for 3223 for any amount of time. Researchers have suggested that ruminal fluid pH less than 6.2 will decrease fiber digestion through decreased growth and activity of fibrolytic bacteria (Therion et al., 1982; Grant and Mertens, 1992). Although the overall mean pH was above this level, increased time below pH 6.2 for heifers fed 3335 may have affected ruminal fiber digestion. Greater starch content of 3335 likely resulted in greater organic acid production, which decreased ruminal pH more for heifers fed 3335 than for those fed 3223. No differences were observed in ruminal pH between HI and LO, suggesting that the absence of the LS from the high-cut corn plants did not affect pH as much as the hybrid main effect.
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Table 4. Main effects of corn hybrid and cutting height on ruminal fluid pH, VFA, and ammonia concentrations in beef heifers
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Total VFA concentration was greater (P = 0.05) for 3335 than for 3223 treatments; however, no differences were observed between LO and HI treatments (Table 3). The greater VFA concentration in diets containing 3335 was likely due to the greater starch content in those diets, which would stimulate increased ruminal organic acid production compared with 3223 diets with less starch. The total ruminal fluid VFA concentration data are consistent with ruminal fluid pH data.
Molar proportion of acetate was greater (P = 0.01) for diets containing 3223 compared with 3335 and for diets containing LO than HI corn silage. Conversely, the molar proportion of propionate was greater (P = 0.02) for diets containing 3335 than those containing 3223, and for diets containing HI vs. LO corn silage. These differences resulted in 3223 and LO treatments having a greater (P = 0.01) acetate:propionate ratio compared with 3335 and HI treatments. Main-effect differences in molar proportion of acetate and propionate reflected differences in starch and fiber contents of silage treatments (Table 2); higher starch content silage resulted in greater proportions of propionate, and higher fiber content silage resulted in greater proportions of acetate. Ruminal molar proportion of butyrate was greater (P < 0.05) for 3335 compared with 3223 treatments and for HI compared with LO treatments. Although statistically significant, these main-effect differences in molar proportion of butyrate were small and probably of little biological significance.
Concentrations of valerate were greater (P < 0.05) for 3335 than for 3223 treatments and for HI than LO treatments. The opposite effect was observed for isovalerate, with 3223 and LO treatments having greater (P < 0.05) concentrations than 3335 and HI treatments, respectively. No differences were observed in isobutyrate concentration. It has been suggested that these minor organic acids are important growth factors for fibrolytic organisms (Gorosito et al., 1985).
Intake and Digestibility
Intake of OM was greater for heifers fed 3335-HI compared with 3335-LO (11,056 and 10,754 g/d); however, OM intake was greater for heifers fed 3223-LO vs. 3223-HI (hybrid x cutting height; P = 0.01; Table 5). Cutting height did not affect OM digestibility; however, OM digestibility was greater (P = 0.04) for 3223 compared with 3335 treatments. The lack of a cutting height effect on OM digestibility was not expected due to the absence of the LS portion that was high in NDF content; however, as stated earlier, the absence of the LS caused an increase in starch content for HI compared with LO treatments. The numerically lower pH for HI treatment may have been sufficient to decrease ruminal fibrolytic activity. A similar effect was observed by Hart (1990), who found that increasing sorghum silage cutting height from 12 to 65 cm did not increase DM digestibility. In contrast, Garcia et al. (2000) reported a tendency (P = 0.07) for corn silage cut at 90 cm to have more digestible DM than corn silage cut at 15 cm (61.9 vs. 58.9%).
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Table 5. Main effects of corn hybrid and cutting height on intake and total-tract digestibility of organic matter, N, starch, neutral detergent fiber, and acid detergent fiber in beef heifers
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Starch intake was greater (P < 0.01) by heifers fed 3335 vs. 3223 treatments (Table 5). This effect was a result of 3335 having numerically greater starch content than 3223 silage. Also, cutting corn silage at HI compared with LO resulted in greater (P < 0.01) starch intake because starch content as a percentage of the total plant increased due to the exclusion of 41 cm of the lower stalk. Starch digestibility was greater (P = 0.01) for diets containing 3223 compared with 3335 treatments. Andrae et al. (2001) and Doggett (1998) found differences in starch digestion between corn silage hybrids. They suggested corn genotypes may differ in vitreous starch content within the kernel endosperm, which is more resistant to microbial and enzymatic attack than floury starch (Philippeau et al., 1999). Although vitreousness was not measured in our study, differences in starch characteristics within the corn kernels of our hybrids may have been a contributing factor to starch digestibility.
Intake of NDF was greater (P = 0.05) for 3223 compared with 3335 treatments, and ADF intake followed the same numerical trend (P = 0.12; Table 5). This effect was due to greater NDF and ADF content of 3223 compared with 3335 silage. Likewise, heifers fed LO treatments had greater (P < 0.05) NDF and ADF intake compared with heifers fed HI treatments due to greater NDF and ADF content of LO treatments. Treatments containing 3223 silage had greater (P = 0.01) total-tract digested NDF and ADF (g/d) and digestibility (percentage of intake) compared with treatments containing 3335 silage (Table 5). Al-Jobeile et al. (2000) also found that high-NDF corn silage-based diets had greater total-tract NDF digestibility than low-NDF diets. Total-tract ADF digested (g/d) was greater (P = 0.04) for LO than for HI treatments, and the same trend (P = 0.07) was observed for NDF digested (g/d). Cutting height main effects were not observed for NDF or ADF digestibility; however, numerical trends (P = 0.25 and 0.19, respectively) were for greater fiber digestibility for LO vs. HI treatments. Hybrid main effect differences may be partially attributed to the disruptive effects of starch on ruminal fermentation (Kaufman, 1976). Compared with 3223 treatments, 3335 treatments had lower fiber digestion but had greater starch digestion (amount per day and percentage of intake), which was associated with lower ruminal pH values. Hybrid main effect differences for fiber digestibility also may be attributed to genetic differences in degradability of the fiber present in the silages. Other researchers have reported genetic differences in fiber digestibility of corn silage (Hunt et al., 1992; Xu et al., 1995; Doggett et al., 1998). Specific differences in the architecture of fiber that account for genetic differences in fiber digestibility of corn silage have not been identified. Understanding of these differences has likely been obscured by the confounding effect of starch in corn silage. As an example, it is logical to assume that fiber in the lower stalk is less digestible than fiber in the remainder of the corn plant; however, we observed fiber digestibility to be equal in LO compared with HI treatments. We hypothesize that this contradiction was the result of greater amounts of starch confounding fiber digestion in the HI treatments. This hypothesis is supported by Hart (1990), who reported greater NDF and ADF digestibility when sorghum silage was harvested at 12 cm compared with 65 cm cutting height.
A hybrid main effect (P = 0.04) for N intake and hybrid x cutting height interactions (P < 0.05) for N intake and N digested were observed. Nonetheless, the magnitude of difference among treatments for N intake and digestion suggests that biologically significant differences did not occur.
Ruminal In Situ Degradability
Neither hybrid type nor cutting height affected rate of DM degradation (Table 6). Ruminal in situ degradability of DM was not different for 3335 and 3223 for any time of incubation. However, degradability of DM was greater (P < 0.05) at 8, 16, and 24 h of incubation for HI compared with LO substrates. Substrates consisting of HI had greater starch content and less NDF (Table 2) compared with LO corn silage, which is a likely explanation for HI having more degradable DM in the first 24 h compared with LO treatment. Thereafter, at 36, 48, and 96 h, no differences were observed in DM degradation between HI and LO, which would be expected due to starch in both HI and LO treatments being essentially thoroughly degraded (95.6 and 95.4%, respectively) by this time and therefore not affecting further degradation. In addition, stalk quality was not as poor as expected; DM degradability of 3335 and 3223 was 51.3 and 47.7% at 24 h, and 64.3 and 62.4% at 72 h, respectively. These numbers would indicate that the stalk portion of the stover might not decrease the digestibility of the total plant as much as expected. Hybrid main effect differences in DM in situ degradability were not consistent with our in vivo data, which showed 3223 to have greater total-tract OM digestibility than 3335. This lack of hybrid difference suggests that rate of passage and mean retention time (not measured) and the subsequent postruminal digestion of DM constituents may have been important factors affecting our in vivo results. Main effects of hybrid and cutting height were not observed for starch degradability or rate of ruminal starch digestion, suggesting that within the ruminal environments, starch from both corn hybrids was digested similarly. This would suggest that the greater total-tract starch digestibility of 3223 compared with 3335 was due to greater postruminal starch digestion.
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Table 6. Main effects of corn hybrid and cutting height on in situ dry matter and starch degradability in beef heifers
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Due to large variability in NDF and ADF degradability before 24 h, only values for 24, 36, 48, and 96 h are presented (Table 7). Degradability of NDF and ADF was greater (P < 0.05) for 3223 compared with 3335 treatments at 24, 36, and 48 h, but did not differ at 96 h. Rate of potentially degradable ADF disappearance was greater (P = 0.04) for 3223 compared with 3335 treatments; the same numerical trend (P = 0.08) was observed for rate of NDF disappearance. These data support our in vivo total-tract NDF digestion findings, which showed 3223 to have increased fiber digestibility compared with 3335. In addition, residual effects of lower ruminal pH for 3335 compared with 3223 treatments due to greater starch intake may have affected fiber digestion. Moreover, in situ data lend further support to our hypothesis that 3223 had fiber architecture, which possibly is more conducive to microbial attachment and degradation than 3335. Other research has shown that lower fiber content of corn silage does not necessarily translate to greater fiber digestion. For example, Thomas et al. (2001) selected two corn silage hybrids for dairy cows: one normal variety and one containing a high proportion of leaves and stalks. They reported the leafy hybrid had greater in vitro DM and NDF disappearance than the normal hybrid. We hypothesize that other factors, such as differences in fiber architecture previously alluded to, may have a greater effect on fiber quality than NDF or ADF content. Although no differences in NDF and ADF degradation were observed between HI and LO treatments at 24, 36, and 96 h, NDF and ADF degradation was greater (P < 0.05) for LO than HI treatments at 48 h (Table 7). Consequently, rate of ADF degradation was greater (P = 0.01), and rate of NDF degradation tended to be greater (P = 0.08), for LO compared with HI treatments. Again, the effect of low ruminal pH may have decreased fiber digestion in the HI but not the LO treatment due to greater starch intake for HI compared with LO. Although the starch within the bag would have been largely degraded by 24 h, overall ruminal pH throughout the entire rumen may still have affected fiber degradation. Degradability of NDF contained within the stalk for 3335 and 3223 was 56.3 and 57.2% at 24 h, and 58.1 and 59.3% at 72 h. This finding would indicate the stalks were of fair fiber quality, which would indicate that including the stalk in the silage would not dilute the digestibility of the WP corn silage as much as would be expected based on stalk NDF and ADF content.
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Table 7. Main effects of corn hybrid and cutting height on in situ neutral detergent fiber and acid detergent fiber degradability in beef heifers
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Implications
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Results of our study suggest the nominal benefits in silage quality from harvesting at a greater cutting height would not justify the loss of yield due to additional stalk left in the field. Hybrid digestibility differences that existed in the present study indicate that plant genetics affect digestibility of corn silage. Based on our results, we conclude that genetics had a more important role in digestibility of corn silage than cutting height at harvest. This conclusion would indicate there are possibilities for selecting corn hybrids that are more digestible and therefore more economical for the producer. Importantly, hybrid x cutting height interactions were infrequent, indicating that selection for genetically superior corn silage should be independent of cutting height management.
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Footnotes
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1 Funding for this research was provided by the Idaho Agric. Exp. Stn. and by Pioneer Hi-Bred International Inc., Johnston, IA. 
2 Correspondence—phone: 208-885-6932; fax: 208-885-6420; e-mail: chunt{at}uidaho.edu.
Received for publication September 11, 2004.
Accepted for publication February 10, 2005.
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Literature Cited
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Al-Jobeile, H., M. A. Bal, R. D. Shaver, and J. G. Lauer. 2000. Influence of corn silage fiber content and level of dietary concentrate supplementation on intake, digestion, and milk production by dairy cows. J. Anim. Sci. 78(Suppl. 1):118. (Abstr.)
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Abstract
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