J. Anim Sci. 2007. 85:527-535. doi:10.2527/jas.2006-358
© 2007 American Society of Animal Science
Effect of crabgrass (Digitaria ciliaris) hay harvest interval on forage quality and performance of growing calves fed mixed diets1
P. A. Beck2,
S. Hutchison3,
C. B. Stewart,
J. D. Shockey and
S. A. Gunter
University of Arkansas, Division of Agriculture, Southwest Research and Extension Center, Hope 71801
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Abstract
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Twelve 0.81-ha crabgrass (Digitaria ciliaris [Retz.] Koel.) hay fields were harvested at 21, 35, and 49 d of regrowth (average phonological growth stage of 30, 51, and 56, respectively). Increased harvest interval exhibited a linear decrease (P < 0.01) in CP (14.1, 13.7, and 10.6% of DM, respectively) and increase (P < 0.01) in NDF (65.3, 70.6, and 70.2% of DM, respectively) and ADF (35.7, 38.9, and 42.7% of DM, respectively). Hays were incorporated into 3 diets that contained 20% (DM basis) crabgrass hay, ground corn (33%), and soybean hulls (32%). Diets contained 14.4, 14.4, and 13.6% CP; 1.83, 1.72, and 1.81 Mcal of NEm/kg; and 1.21, 1.10, and 1.17 Mcal of NEg/kg; respectively. Diets were fed to beef calves in 12 pens at a rate of 2.3% (DM basis) of BW in 1 experiment (n = 120, initial BW 210 ± 4.4 kg) and ad libitum in another experiment (n = 60, initial BW 207 ± 4.4 kg). To measure passage rate of the hay and concentrate portions of the diets, 12 heifer calves (BW = 145 ± 4.5 kg) were individually fed at 2.3% of BW for 14 d and dosed with Dy-labeled soybean hulls and Yb-labeled hay. In situ DM digestibility of the hays and diets were determined using 3 ruminally cannulated steers (BW = 584 ± 10.4 kg). Harvest interval did not affect (P 
0.11) ADG of limit-fed calves during the diet acclimation or growing phases (average 0.32 and 0.80 kg, respectively) or ADG of calves fed ad libitum (average 1.21 kg). Dry matter intake of calves fed ad libitum averaged 7.9 kg/d (3.28% of BW) and was not affected (P 0.22) by harvest interval. Gain:feed was not affected (P 0.20) by harvest interval (0.13 and 0.15 for limit-fed and ad libitum-fed calves, respectively). Increased harvest interval linearly increased (P < 0.01) ruminal retention time of the hay and tended (P = 0.06) to linearly increase ruminal retention time of the concentrate portions of the diet. Harvest interval linearly decreased (P 
0.05) the extent of degradability and effective degradability of DM and NDF of hays, but DM disappearance of the total diet did not differ (P 0.35). In the conditions of this study, increasing harvest interval of crabgrass hay from 21 to 49 d had no deleterious impact on animal performance or efficiency of gain when fed to growing calves in a high-concentrate mixture.
Key Words: cattle • Digitaria ciliaris • growth • hay quality
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INTRODUCTION
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Maturity at harvest has a large impact on hay quality; as forages mature, concentrations of fiber and lignin increase. Highly lignified forages remain in the rumen longer because of their slow rate of digestion, which results in decreased DMI and reduced animal performance (Jung and Allen, 1995
; Allen, 2000). Crabgrass (Digitaria ciliaris) is a warm-season annual that is usually of greater nutritive value than most other warm-season grasses, is highly palatable, and makes excellent-quality hay when properly managed (Dalrymple et al., 1999). Ogden et al. (2005) reported that crabgrass harvested on 7 dates had greater DM and NDF digestion rates and less fiber content than Bermudagrass (Cynodon dactylon [L.] Pers.).
When feeding mixed diets to growing calves, producers often use low-quality forages because of the perception that forage quality is insignificant relative to the total diet. In research with lactating dairy cows, Undersander (2004) reported that DMI and milk yield were increased when forage quality was increased in mixed diets. Galyean and Defoor (2003) reviewed 11 published experiments investigating roughage source and level in feedlot cattle and found that dietary NDF explained 92% of the variation in DMI of diets containing roughage levels between 0 to 30% of DM. However, little research has reported the effects of harvest interval of warm-season grass hays on DMI and performance of growing beef calves fed mixed diets.
The following research was designed to determine the impact of harvest interval (21, 35, and 49 d of re-growth) of crabgrass hay on forage DM yield, forage chemical composition, and performance of growing calves fed mixed diets in drylot.
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MATERIALS AND METHODS
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All animal procedures were conducted in accordance with the recommendations of the Consortium (1988) guidelines and were approved by the University of Arkansas Institutional Animal Care and Use Committee.
Forage Harvest Management
Twelve 0.81-ha, naturally reseeded, crabgrass hay fields were harvested on 8 July 2004 and baled on 12 July 2004, fertilized with 57 kg of N/ha as ammonium nitrate and 100 kg of K/ha as muriate of potash on 21 July 2004, then randomly assigned to a harvest interval of 21, 35, or 49 d (n = 4/treatment). These fields consisted primarily of common crabgrass (Digitaria ciliaris [Retz.] Koel.; 95%) and other warm season grasses (5%) as determined by the dry weight rank method (Gillen and Smith, 1986) and were located at the University of Arkansas Division of Agriculture, Southwest Research and Extension Center (33°42'N, 93°31'W) near Hope, AR. Soils of these pastures were primarily Smithdale fine sandy loam but also included areas of Sawyer loam. These soils are deep and moderately well drained, low in native fertility, and have a low soil pH and OM (Hoelscher and Laurent, 1979).
After the assigned regrowth interval, standing height and phenological growth stage were assessed (Stauss, 1994) on plants in 10 random 0.5 m2 quadrants per pasture before cutting with a disc mower (Kuhn, model GMD 700 HD, Vernon, NY) and baling into round packages (John Deere 466, John Deere Tractor Works, Moline, IL). Phenological growth stage was based on the Biologische Bundesanstalt Bundessortenamt and Chemical Industry scale (Stauss, 1994), where a growth stage of 30 corresponds to the beginning of stem elongation, 40 to the early boot stage of development, 50 to heading, and 60 to ripening seed heads.
Individual bales were weighed, labeled, and sampled. Samples were dried at 50°C in a forced-air oven, ground to pass a 2-mm screen (Model 4 Thomas A. Wiley Laboratory Mill, Thomas Scientific, Swedesboro, NJ), and composited within pasture. Composite samples were analyzed for DM and ash (AOAC, 1990) and for NDF and ADF by the batch procedures outlined by Ankom Technology Corp. (Fairport, NY). Concentration of N was determined by rapid combustion (FP-528, Leco Corp., St. Joseph, MI), and CP was calculated as the percentage of N in the sample x 6.25. Crabgrass hays were stored in a 17 x 23-m barn and composited across pastures within harvest interval for 2 feeding studies.
Performance Studies
In the first experiment, 120 steer and heifer calves (75 to 87% Angus ancestry; initial BW = 210 ± 4.4 kg) from the Southwest Research and Extension Center cow herd were used to test the effects of forage harvest interval on performance of calves that were limit-fed mixed diets at a rate of 2.3% (DM basis) of BW. After weaning, calves were weighed after a 16-h shrink, stratified by sex (5 steers and 5 heifers/pen) and BW, and assigned to 1 of 12 pens. Pens were then randomly assigned to 1 of 3 treatment diets based on the crabgrass hays harvested after 21, 35, and 49 d of regrowth (n = 4 pens/treatment).
Diets (Table 1) contained 20% (DM basis) of the crabgrass hay, and the primary energy sources for the diets were from ground corn (33%) and soybean hulls (32%). Round bales of hay were processed in a feed mixer (model 1060, Kuhn, Vernon, NY) before blending with the concentrate. Diets were stored in the feed alley until delivery to the feedbunks at 0800 daily. Calves were acclimated to treatment diets over a 14-d period. Calves were allowed ad libitum access to Bermudagrass hay for 1 wk and fed 1 kg/calf of the assigned treatment diets on d –1; the feeding level was increased daily until the target intake rates were reached (2.3% of BW, DM basis). Diets were then limit-fed for a 70-d growing period.
At weaning, the calves were treated for internal and external parasites (Ivomec Plus, Merial Inc., Duluth, GA) and were vaccinated for bovine respiratory disease complex (Cattlemaster 4, Pfizer, Inc., New York, NY) and 7-way Clostridial plus Haemophilus somnus (Vision 7 somnus, Bayer Corp., Shawnee Mission, KS). Revaccination occurred 14 d after the initial vaccination. Calves were weighed once every 2 wk before the morning feeding after a 16-h removal of feed and water. Feed was offered once daily at 0800 in quantities equaling 2.3% (DM basis) of the pen BW, which was adjusted once every 2 wk. Diet samples were collected weekly for DM, ash, N, NDF, and ADF analysis. Calves were housed in pens (7 x 33 m) that were partially covered by an open-sided barn and constructed of metal panels with interior pens split by electrified fencing. Each pen offered 7 m of space in the feed bunk.
In the second experiment, 60 heifers (75 to 87% Angus ancestry; initial BW = 207 ± 5.81 kg) from the Southwest Research and Extension Center cow herd were used to evaluate the effects of forage harvest interval on performance and DMI of calves fed mixed diets in amounts sufficient for ad libitum intake. Calves were acclimated to ad libitum intake of a common diet during a 14-d period. The diet fed during the acclimation period was similar to the research diets but included Bermudagrass hay in place of the crabgrass hay used in the study.
After the diet acclimation period, the calves were weighed (unshrunk) and assigned to 1 of 12 pens that were stratified by BW (n = 5 heifers/pen). Pens were then randomly assigned to 1 of the 3 treatment diets previously described (n = 4 pens/treatment) for a 56-d feeding period. Feed was offered once daily at 0800 in quantities adequate to allow ad libitum consumption. Diet samples were collected weekly for DM, ash, N, NDF, and ADF analysis. Calves were weighed (unshrunk) once every 2 wk before the morning feeding. Orts were collected weekly at 0700 (before weighing) and were weighed and sampled for DM determination.
In Situ Disappearance and Passage Rate of Hay and Concentrate
Subsamples of forage were composited within pasture to determine in situ DM and NDF disappearance. One-gram aliquots were weighed into nylon bags (7 x 7 cm; 25-um pore size) and heat-sealed using an impulse sealer (model CD-200, National Instrument Co. Inc., Baltimore, MD). Duplicate samples were placed sequentially in the ventral rumen of 3 ruminally cannulated steers (BW = 584 ± 10.4 kg) maintained on free-choice mixed-grass (Paspalum dilatutum Poir. and Cynodon dactylon [L.] Pers.) hay (10% CP, 62% NDF, and 34% ADF; DM basis) in a completely randomized design.
Samples were soaked in water for 20 min before incubation for 6, 12, 24, 36, 48, 72, 96, and 120 h in a mesh nylon bag. Upon removal, samples were washed in a hand-operated washer (Wonder Clean, Wonder Wash Corp., Bala Cynwyd, PA) 10 times for 2 min each and then were dried at 50°C for 48 h. Forage samples for each pasture and in situ samples from each steer were analyzed for NDF by the batch procedures outlined by Ankom Technology Corp. (Fairport, NY).
Disappearance curves for in situ data set were analyzed by nonlinear regression using the NLIN procedure (SAS Inst. Inc., Cary, NC). Fractions of DM and NDF were partitioned based on relative susceptibility to ruminal degradation (Ørskov and McDonald, 1979). The A fraction was defined as the immediately soluble fraction, the B fraction was composed of DM and NDF that disappeared at a measurable rate, and the C fraction was considered undegradable in the rumen (NRC, 1996). For the B and C fractions, disappearance rate (Kd), and lag time were determined by the nonlinear regression model. The A fraction was calculated as 100 – (B + C). The effective degradability of DM was calculated as A + {B x [Kd/(Kd + Kp)]}, where Kd is defined as the rate of disappearance and Kp is the ruminal particulate passage rate (Ørskov and McDonald, 1979).
To measure passage rate of the hay and concentrate portions of the diet, 12 heifer calves (BW = 145 ± 4.5 kg) were individually fed, at 2.3% of BW, 1 of the 3 treatment diets previously described (n = 4/diet) for 14 d in a completely randomized design. At 0800 on the first day of a 5-d collection period, heifers received a pulse dose of Yb-labeled hay and Dy-labeled soybean hulls. Fecal samples were collected at 4, 8, 12, 16, 20, 24, 30, 36, 48, 54, 60, 72, 84, and 96 h postdosing. Labeled forage and fecal grab samples were dried for 72-h in a forced-air oven at 50°C and ground to pass a 2-mm screen (Model 4 Thomas A. Wiley Laboratory Mill). Ground forage and fecal samples were then dried overnight at 100°C to determine DM. Extraction of Yb and Dy were completed using an EDTA procedure described by Hart and Polan (1984). Fecal Yb and Dy excretion curves were analyzed by using the NLIN procedure of SAS with a 1-compartment model (Marquardt method) described by Krysl et al. (1985). Ruminal particulate passage rate (PPR) was determined as the rate of ruminal mixing x 0.59635, mean ruminal retention time was calculated as 1/PPR, and fecal output was determined as dose/K0, the initial concentration of marker in the compartment.
Statistical Analysis
Forage quality at harvest was analyzed as a completely randomized design (Lentner and Bishop, 1986) with the GLM procedure of SAS using pasture as the experimental unit and residual error as the error term. The G:F as well as DMI of calves in the performance studies also were analyzed as a completely randomized design (Lentner and Bishop, 1986) with the GLM procedure of SAS using pen as the experimental unit and residual error as the error term. Effects of diet fed in drylot on performance of limit-fed or ad libitum-fed calves, in situ DM and NDF disappearance characteristics, and passage rate data for each forage and diet were analyzed as a completely randomized design (Lentner and Bishop, 1986) with the MIXED procedure of SAS using pen as the experimental unit and pen within treatment as the random effect. Least-squares means for forage quality, calf performance, passage rate, in situ digestion kinetics, DMI, and G:F were separated using orthogonal contrasts, which included the linear and quadratic effects of forage harvest interval (Steel and Torrie, 1980).
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RESULTS AND DISCUSSION
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Forage Yield and Quality
Forage DM yield/ha (Table 2) increased linearly (P < 0.01) from 2,872 kg to 9,788 kg as the regrowth interval between hay harvests increased from 21 to 49 d. This increase in DM yield was accompanied by a quadratic (P < 0.01) increase in growth stage at harvest from 29 (late tillering) with a 21-d harvest interval to 51 (10% head emergence) with a 35-d harvest interval and finally 56 (60% head emergence) for a 49-d harvest interval. In northern Arkansas, Ogden et al. (2005) reported that common crabgrass increased in maturity from stem elongation with 3 nodes (growth stage = 33) to milk stage (growth stage = 73) over a 7-wk period and DM yield/ha increased from 3,448 to 4,202 kg.
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Table 2. Effect of harvest interval of crabgrass hay on DM yield, phenological growth stage, and nutritive characteristics
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With the increase in maturity, there were concomitant linear (P < 0.01) decreases in CP and calculated TDN concentrations and increases in detergent fiber concentrations (Table 2
). Crude protein concentration was reduced (P < 0.01) from 15.6 to 11.0%, and TDN concentration was reduced from 62.6 to 54.8 when harvest interval increased from 21 to 49 d. Ogden et al. (2005) reported pronounced linear reductions in whole plant CP concentration of crabgrass (from 21.0 to 15.9%) when growth stage increased from stem elongation (3 node) to milk stage. Tifton-85 and Coastal Bermudagrass decreased from 20.8 and 21.1% CP, respectively, to 11.1 and 13.6% CP, respectively, when harvested after 2- to 7-wk regrowth (Mandebvu et al., 1999). Concentration of NDF increased linearly (P < 0.01) from 61.3 to 69.8% and ADF increased linearly (P < 0.01) from 35.7 to 42.7% when harvest interval increased from 21 to 49 d. Ogden et al. (2005) also reported linear increases in NDF (from 48.7 to 54.6%) and ADF (from 24.2 to 25.1%) concentrations of common crabgrass harvested with increasing maturity. The NDF concentrations in the current study and reported by Ogden et al. (2005) are less than would be expected with other warm season grasses. Venuto et al. (2003) reported that NDF concentration of dallisgrass (Paspalum dilatatum Poir.) ranged from 68 to 71%, and Johnson et al. (2001) reported NDF concentrations of Bermudagrass ranged from 71.9 to 79.6% of DM. Mandebvu et al. (1999) reported that NDF concentration of Tifton 85 and Coastal Bermudagrass increased from 65.5% at 3 wk to 68.2% at 4 wk of regrowth but NDF content did not differ from 4 to 7 wk, whereas ADF content increased from 29.4 to 32.7 from 3 to 7 wk.
Performance of Calves Fed Mixed Diets
When calves were limit-fed 2.3% of BW in the first experiment (Table 3), there were no differences (P 0.15) in BW before or after acclimation to treatment diets (210 ± 4.4 and 215 ± 4.5 kg, respectively) or after the 70-d growing period (270 ± 4.7 kg) among the treatments. During the 14-d diet acclimation period, ADG averaged 0.32 kg and did not differ (P 0.11) among treatments. Average daily gain during the 70-d growing period averaged 0.80 kg across treatments and also did not differ among treatments (P 0.22). Because ADG did not differ and DMI was limited to 2.3% of BW, G:F did not differ (P 0.35) and averaged 0.13 across treatments.
Body weight of calves fed mixed diets ad libitum (Table 4) did not differ (P 0.29) before or after the 56-d growing study (207 ± 5.8 and 274 ± 6.6 kg, respectively). Average daily gain and total BW gain per calf did not differ (P 0.34) among treatments and averaged 1.21 and 68 kg, respectively. Daily DMI averaged 7.9 kg and did not differ (P 0.21) across treatments. Once again, because ADG and DMI did not differ, efficiency of gain also did not differ (P 0.20) among treatments, averaging 0.15.
Dry matter intake and NDF intake by steers fed long-stem hay were also not affected by harvest interval (3, 5, or 7 wk of regrowth) of Coastal or Tifton-85 Bermudagrass (Mandebvu et al., 1999). Garrett (1974) reported that steers fed diets containing 20% roughage from oat hay, alfalfa hay, rice straw, or barley straw did not differ in ADG, but steers fed oat or alfalfa hay had greater ADG than those fed rice or barley straw when roughage level was increased to 40 or 60% of DM. Guthrie et al. (1996) reported that ADG of finishing heifers was not affected by source of roughage (alfalfa, Sudangrass hay, or cottonseed hulls) or roughage level (7.5 or 15% roughage, 9.2 to 13.9% ADF).
Rinne et al. (2002) reported that dairy cows fed grass silages harvested at progressive maturity experienced linear reductions in DMI and milk yield with increasing forage maturity. Breakdown or passage of medium particles were hypothesized to be constraining to DMI, but reductions of ruminal fill with early cut silage indicates ruminal fill was not solely responsible for DMI control. Beauchemin (1991) compared early and midbloom alfalfa hay included in lactation diets to provide 31, 34, and 37% dietary NDF and reported that maturity of alfalfa hay had no effect on lactation performance of Holstein cows when hays were included in diets at similar NDF contents. Across dietary NDF concentrations, increasing maturity of harvest of alfalfa hay tended to reduce fat corrected milk production, increased eating time, and reduced total ruminal VFA production. Diets fed in the current studies increased in NDF concentration (Table 1) as the maturity of crabgrass hay increased (from 53.1 to 58.9% of DM for hay harvested after 21- to 49-d regrowth, respectively). Although hay concentrations were less in the diets fed in the current study than the diets reported by Beauchemin (1991), diets fed to the growing calves in the current study also included 32% (DM basis) soybean hulls that supplied a readily digestible fiber source. Ipharraguerre et al. (2002) found that when soybean hulls replaced corn in dairy lactation diets containing 46% roughage (DM basis) there were reductions in the amount of nonstructural carbohydrate and increases in the amount of fiber digested, whereas passage of nonammonia N, microbial N, and amino acids was not affected. In limit-fed diets fed to growing calves, Löest et al. (2001) reported that soybean hull-based diets without roughage produced less ADG and G:F than roughage-based or corn-based diets.
In Situ Disappearance of Hay and Diets
The fractionation and kinetics of hay DM and NDF disappearance are shown in Table 5. The A (immediately soluble) fraction of DM decreased linearly (P = 0.02) with increased harvest interval (11.7, 8.0, and 6.2% of DM for 21-, 35-, and 49-d harvest intervals, respectively). The A fraction of NDF also decreased linearly (P = 0.04) with increased harvest interval, from 5.8% with 21-d harvest interval to 1.6% with a 49-d harvest interval. Harvest interval of crabgrass hay had no effect (P 0.26) on the B fraction of DM or NDF. The undegradable or C fraction of DM and NDF linearly increased (P 0.05) with increasing harvest interval. Rate of disappearance DM and NDF as well as lag time between insertion of sample in the rumen and disappearance of DM and NDF from the bags were not affected by harvest interval (P 0.13). When effective DM degradability was calculated using the equation suggested by Ørskov and McDonald (1979), there were linear (P < 0.01) reductions when harvest interval was increased from 21 to 49 d. There were also quadratic (P = 0.02) reductions in effective NDF degradability because increasing harvest interval from 21 to 35 d had no effect on effective NDF disappearance, but extending harvest interval from 35 to 49 d reduced effective NDF disappearance. In Florida, linear and quadratic reductions in IVOMD were found with later harvest dates of bahiagrass, Bermudagrass, and stargrass (Johnson et al., 2001) indicating that warmer temperatures associated with later harvest dates may explain part of the reduction in digestibility. Ogden et al. (2005) reported that DM of crabgrass harvested over a 7-wk period had reduced A and B fractions, along with reduced potential extent of disappearance, disappearance rate, and effective disappearance, whereas the A and B fractions, potential extent, and effective disappearance of NDF increased in a quartic fashion over the same time period. Beauchemin (1991) reported that the soluble fraction of DM, the extent of DM disappearance, and rate of DM disappearance of midbloom alfalfa hay was less than alfalfa hay harvested at the early-bloom stage of maturity. Rinne et al. (2002) reported that increasing maturity of grass silage resulted in linear reductions in potential digestibility and rate of digestion in situ. Mandebvu et al. (1999) reported that in situ A and B fractions of DM and NDF or rate of in situ DM disappearance did not differ in Bermudagrass harvested weekly after 3 to 7 wk of regrowth.
The fractionation and kinetics of total diet DM disappearance are shown in Table 6. Harvest interval of hay had no effect (P 0.22) on the A, B, or C fractions, rate of disappearance, lag time, or effective DM degradability of the diet. Moore et al. (1990) reported that roughage source had no effect on extent or rate of digestion of flaked milo when cottonseed hulls or wheat straw replaced alfalfa hay. Poore et al. (1990) indicated that increasing roughage concentration in flaked grain sorghum diets increased potential fiber digestion of alfalfa hay, wheat straw, and grain sorghum. Also, the rate of fiber digestion was increased in alfalfa hay and wheat straw diet dietary fractions with increasing roughage concentration, whereas the rate of grain sorghum fiber digestion was not affected by roughage level (Poore et al., 1990).
Passage Rate of Hay and Concentrate
Particulate passage rate (Table 7) of the hay portion of mixed diets decreased linearly (P < 0.01) with increased hay harvest interval; ruminal retention time was thus linearly (P < 0.01) increased when harvest interval increased from 21 to 49 d. Along with this reduction in hay PPR with increased forage maturity, there was a tendency (P = 0.06) for attendant linear reductions in PPR of the concentrate portion of the mixed diets associated with increased forage maturity. Ruminal retention time of the concentrate portion of mixed diets tended (P = 0.06) to increase in a linear fashion when harvest interval of hay increased from 21 to 49 d. When PPR of the entire diet was calculated using relative contributions of hay and concentrate to the diet, PPR of the total diet decreased with increased forage maturity (4.06, 3.89, 3.30%/h, for 21-, 35-, and 49-d hay harvest interval, respectively), and the affiliated ruminal retention time increased with forage maturity (24.6, 25.7, and 30.3 h for 21-, 35-, and 49-d hay harvest interval, respectively). In contrast to these results, Poore et al. (1990) reported that increasing roughage concentration from 10 to 70% of DM increased passage rate of alfalfa hay and wheat straw but had no effect on passage of flaked grain sorghum.
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Table 7. Effect of harvest interval of crabgrass hay on digestion parameters and predicted apparent digestibility of mixed diets
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Fecal output did not differ (P 0.14) as a result of hay maturity, averaging approximately 5.8 g/kg of BW, and thus apparent DM digestibility did not differ (P 0.14), averaging 74.8% of DM. Effective degradability of the diets (Table 6) averaged 80.3% of DM and did not differ (P 0.35) due to harvest interval of the hay, which is very similar to apparent digestibility estimated using external inert markers. Mandebvu et al. (1999) reported that long-stem Bermudagrass hay harvested after 3, 5, or 7-wk of regrowth did not differ in apparent total tract DM digestibility when fed to growing steers. Rinne et al. (2002) reported that DMI and apparent OM digestibility of grass silage fed to lactating dairy cows decreased linearly with later harvest date. Moore et al. (1990) reported that replacing alfalfa hay with cottonseed hulls reduced NDF digestibility but increased DMI, whereas replacing alfalfa hay with wheat straw had no effect on dietary digestibility. Finishing diets containing alfalfa hay, wheat straw, ammoniated wheat straw, or corn silage did not affect ruminal or total tract digestibility of OM, starch, or N (Panichnantakul and Stanton, 1998). Cole et al. (1976) reported that total starch digestion was not affected when cottonseed hull concentration was increased from 0 to 21% in corn-based diets, but as roughage concentration increased there was a decrease in ruminal starch digestion and increased intestinal starch digestion.
Voluntary DMI is critical to animal performance and forage fiber concentration is negatively associated with DMI of high forage diets because of contributions to ruminal fill (Jung and Allen, 1995). In a review of lactation experiments with dairy cows, Allen (2000) stated that when in situ or in vitro NDF digestibility increased 1% in corn silage diets, DMI increased 0.17 kg and milk yield increased by 0.25 kg. Oba and Allen (1999) reported that differences in digestibility in vivo were only 40% of those measured in vitro at a constant incubation time; this is because differences in NDF digestibility were depressed as ruminal retention time decreased with increases in DMI.
Results presented by Garrett (1974) and Guthrie et al. (1996) with growing and finishing calves and the impact that forage nutritive quality has on performance in diets fed in dairy lactation studies (Beauchemin, 1991; Rinne et al., 2002) suggest that when roughage level is 20% or less of dietary DM, digestibility of roughage has much less effect on performance than when roughage is fed at a higher level. Thus if roughage content of the diets fed in the current study were greater, nutritive quality of the forages may have had a larger impact on DMI, diet digestibility, and performance of growing calves.
Under the conditions of this research, increasing harvest interval of crabgrass hay has no deleterious impact on animal performance or efficiency of gain when fed in 80% concentrate diets. Even though rate and extent of digestion of the hay fraction of the diet decreased with increased harvest interval, the reduction in passage rate and increased ruminal retention time observed with increased hay harvest interval equalized effective degradability of the diet with no impact on DMI of the mixed diets.
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Footnotes
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1 This project was conducted with funding from the Univ. of Arkansas Agric. Exp. Sta., Hatch Project No. AR001735. The authors wish to express their appreciation to Pat Capps, Josh Loe, and Clint Cornelius for technical assistance in completing this project. 
3 Current address: Kansas State Univ., 216 Weber Hall, Manhattan, KS 66506.
2 Corresponding author: pbeck{at}uaex.edu
Received for publication June 5, 2006.
Accepted for publication October 8, 2006.
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Abstract
INTRODUCTION
