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ANIMAL PRODUCTION |
Department of Animal Science and ARS-USDA, North Carolina State University, Raleigh, NC 27695-7621
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: beef steer • diurnal harvest • gamagrass • silage • switchgrass
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; 2002). These studies and others (Buntinx et al., 1997; Mayland et al., 2000) found that tall fescue or alfalfa hays harvested in the afternoon had less fiber and greater in vitro true dry matter disappearance (IVTDMD) than hays harvested in the morning from the same plots. Multidimensional scaling linked increased TNC concentrations of alfalfa hays harvested in the PM to increased short-term intake preference by goats, sheep, and cattle (Buntinx et al., 1997). Compared with morning harvest, afternoon harvest of alfalfa hay increased voluntary DMI and apparent DM digestibility in goats and increased voluntary DMI in beef steers (Burns et al., 2005).
In a recent study evaluating switchgrass harvested in the afternoon vs. morning, Fisher et al. (2005) found increased short-term DMI in response to increased TNC in 4 of the 9 experiments conducted with sheep, goats, and cattle. In the case of switchgrass, and opposed to the other forage species noted above, multidimensional scaling implicated fiber concentrations as an important selection criteria for the animals in all experiments as well as TNC in some experiments. Although there is information available on morning vs. afternoon harvest of cool-season grasses and legumes and some information on short-term intake (discrete meals) effects with switchgrass, a warm-season grass, we found no published information demonstrating a link between TNC concentration and a sustained ( < 10 d) increase in DMI of warm-season grasses.
The objectives of this study were to compare voluntary DMI and apparent digestibility and N metabolism of steers fed switchgrass and gamagrass baleage and to determine if harvest in the morning or afternoon altered baleage composition and subsequent animal intake preference, voluntary DMI, digestibility, and N metabolism.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Black-haired steers, presumably of predominantly Angus breeding, were purchased at local auctions for the experiment and quarantined for 30 d after arrival at the research location in Raleigh, NC. The design was a completely randomized block with a 2 x 2 factorial arrangement of treatments and 5 replicates (steers) per treatment. The 4 treatments were Alamo switchgrass (Panicum virgatum L.) and Iuka gamagrass (Tripsacum dactyloides L.), each harvested in the PM (1800) of August 5, 2002, and the following AM (0600). Forages were cut with a mower-conditioner, immediately round-baled, wrapped in 4 layers of plastic, and stored outdoors as baleage until the experiment began in December 2002.
At the initiation of the experiment, large round bales of each treatment, with the plastic removed, were placed one at a time into a Tomahawk Model 5050 Shredder (Teagle Machinery Ltd., Blackwater, Cornwall, UK). Bales were shredded into predominately 2.5-to 15-cm lengths, with the shredded baleage blown onto plastic sheets placed over a cement floor and subsequently kept covered, except when feeding, with a second plastic sheet to prevent spoilage. Thereafter, large round bales of baleage were similarly processed as required for each treatment. Time between shredding and feeding for a given bale ranged among treatments from 3 to 7 d.
The steers were blocked into 5 groups on the basis of BW and then randomly assigned to treatments within blocks. There were 2 pens with 8 steers each. A third pen was shared by the other 4 steers. Pens had individual Calan gates (American Calan Inc., Northwood, NH), elevated floors of expanded, coated metal, and were under a roof with the north, south, and east walls open. Steers were adapted to the Calan gate, feeding system, and fed a common diet of gamagrass baleage ad libitum for 14 d before the experimental ad libitum intake phase began.
Ad libitum intake of the 4 baleage treatments was measured for 21 d (Burns et al., 1994), consisting of a 7-d adjustment and 14-d intake estimate, during which each steer was offered daily 115% of his previous days’ intake. Steers were fed approximately one-third of their daily ration at 0930 and the remaining two-thirds at 1600; exact proportions varied among steers and days. Orts were removed and weighed before the morning feeding.
After the ad libitum phase, steers were housed indoors in individual metabolism crates for 12 d, allowing 7 d of adjustment followed by 5 d to obtain chewing behavior measurements and to collect orts and excreta. Chewing behavior was monitored with an electronic system during the last 4 d of the digestion phase. Jaw movements were recorded continuously and stored on computer diskette (Luginbuhl et al., 1987). Lights were timed to allow 12 h of light and 12 h of dark daily. Mean ± SD of 9 measures of barn temperature during the collection, measured between 0800 and 0900, was 9 ± 0.9 ° C. Steers had ad libitum access to water and a trace-mineralized salt block throughout the study. Grab samples of baleage fed and subsequent orts were collected for each steer, composited for the ad libitum phase and balance trial phase for each steer, subsampled, and stored frozen for later analysis. One of the steers fed gamagrass, AM harvest, would not accept confinement in the balance crate, so no data were collected from him during the balance phase.
There were 2 balance study collection periods with 8 steers, and 1 collection period with 4 steers. Before the balance study collection, all crates were thoroughly scrubbed and washed. Plastic tarps were placed directly behind the crates and urine collection containers were placed under the crates.
Jugular venous blood samples for determination of plasma urea N concentrations were collected into heparinized tubes twice from each steer during the experiment, once at the end of the ad libitum intake phase, and once at the end of the balance study. Samples were centrifuged, and plasma was removed and stored frozen until analyzed. Orts, urine, and feces were collected for 5 d during the balance study. A daily allotment of water, to dilute the urine, and 6 N HCl, for acidification, was added to the urine pans to ensure that the pH of the collected urine ranged from 4 to 6. This was verified using pH-sensitive paper before collection of the aliquot. The amount of HCl added to the urine was determined empirically for each steer. Feces and urine were collected daily, weighed, thoroughly mixed, and a 5% daily aliquot was retained. The urine aliquots were pooled by steer and stored frozen at < – 4 ° C. At the end of the balance study, steers were removed from their crate, and the crate was thoroughly scraped. Feces recovered from the crates were added to the fecal collection for that day.
Masticate Collection
The potential intake preference or selection of the 4 experimental baleages was evaluated in a separate experiment, with a randomized, complete block design, using 6 mature, esophageally fistulated Angus steers (blocks) weighing 454 to 590 kg. Steers were given free access to gamagrass and switchgrass baleage for 3 d before masticate collection. The masticate was collected in 4 periods from each steer over 2 d, with 2 feedings daily at approximately 0900 and 1400. A random sequence of the 4 baleages was assigned to each steer. Steers were offered approximately 1.2 kg of baleage in each period. The cannulas were removed and the boluses collected by hand to ensure complete collection. The first 3 to 4 boluses were discarded and the following 5 to 8 boluses were collected. The boluses were gently mixed on a tray, placed in a plastic bag, and immediately frozen in liquid nitrogen. Boluses were stored in a freezer (– 15 ° C) until they were freeze-dried.
Sample Drying and Grinding
All baleage samples collected during the intake and digestion studies were stored in a freezer (– 15 ° C), subsequently freeze-dried, and then returned to the freezer until they were ground in a Wiley mill (Thomas Specific, Swedesboro, NJ) to pass through a 1-mm sieve and returned to the freezer until analyzed. Fecal samples from the digestion phase were dried in a forced-air oven at 55 ° C and then ground in a Wiley mill to pass through a 1-mm sieve. Masticate samples collected during the mastication phases were stored frozen, subsequently freeze-dried, and then returned to the freezer until they were ground in an Udy mill (Udy Corporation, Fort Collins, CO) to pass through a 1-mm screen, and returned to the freezer until analyzed.
Chemical Analysis
Samples of baleage and orts from each steer (5 per treatment) from the intake and digestion studies, feces from the digestion study, and masticates (6 per treatment) from the separate mastication study were analyzed for DM and N (Kjeldahl) using AOAC (1999) procedures. Concentrations of NDF and ADF (only NDF on masticates) were sequentially determined using the method of Van Soest et al. (1991) in a batch processor (Ankom Technology Corp., Fairport, NY).
The IVTDMD of feed and orts was determined by an in vitro fermentation of 0.25 g of the samples in Ankom fiber bags (Ankom Technologies, Fairport, NY) for 48 h. Fermentation vessels containing a low (550 g/kg) and high (820 g/kg) IVTDMD grass-hay standard, a blank, and 21 bags of samples were inoculated with 1,600 mL of McDougal’s buffer (Tilley and Terry, 1963) as modified by Burns and Cope (1974) and 400 mL of strained ruminal fluid using the Ankom II Daisy batch fermentor (Ankom Technologies, Fairport, NY). In vitro fermentations were terminated with the NDF procedure in the Ankom 200 fiber analyzer to remove the residual microbial fraction.
Total nonstructural carbohydrate and its constituent starch, mono-, di-, and polysaccharide concentrations in baleages and masticates were determined as described by Burns et al. (2006). Protein fractions of the baleages were determined as described by Licitra et al. (1996). Protein fraction A (nonprotein, readily soluble N) was determined using borate phosphate buffer. Protein fractions B1 (true protein, rapidly degraded in the rumen), B2 (true protein, more slowly degraded in the rumen), and B3 + C (true protein, not degraded in the rumen but digested and absorbed postruminally, and undigested protein) of samples collected during the balance study phase were determined using filtration procedures (Licitra et al., 1996) and the NDF procedures described above, followed by Kjeldahl N determination (AOAC, 1999). Fraction B3 + C is NDF-N, and will be identified as such in the Results and Discussion.
Concentrations of short-chain fatty acids, alcohols, and lactic acid in baleages were determined on aqueous extractions of the samples. Fifteen grams of frozen baleage collected during the intake phase were blended for 30 s in a commercial blender (Waring Laboratory, Torrington, CT) with 100 mL of deionized water, transferred into a 250-mL beaker, covered tightly with laboratory film, and then placed overnight in a refrigerator. The following day, samples were filtered through cheesecloth, filtered again through Whatman #1 filter paper, and an aliquot was transferred to 30-mL, Nalgene, polyethylene plastic bottles and stored frozen until further analysis.
For short-chain fatty acids and alcohol determinations, extracts were thawed, 5 mL of extract was mixed with 1 mL of 25% (wt/vol) of metaphosphoric acid, vortexed, and allowed to set for 30 min. The samples were then centrifuged for 10 min, and the supernatant was decanted into 5-mL, polypropylene storage tubes and stored frozen until they were thawed and analyzed. On the day of short-chain fatty acid and alcohol analysis, the acidified sample was thawed, mixed, and 1 mL of sample was combined with 100 µL of internal standard (1% 1-butanol, vol/vol) and transferred to the injection vials. For lactic acid determination, 900 µL of extract was combined with 100 µL of internal standard (1% pivalic acid vol/vol) and 100 µL of 0.3 M oxalic acid, then transferred to the injection vials.
Concentrations in the injection vials were determined with a Varian 3800 gas chromatography system that included a Varian 8200 autosampler (Walnut Creek, CA), and a flame ionization detector and integrator (Chromatography Systems Business, Sugar Land, TX). The short-chain fatty acids and alcohols were analyzed using a Nukol, fused silica, capillary column (15-m x 0.53-mm x 0.5-µm film thickness, Supelco, Supelco Park, Bellefonte, PA). One microliter of sample was injected and then was split 1:25 with carrier gas before entering the column. The injector and the detector were maintained at 240 ° C. The carrier gas was N2. Column flow varied automatically to maintain 2.5 psi of pressure. Column temperature was held at 50 ° C for 30 s, then increased by 100 ° C/min to 155 ° C and held for 5 min. The column temperature was then raised to 180 ° C for an additional 1 min. Lactic acid was determined using a 2.46-m glass column (Supelco 4% carbowax 20 M, mesh size 80/120, Carbopack B-DA, Supelco Park). The injector temperature was 200 ° C, and the detector temperature was 225 ° C. The column was isothermal at 175 ° C, and the carrier gas (N2) flow was 25 mL/min.
Concentrations of acids and alcohols from both assays were calculated from ratios derived from the injections of a range of known concentrations of acids and alcohols and internal standards. Mean concentrations were calculated from duplicate injections of duplicate extractions.
The urine concentration of N was determined by Kjeldahl N procedures (AOAC, 1999). Urea content of plasma was determined using the diacetyl monoxime method of Marsh et al. (1957), adapted to a Technicon Auto Analyzer (Industrial Method #339-01, Technicon Instrument Corp, Tarrytown, NY).
Statistical Analysis
Compositional data for masticate samples were analyzed as a randomized, complete block design, with a 2 x 2 factorial arrangement of treatments, using the MIXED procedure (SAS Inst. Inc., Cary, NC). The model included baleage, harvest time, and their interaction as fixed effects, and periods and steers as random effects.
Due to equipment malfunctions and an electric power failure, only 53 of the possible 76 eating behavior measures (19 steers multiplied by 4 d of measurements) were available for interpretation. All 4 d were available for 6 steers, 3 d for 6 steers, 2 d for 4 steers, and 1 d for 3 steers. All available data for each hourly measurement were averaged within steer before subsequent statistical analysis.
Data for intake and balance study phases from the steers were statistically analyzed as a randomized, complete block design, with a 2 x 2 factorial arrangement of treatments using the MIXED procedure of SAS. The model included baleage, harvest time, and their interaction as fixed effects, and steer as a random effect. Statistical significance was declared at P < 0.10. One steer fed gamagrass, AM harvest, refused to adapt to the collection crates, so n = 4 for the balance study for that treatment.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) and the balance study phase (data not shown) were very similar. Compared with gamagrass baleage, switchgrass baleage had a greater pH, less DM, greater IVTDMD and fiber components (except for hemicellulose), and less CP (Table 1). Switchgrass baleage had less fraction A and NDF-N, but greater fraction B1 and B2 CP than gamagrass baleage (Figure 1). Additionally, compared with gama-grass baleage, switchgrass baleage had less methanol, greater ethanol and short-chain fatty acids, less lactate, and less TNC (Table 1). Compared with AM harvest, PM harvest had greater DM and IVTDMD, less CP and fraction A CP, greater fraction B1 and NDF-N CP, less methanol, less short-chain fatty acids and lactate, and greater TNC and its components starch and monosaccharides (Table 1, Figure 1).
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). Concentrations of NDF, ADF, and cellulose increased from PM to AM harvest for switchgrass baleage but decreased for gamagrass baleage. Concentrations of CP and lactate decreased from PM to AM harvest for switchgrass baleage but increased for gamagrass baleage. The decrease in IVTDMD from PM to AM harvest was greater for switchgrass than for gamagrass baleage. The decrease in concentrations of TNC and monosaccharides from PM to AM harvest was greater for gamagrass than for switchgrass baleage. Concentrations of acetate increased from PM to AM harvest more for switchgrass than for gamagrass baleage. Concentrations of propionate, and isobutyrate likewise increased from PM to AM harvest for switchgrass baleage, but those acids were not detected in gamagrass baleage.
Ad Libitum Intake Phase
Steers fed PM harvest ate more (P = 0.03) than steers fed AM harvest (Table 2). Ad libitum DMI was not affected (P = 0.61) by baleage, and no interaction between baleage and harvest time was detected (P = 0.62). Plasma urea N was greater (P = 0.07) for steers fed gamagrass than steers fed switchgrass baleage and less (P = 0.01) for steers fed PM harvest than steers fed AM harvest (Table 3).
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), or N (Table 3). Digestible DMI during the ad libitum phase and DMI during the balance trial phase were greater (P = 0.02) for steers fed PM harvest than steers fed AM harvest. Steers fed gamagrass baleage had greater plasma urea N concentrations, ate more N, excreted more N in feces and urine, digested more N, but had similar N retention as steers fed switchgrass baleage (Table 3). Steers fed PM harvest ate more (P = 0.03) N, digested more (P = 0.03) N, and retained more grams of N/day (P = 0.01) or N as a percentage of intake (P = 0.01) or percentage of N digested (P = 0.01) than steers fed AM harvest. There were baleage x harvest time interactions for daily amounts of N intake (P = 0.01), amounts of N digested (P = 0.01) or N digested as percentage of intake (P = 0.05), amounts of N retained (P = 0.01), and N retained as a percentage of intake or digestion (P = 0.01, Table 3).
Eating Behavior
Steers fed PM harvest spent fewer minutes per day resting (P = 0.03) and more minutes per day ruminating (P = 0.08) than steers fed AM harvest (Table 4, Figure 2). Similarly, on a percentage basis, steers fed PM harvest spent less (P = 0.03) time resting (47 vs. 57%) and more (P = 0.09) time ruminating (24 vs. 19%), than steers fed AM harvest. The difference between steers fed AM or PM harvest was similar across time of day (Figure 2). The number of boluses per day per kilogram of NDF intake (131 ± 11) and number of boluses per day per minute of rumination time (1.24 ± 0.04) were similar among treatments. Time spent eating and number of chews during eating were similar between baleages or between harvest times (Table 4). Time spent ruminating and number of chews during rumination were greater for PM than for AM harvest. During the afternoon meal and during the time of both meals, steers fed PM harvest spent more time eating and less time resting than steers fed AM harvest (Table 4). Interactions between baleages and harvest times for chewing times expressed as a proportion of DM or NDF intake, and the interaction for time spent ruminating during both meals (Table 4) coincide with similar interactions between baleage and harvest for DM or NDF intake (Table 2).
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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and 4) indicate that steers did not select, or discriminate, in favor of more digestible components of the baleage. Figure 4 illustrates a slight decrease in concentration of TNC and its components starch and monosaccharides in baleage compared with masticate in all treatments except switchgrass, AM harvest, but total TNC concentration was less than 4% of DM in all samples.
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, 1981; Woodford and Murphy, 1988). Chewing time/kg of DMI decreases as DMI increases (Sudweeks et al., 1980; Beauchemin and Buchanan-Smith, 1990; Luginbuhl et al., 2000). Steers fed PM harvest tended (P = 0.23) to spend more time eating over the entire day and spent more (P = 0.04) time eating during meal times (Table 4). Increased time eating during meals plus greater (P = 0.09) time spent ruminating and less (P = 0.03) time spent resting for steers fed PM harvest (Figure 2, Table 4) correlates with increased TNC in the PM harvest (Table 1) and increased DMI (Table 2). Albeit in a subtle fashion, more time spent eating and ruminating and less time spent resting should diminish variations in ruminal concentrations of supplies of readily fermentable carbohydrate and nitrogen, thereby enhancing capture of ammonia as microbial protein.
Dry matter intake of steers fed switchgrass, AM harvest during the balance study was less than DMI of other steers and less than DMI of the same steers during the ad libitum intake phase (Table 2). Examination of the individual steer’s DMI (data not shown) revealed that during the balance study 2 steers fed switchgrass, AM harvest, consumed baleage equivalent to approximately 50% of their DMI during the ad libitum phase. Composition of baleage samples taken during the ad libitum phase and balance study phase were similar (data not shown). The only components of switchgrass, AM harvest during the ad libitum phase that appear to be different were slightly greater grams per kilogram of concentrations of methanol (28.8 vs. 25.1) and TNC (10.3 vs. 8.6) and slightly less concentrations of acetate (23.4 vs. 25.4) and butyrate (9.6 vs. 11.5) than during the balance study phase. Data from grass silages and data from silages characterized as high butyrate silages (McDonald, 1981; Harrison et al., 2003) indicate that our silages are within criteria of normal, well-preserved silages.
We conclude that reduced DMI was attributed, in part, to the steers’ behavior. Dry matter intake, digestibility, and retention means for steers fed PM and AM harvest of gamagrass are similar (Table 2), indicating that depressed DMI during the balance study phase for steers fed switchgrass, AM harvest, likely is responsible for the statistically significant responses to harvest source. Further, depressed DMI for 2 of the steers in that treatment likely explains the grass by harvest time interactions in N balance components (Table 2), the lack of response to harvest in plasma urea N concentrations during the balance study (Table 3), and the baleage x harvest time interactions in chewing time per kilogram of DM or NDF intake (Figure 2).
Davis (2001) used NRC (1996) equations to calculate the nutrient requirements of growing beef steers (272 kg of BW, and 545 kg of BW with 28% body fat at finishing). A growing steer with ADG of 0.23 or 0.45 kg/d requires 79 or 95 g of CP/d. We used measured N intakes and N retentions (Table 3, excluding steers fed switchgrass, AM harvest), and the equation of Ferrell and Jenkins (1985) to calculate ADG that ranged from 0.32 to 0.52 kg/d and averaged 0.40 kg/d.
Comparison of our protein fractions from baleage (Figure 1) to those reported for switchgrass or gamagrass hay grown in the same environment (Archibeque et al., 2001; Magee, 2004), shows similar values for protein fraction B1 and B2 between hays and baleage, but a shift from protein fraction A to NDF-N in hays. Protein fraction B1 ranged from 0 to 2.9% of CP in our baleages (Figure 1) and ranged from 2.3 to 7.6% of CP in hays reported by Archibeque et al. (2001) and Magee (2004). Protein fraction B2 ranged from 20 to 30% of CP in our baleages and from 25 to 29% of CP in the hays. However, protein fraction A in our baleages (51 to 63% of CP, Figure 1) was much greater than in the hays (16 to 27% of CP), and NDF-N in our baleages (17 to 21% of CP, Figure 1) was much less than in the hays (39 to 52% of CP, Archibeque et al., 2001; Magee, 2004). Protein fraction C (ADIN) was measured in the hays and ranged from 4 to 5.9% of CP (Archibeque et al., 2001; Magee, 2004); we expect that fraction C in our baleages would be no greater than in the hays and decided not to measure fraction C because it would not affect N metabolism in our steers. We conclude from this comparison that during the drying process of conservation of grass as hay, there was a shift in herbage N from protein fraction A (nonprotein N) to fraction B3 (true protein, partially degradable in the rumen, available for digestion and absorption in the small intestine). A substantial loss of N during the drying process would not allow fractions B1 and B2 to retain their proportions of CP after drying. We also conclude that preservation of grasses as baleage vs. hay increases the nonprotein N load on the ruminant, thereby increasing the need for readily fermentable dietary energy to support conversion of that N to microbial protein in the rumen. Data presented by Harrison et al. (2003) support that conclusion. Increased concentrations of TNC in PM vs. AM harvest provide, at least in part, the fermentable energy needed for dietary N conversion to microbial protein.
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Footnotes |
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2 The authors thank Sharon Freeman and Tina Starr for care and feeding of the steers, and Lucile Smith and Ellen Leonard for their able technical assistance in data collection, laboratory analyses, and preparation of this manuscript.
3 Corresponding author: Gerald_Huntington{at}ncsu.edu
Received for publication June 8, 2006. Accepted for publication August 28, 2006.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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