J. Anim Sci. 2008. 86:159-166. doi:10.2527/jas.2006-701
© 2008 American Society of Animal Science
The interaction of harvesting time of day of switchgrass hay and ruminal degradability of supplemental protein offered to beef steers1,2
G. B. Huntington3 and
J. C. Burns
Department of Animal Science and ARS-USDA, North Carolina State University, Raleigh 27695-7621
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Abstract
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The objective of this study was to evaluate an interaction between harvest at 0600 (AM) vs. 1800 (PM) with high (HI) or low (LO) ruminal degradability of a protein supplement to change voluntary intake, digestion, or N retention by steers offered switchgrass (Panicum virgatum L.) hay. Black steers (255 ± 14 kg of BW) were blocked by BW, and then randomly assigned (5 steers each) to AM/HI, PM/HI, AM/LO, or PM/LO treatment groups. Steers were group-housed in covered, outdoor pens with individual feeding gates. After adaptation and standardization, intake was measured for 21 d followed by a digestion trial (5 d of total collection). Steers were offered 767 (LO) or 825 (HI) g/d of supplement to provide 268 g of CP/d. Compared with AM, PM had greater (P = 0.01) concentrations of total nonstructural carbohydrate (TNC, 71 vs. 56 g/kg of DM), and lesser concentrations of NDF (760 vs. 770 g/kg of DM, P = 0.02), ADF (417 vs. 427 g/kg of DM, P = 0.02), and CP (55.9 vs. 58.6 g/ kg of DM, P = 0.07). Protein fractions A, B2, and B3 were similar for AM and PM, but HI contained more (P < 0.02) A (694 vs. 296 g/kg of protein) and less B2 (174 vs. 554 g/kg of protein) fraction than LO. Harvest interacted with supplement to increase (P = 0.07) ad libitum digestible DMI for steers offered PM/HI (11.4 g/kg of BW daily) compared with steers offered PM/LO (10.2 g/kg of BW daily), but there was no difference for steers offered AM/LO or AM/HI (10.7 g/kg of BW). Apparent digestibilities of DM (594 vs. 571 g/kg of intake), NDF (591 vs. 562 g/kg of intake), ADF (585 vs. 566 g/kg of intake), and N (651 vs. 632 g/kg of intake) were greater (P < 0.04) for PM than for AM. Apparent digestibility of N was greater (P = 0.02) for HI (652 g/ kg of intake) vs. LO (631 g/kg of intake). Interactions between harvest and supplement for apparent digestibilities of NDF (P = 0.09) and ADF (P = 0.03) were due to no change or an increase in digestibility in response to increased ruminal degradability of supplement in steers offered PM harvest, whereas increased ruminal degradability of supplement decreased digestibility of NDF and ADF in steers offered AM harvest. Treatments did not affect hay intake (3.93 kg/d), N retained (15.8 g/d), or plasma urea N (5.25 mM) during ad libitum intake. Greater TNC in PM vs. AM harvest was not sufficient by itself to increase total voluntary DMI, but greater protein degradability interacted with harvest time to increase ruminal fiber digestibility and digestible DMI of beef steers offered PM vs. AM harvest.
Key Words: beef steer • protein degradability • switchgrass • Panicum virgatum
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INTRODUCTION
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Hays from tall fescue (Festuca arundinacea L.) or alfalfa (Medicago sativa L.) harvested in the late after-noon (PM) have greater concentrations of total non-structural carbohydrates (TNC), lesser concentrations of fiber, and greater in vitro true DM disappearance (IVTDMD) than hays harvested in the early morning (AM, Fisher et al., 1999
, 2002; Mayland et al., 2000). Increased TNC concentrations of alfalfa hays harvested in the PM were linked by multidimensional scaling procedures to increased short-term intake preference by goats, sheep, and cattle (Buntinx et al., 1997; Fisher et al., 2002). A similar relationship was reported for sheep grazing ryegrass (Lolium perenne L.) or white clover (Trifolium repens L.; Orr et al., 1997). Compared with AM harvest, PM harvest of alfalfa hay increased voluntary intake and apparent DM digestibility in goats, and increased voluntary intake in beef steers (Burns et al., 2005). Fisher et al. (2005) produced PM switchgrass hays with 0.7 to 1.2 percentage units more TNC than AM hays. Increased short-term intake of sheep, goats, and cattle in response to increased TNC was detected in 4 of the 9 experiments conducted. Huntington and Burns (2007) found that PM vs. AM harvest contained more TNC and increased voluntary DMI and digestible DMI in beef steers offered switchgrass or gamagrass baleage. Synchronizing supply of N and energy in the rumen should increase rumen microbial protein production, increase fiber digestion, and decrease ammonia absorption (Hoover and Stokes, 1991; Witt et al., 2000; Lee et al., 2002). However, we can find no published information that links degradability of dietary N to time of harvest, TNC concentration, fiber digestion, and sustained increased intake of warm-season grasses, which have distinct tissue structural and chemical differences from cool-season grasses.
The objective of this study was to evaluate the interaction between N degradability in a supplement and harvest time of a warm-season grass hay.
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MATERIALS AND METHODS
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The experiment was conducted under the supervision and approval of North Carolina State University’s Institutional Animal Use and Care Committee.
Black-haired steers, presumably of predominantly Angus breeding, were purchased at local auctions for the experiment, and were quarantined for 30 d after arrival at the research location. The design was a randomized, complete block with a 2 x 2 factorial arrangement of treatments and 5 replicates (steers) per treatment. The 4 treatments were PM- or AM-harvested hay supplemented with high or low ruminal degradable protein. A well-established stand of switchgrass (Panicum virgatum L. cv. Alamo), previously used as a source of harvested feed, provided the experimental hays. The field was uniformly topdressed annually with P and K according to soil test, and pH was maintained between 6.1 and 6.3. The initial growth was removed as hay on June 23, 2003. The field was subsequently topdressed with 78 kg/ha of N from ammonium nitrate. At the time of harvest, the field of regrowth was divided in half and each half was further divided in half, creating 4 strips of field. Each strip was randomly assigned to PM or AM treatment. The PM treatment was harvested beginning at 1800 on August 24, 2003, and the AM treatment was harvested beginning at 0600 on August 25, 2003. Both treatments were sun cured in the field, baled in square bales (approximately 20 kg each), and stored indoors until the experiment began in January 2004. The bales were sliced with a 5600 Van Dale bale processor (J. Starr Industries, Ft. Atkinson, WI) and stored in feed carts before feeding. The supplements (Table 1; Figure 1) were formulated (G. Huntington, North Carolina State University, unpublished data on feedstuffs; NRC, 1996; Archibeque et al., 2001) and offered to provide either 32% of dietary protein (including the switchgrass hay) as fractions A plus B1 and 63% as fractions B2 plus B3 (i.e., low ruminal degradability) or 49% of dietary protein as fractions A plus B1 and 47% as fractions B2 plus B3 (i.e., high ruminal degradability). The supplements were formulated to obtain the greatest possible difference in ruminal degradability of protein from commonly available feedstuffs, and offered at amounts intended to supply similar amounts of CP. Dry molasses (5% of supplement) was added to the supplement before feeding to encourage rapid and complete consumption of the supplement.
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Figure 1. Calculated (C) and measured (M) protein fractions (A, B1, B2, B3, and C) in hay plus supplement of switchgrass hay harvested in the afternoon (PM) or in the morning (AM) and supplemented with a high or low ruminally degradable protein supplement. The SEM for the measured protein fractions are given in Table 1 .
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The steers (255 ± 14 kg of BW) were blocked into 5 groups of 4 steers per group on the basis of BW, randomly assigned to treatments within blocks, and housed in pens that allowed measures of individual feed intake (Huntington and Burns, 2007). Steers were adapted to the Calan gate feeding system (American Calan Inc., Northwood, NH) and the metabolism crates for several weeks before the experiment began. They were offered ad libitum access to a common adaptation diet (hay harvested on June 23, 2003, from the same field that supplied the treatments for the study) for 14 d before the ad libitum intake phase began. Ad libitum intake of the 4 treatments was measured for 21 d (7 d of adjustment and 14 d to measure intake), during which each steer was offered daily 115% of its hay intake from the previous day. Steers were offered approximately one-third of their daily hay ration at 0930 and the remaining two-thirds at 1600; exact proportions varied among steers and days. Supplement was offered once daily, before the 0930 feeding of hay. Steers assigned the low ruminal degradability treatments were offered 770 g of DM/d of supplement plus dried molasses, and steers assigned the high ruminal degradability were offered 825 g of DM/d of supplement plus dried molasses. After the ad libitum intake phase, steers were housed indoors in individual metabolism crates for 12 d: 7 d of adjustment followed by 5 d to collect orts and excreta. Steers had ad libitum access to water and a trace-mineralized salt block (94% NaCl, 3,500 mg/kg of Zn, 2,000 mg/kg of Fe, 2,000 mg/kg of Mn, 300 mg/kg of Cu, 70 mg/kg of I, and 50 mg/kg of Co) throughout the study.
Daily grab samples of hay offered were collected and composited separately for the ad libitum intake and digestion phases for each steer, subsampled, and stored for later analysis. This procedure allowed for variation in composition of the hays actually offered to be represented within the error term. Orts were weighed twice daily, saved for each steer, and composited within experimental periods. Hay samples from the ad libitum intake phase and the digestion phase, and orts from both phases were dried at 55°C until they reached a constant weight. Supplements (Table 1) were mixed 3 times during the study. Grab samples were collected from each mix, subsampled, and stored for later analysis.
Before the digestion phase, all metabolism crates were thoroughly cleaned. Plastic tarpaulins were placed directly behind the crates, and urinal collection containers were placed under the metal crate urine pans.
Jugular venous blood samples (10 mL) were collected into heparinized tubes from each steer, once at the end of the ad libitum intake phase and once at the end of the digestion phase. The samples were centrifuged at 1,000 x g for 20 min, and plasma was removed and stored at <–4°C until analyzed approximately 3 mo later. Orts, urine, and feces were collected for 5 d during the digestion phase. The urine pans contained a daily allotment of H2O and 6 M HCl for acidification 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. 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 at <–4°C. Fecal aliquots were dried at 55°C and pooled by steer. At the end of each collection phase, each steer was removed from the crate and the crate was thoroughly scraped. Feces recovered from the crates were added to the fecal collection for the final day.
Laboratory Analysis
All forage, supplement, orts, and fecal samples collected during the intake and digestion trials were ground in a Wiley mill (Thomas Scientific, Swedesboro, NJ) to pass through a 1-mm sieve. Samples were scanned in a near-infrared reflectance spectrophotometer (Model 5000, Perstorp Analytical, Silver Spring, MD) and the "H" statistic (0.6) was used to identify samples with different spectra. These samples were analyzed chemically and subsequently included in the appropriate calibration data sets. The switchgrass and orts samples (n = 78) were compared with a library containing previously analyzed switchgrass hay and silage. The "H" statistic selected 5 of the 78 samples for laboratory analysis (NDF, ADF, CP, and IVTDMD), and an additional 5 samples were selected at random for inclusion in the calibration data sets. Fecal samples (n = 19) were compared with a library containing fecal samples. Only 2 samples were selected for laboratory analysis (NDF, ADF, CP) and an additional 5 samples were selected at random for inclusion in the appropriate calibration data sets. We had no libraries available for supplements or TNC concentration in switchgrass hay; consequently, 30 of the 49 supplement samples were selected at random, chemically analyzed, and used to develop calibration equations. Samples of switchgrass hay offered during the ad libitum intake and digestion phases (n = 39) were analyzed for TNC and its constituent monosaccharide and starch concentrations. Data from these analyses were used to develop the appropriate calibration data sets. Calibration and validation statistics are presented in Table 2.
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Table 2. The range for each forage, fecal, and supplement constituent predicted by near-infrared reflectance spectroscopy, the SE of calibration (SEC) and SE of cross-validation (SECV), and the associated R2 for appropriate calibration data sets
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Selected samples of feed, orts, and feces were analyzed for DM and Kjeldahl N using AOAC (1999) procedures. Concentrations of NDF and ADF were sequentially determined using the method of Van Soest et al. (1991) in a batch processor (Ankom Technology Corp., Fairport, NY). In vitro true DM disappearance was determined on 0.25-g samples in Ankom fiber bags using the Ankom II Daisy batch fermenter (Ankom Technology Corp.). Samples were inoculated with 1,600 mL of McDougall’s buffer (Tilley and Terry, 1963) and 400 mL of strained ruminal fluid. The ruminal fluid came from a fistulated steer maintained on a mixture of alfalfa (Medicago sativa L.) and orchardgrass (Dactylis glomerata L.) hay. After 48 h at 38°C, the in vitro fermentations were terminated with the NDF procedure cited above to remove the residual microbial fraction. Total nonstructural carbohydrate and its constituent starch and monosaccharides in hays were determined as described by Burns et al. (2006). The di- and polysaccharides were calculated by subtracting the concentrations of starch plus monosaccharide from the TNC.
Urine concentration of N was determined by Kjeldahl N procedures (AOAC, 1999). Urine concentration of ammonia N was determined by the hypochlorite method adapted to a Technicon Auto Analyzer (Industrial Method No. 334-74 A/A, Technicon Instrument Corp., Tarrytown, NY), and urea content of plasma and urine was determined using the diacetyl monoxime method of Marsh et al. (1957) adapted to a Technicon Auto Analyzer (Technicon Instrument Corp.). Protein fractions of supplement and forage samples were determined as described by Licitra et al. (1996). Protein fraction A was determined using borate phosphate buffer.
Statistical Analysis
With the exception of protein fractions, all compositional data for forage, supplement, orts, and feces used in calculations and statistical analyses were predicted by near-infrared reflectance spectroscopy. Data for intake and digestion phases from the steers were statistically analyzed as a randomized, complete block design using the MIXED procedure (SAS Inst. Inc., Cary, NC). The model included treatment as a fixed effect and steer, or block, as a random effect. Treatment contrasts were used to test for the main effects of harvest time and supplement and their interaction. Statistical significance was declared at P < 0.10. One steer assigned to PM harvest and low ruminal degradability injured a rear leg during the ad libitum intake phase, so its data were removed from the experiment.
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RESULTS
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Hay and Supplement Composition
The composition of samples collected during the ad libitum intake phase (Table 3) and the digestion phase (data not shown) were very similar. Compared with the AM harvest, the PM harvest of switchgrass hay had greater concentrations of fraction B1 of CP (P = 0.05) and nonstructural carbohydrates (P = 0.01), but lesser concentrations of CP (P = 0.07), NDF (P = 0.02), and ADF (P = 0.01). Dry matter and IVTDMD were similar for the 2 hay harvests (Table 3). By design, the protein supplements varied in protein fractions, with the high ruminal degradability supplement having greater concentrations of fraction A and lesser concentrations of fractions B1 and B2 and C compared with the low ruminal degradable supplement (P < 0.02). The profiles of protein fractions in the total diet (hay plus supplement) were similar to those calculated before the experiment (Figure 1).
By design, the DMI of the high ruminal degradable protein supplement was slightly greater (P = 0.01) than that of the low ruminal degradable supplement during both the ad libitum intake and digestion phases (Table 4). Hay intake and total DMI were not affected by time of harvest (Table 4). Supplement intake within treatments was similar during both phases of the experiment, but hay intake during the digestion phase was, on average, 88% of hay intake during the ad libitum intake phase. Total N intake was similar among treatments, with approximately 55% of total N from the supplement during the ad libitum intake phase and 58% of total N intake from the supplement during the digestion phase. Per kilogram of BW, steers offered the high ruminally degradable supplement had greater (P = 0.10) intake during the ad libitum intake phase compared with steers offered the low ruminally degradable supplement (Table 4).
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Table 4. Body weight, intake, apparent digestibilities, and N balance of steers offered switchgrass hay harvested in the afternoon (PM) or in the morning (AM) and supplemented with a high or low ruminally degradable protein supplement
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Steers offered the PM harvest had greater apparent digestibilities of DM (P = 0.02), NDF (P = 0.04), and ADF (P = 0.04) compared with steers offered AM harvest (Table 4). Digestible DMI, calculated by multiplying intake during the ad libitum intake phase by digestion coefficients obtained during the digestion phase and expressed on a daily basis, was similar among treatments. The harvest by supplement interaction (P = 0.07) for digestible DMI per kilogram of BW was due to no change in intake attributable to supplement degradability in steers offered the AM harvest, but increased digestible DMI per kilogram of BW with increased ruminal degradability of supplement in steers offered the PM harvest (Table 4). Interactions between harvest and supplement for apparent digestibilities of NDF (P = 0.09) and ADF (P = 0.03) were due to no change or an increase in digestibility in response to increased ruminal degradability of supplement in steers offered the PM harvest, whereas increased ruminal degradability of supplement decreased digestibility of NDF and ADF in steers offered the AM harvest.
Steers offered the PM harvest tended (P = 0.15) to excrete less N in feces and had greater (P = 0.03) apparent N digestibility than steers offered the AM harvest (Table 4). Apparent N digestibility was greater (P = 0.02) for steers offered the high vs. low ruminal degradable supplement, and that response tended (P = 0.17) to be greater in steers offered the AM harvest than in steers offered the PM harvest. Steers offered the PM harvest excreted more (P = 0.09) N in urine than steers offered the AM harvest, but daily amounts of N digested and retained, and N retained per kilogram of N intake or kilogram of N digested were similar among treatments (Table 4).
Urine from steers offered the AM harvest had greater concentrations of ammonium N (P = 0.08) than urine from steers offered the PM harvest (Table 5). Increased degradability of supplement increased urine concentration of urea N (P = 0.09) and the sum of urea N plus ammonium N (P = 0.04). Steers offered the AM harvest tended (P = 0.16) to have a greater percentage of total urine N as ammonium N, and had a greater (P = 0.01) percentage of total urine N as the sum of urea N and ammonium N compared with steers offered the PM harvest. Increased ruminal degradability of supplement tended (P = 0.17) to increase urea N as a percentage of total urine N, and increased (P = 0.01) urea plus ammonia N as a percentage of total N with a trend (P = 0.14) for a greater effect with AM harvest than PM harvest (Table 5). As stated previously, steers offered the PM harvest excreted more (P = 0.09) N daily in urine, but tended (P = 0.18) to excrete less ammonium N than steers offered the AM harvest (Table 5). Increased ruminal degradability of supplement increased (P = 0.02) daily excretion of the sum of urea N plus ammonium N.
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Table 5. Concentrations of urinary N components, urinary excretion rates, and plasma urea N of steers offered switchgrass hay harvested in the afternoon (PM) or in the morning (AM) and supplemented with a high or low ruminally degradable protein supplement
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Plasma urea N concentrations were similar among treatments at the end of the ad libitum intake phase (Table 5). At the end of the digestion phase, steers offered the AM harvest had greater (P = 0.03) plasma urea N concentrations than steers offered the PM harvest, and increased ruminal degradability of supplement increased (P = 0.03) plasma urea N concentrations. Increased ruminal degradability of supplement decreased concentration of plasma urea N in steers offered the PM harvest, but increased it in steers offered the AM harvest, causing a trend (P = 0.20) for a harvest by supplement interaction (Table 5).
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DISCUSSION
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The effects of harvest time and composition of the protein supplement caused predicted changes in nutrient composition of the diet (Table 3; Figure 1). Increased ruminal degradability of the supplement increased apparent digestibility of N (Table 4), which was similar to results of Bodine and Purvis (2003), Bodine et al. (2000), and Olson et al. (1999) with warm-season grass hay or pasture supplemented with ruminally degradable protein supplement. However, Bodine et al. (2000) and Magee et al. (2005) reported that a supplement containing corn and soybean hulls reduced hay intake by steers, which might negatively affect increased intake of hay harvested in the PM vs. AM (Fisher et al., 1999; 2002; Burns et al., 2005). The lack of a positive response to increased ruminally degradable protein in digestion of NDF and ADF and the interactions in digestibility of NDF and ADF between harvest and ruminal degradability of supplement (Table 4) were not expected. Furthermore, they are not consistent with the concept of increased fiber digestion associated with increased degradable protein intake (Olson et al., 1999). However, the concept of a positive response to the synchronization of readily fermentable carbohydrate and protein is supported. High ruminal degradability of supplemental protein positively influenced NDF and ADF digestibility of PM harvest (which had greater TNC concentrations) as opposed to negatively influencing NDF and ADF digestibility of AM harvest (which had lesser TNC concentrations, Table 4).
Comparison of hay compositions in this study (Table 3) with those reported for hays harvested in an earlier study from the same field (Archibeque et al., 2001) provides some insight into variability in CP and in protein fractions as a percentage of CP in switchgrass hay. Crude protein in hay harvested by Archibeque et al. (2001) ranged from 56 to 110 g/kg of DM. Fraction A ranged from 213 to 247 g/kg of CP, fraction B1 ranged from 47 to 76 g/kg, fraction B2 ranged from 276 to 287 g/kg, fraction B3 ranged from 342 to 392 g/kg, and fraction C ranged from 41 to 82 g/kg. The values in Table 3 are within the ranges reported by Archibeque et al. (2001). Moreover, Archibeque et al. (2001) reported that increased CP was associated with slightly greater contributions of B1 and B3 fractions and a smaller contribution of the C fraction. The A and B2 fractions were not associated with changes in CP concentration. The slightly greater (P < 0.05) B1 fraction in PM vs. AM harvest in the current study (Table 3) was also seen in switchgrass baleage (Huntington and Burns, 2007).
Based on NEm and NEg values for our feedstuffs and similar forages in NRC (1996), we calculated daily NEm intake of 5.5 Mcal/d and NEg intake of 3 Mcal/d, sufficient to support 1.0 kg of ADG. The N intake of the steers (Table 4) exceeded their maintenance requirement (NRC, 1996) and provided N to support approximately 0.25 kg of ADG. The average N balance was 15.8 g/d, indicating that the steers were gaining weight and that protein supply was likely the principal restraint to growth.
Albeit in a subtle way, PM vs. AM harvest increased nonstructural carbohydrate content and digestible DMI of switchgrass hay offered to beef steers. Moreover, concentrations of NDF were lesser in the PM harvest, whereas ADF was similar between AM and PM harvests. Increased ruminal degradability of supplemental protein did not increase fiber digestibility as expected. High ruminal degradability of supplemental protein positively influenced NDF and ADF digestibility of PM harvest (greater TNC concentrations) as opposed to negatively influencing NDF and ADF digestibility of AM harvest (lesser TNC concentrations).
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Footnotes
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1 Use of trade names in this publication does not imply endorsement either by the North Carolina ARS or USDA_ARS or criticism of similar products not mentioned. 
2 The authors thank Sharon Freeman 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 October 25, 2006.
Accepted for publication September 17, 2007.
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LITERATURE CITED
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Abstract
INTRODUCTION
