Feed intake and digestion by Holstein steer calves consuming bermudagrass or ryegrass-wheat hay and supplemented with alfalfa, corn or monensin

241

Feed intake and digestion by Holstein steer calves consuming bermudagrass or ryegrass-wheat hay and supplemented with alfalfa, corn or monensin*

ABSTRACT

Sun, W., Goctsch. A.L.. Forster. Jr., L.A.. Ga!lowsy. Sr.. D.L. and Johnson. Z.B., 1991. Feed intake and digestton by Holstein steer calveswnsuming bermudagrassw ryegrass-wheat hay and supplemcnted with alfalfa, corn or monensin. Aw?z. Feed.%. Teho/., 34: 241-254. Effects of rupplcmenting with legume and/or gram or monensin on intake and dIgestion by Holslcin swer calves fed warm or cool seasongrasshay lbermudacrass or wemass-wheat) were determmcd. In Expcrimcnt I, cighl steers (140 kg body-weight (Bw) ). in t&&nultaneo& 4X4 Latin squares. were fed bermudagrass (72% neutral detergem fiber. NDF) or ryegrass-wheat hay (59% NDF) ad libilom and nosupplcmem (BGand K’~tr&ments) orapproximatelyO.S*BW ofground alfalfa lhsy (BG-A and RW-A lreatmena). I .O% SW of ground corn (BG-C and RW-C treatments) or both once daily (BG-AC and RW-AC treatments ). Supplement treatments affected organic matter !OM) intake difTcrcntly with each forage (2.97 kg day-‘, 3.35 kg day-‘, 4.05 kg day-‘, 4.07 kg day-‘. 4.07 kg day-‘. 4.28 kg day-’ and 4.54 kg day-’ for BG, BG-A, BG-C, BG-AC, RW, RW-A. RW-C and RW-AC. respectively; SE=O.l Total lracl NDF digestibility was 63.2%. 62.3%, 61.1%. 59.7%,68.9%. 67.3%. 60.99band 54.2% (SE=3.0) for BG. BG-A. BG-C, BG-AC, RW, RW-A, RW-C and RW-AC. respectively (greater with than without supplementation and for alfalfa and corn given together vs. alone; PcO.05). Supplementation with alfalfa or cora alone increased digestible OM intake with bermudagrassbut not with ryegrass-wheat: &wing alfalfa and corn together increased digestible OM intake with both foragesas compared with cum alone (1.72 kg day“, 2.0, kg day-‘, 2.57 kgday-‘.2.74kgday-‘-2.71 kgday-‘,2.71 kgday-‘.2,77kgday-‘and3,02kgday-’forBG. BG-A. BG-C. BG-AC, RW, RW-A, RW-C and RW-AC, respectively: S&0.13). In another experiment (4x4 Latin square). four steers (228 kg BW) fed bermudagrass(75% NDF) or orchardgrass(64kNDF) hay ad libitumwith 0.5%BW ofgroundcornandOor200mgofmonensin.Neither intake nordigestibilitywere signilicantlyaffected by moncnsin, but monensin tended to depressboth measureswith bermudagrass.Characteristics of foragessuch as those unique to mol and warm season grassescan affccl changesin feed intake and digestion with rupplemen~ation.

I).

were

‘Approved for publication by the Director of the Arkansas Agricultural Experiment Station. ‘Author to whom reprint requests should be addressed.

0377-840

I/9

I /SOS.50 0 199 I Elsevier Science Publishers B.V. All rights reserved.

W.SUNET AL.

242 INTRODUCTION

Warm and cool season grasses are basal dietary ingredients in ruminant production systems throughout much ofthe world. For acceptable production levels of some classes of ruminants, consumption of grass alone does not provide adequate energy and nutrients. Thus, often legumes and/or concentrates are added to grass diets. Inclusion of legumes and grain in the diet affects the digestion and metabolic conditions of the ruminant (Beever and Siddons, 1986; Weston and Poppi, 1987). Likewise, ionophores are used in many ruminant production settings. Warm and cool season grasses differ in chemical and physical characteristics that govern ruminal conditions (Akin, 1986). Jarrige et al. (1986) and Minson (1990) reported that feed intake increases are less for high than for low quality forage when grains are added to the diet. Effects of monensin on performance vary more with forage-based than with concentrate-based diets (Lemenager et al., 1978). Generally, performance responses to monensin appear to be lower with low than with high quality forage (Zorrilla-Rios et al., 1985; Elliott et al., 1987), and the level of monensin may interact with forage quality as well (Oliver, 1975). Thus, characteristics of forage such as those associated with cool and warm season grasses may affect changes in feed intake and digestion occurring when legumes, cereal grains and ionophores are added to the diet. This experiment was conducted to determine if dietary additions of legumes, grain or monensin have similar effects on feed intake and digestion by Holstein steer calves with bermudagrass and ryegrass-wheat or orchardgrass hay. MATERIALS

AND METHODS

Experitnenf I An experiment with two simultaneous 4x4 Latin squares was conducted with eight Holstein steers (average initial and final body weights of 118 kg and 161 kg, respectively). Steers were assigned to squares according to mean body weight (BW) and variation in BW. Steers were treated for internal and external parasites and injected with 500 000 IU vitamin A and 75 000 IIJ vitamin D, 7 days before the trial. Steers were tethered in a partially enclosed barn with free access to water. The trial periods lasted 14 days; steers were weighed at the beginning of the trial and on Day 14 of each period at I3:OO h. Four steers of one Latin square were fed bermudagrass ( Cynodon dactylon; early heading) hay ad libitilm (105-l 10% of consumption on previous days) while the other four received a ryegrass (Lolium multiforum; eariy head emergence)-wheat (Trilicum aeslivum; anthesis) mixed hay (approximately I : I with trace amounts of fescue in early head emergence). Basal hays were

FEED INrAKE

AND oIGESTcON BY rlOLSTE,N CALVES

243

fed alone (BG and RW treatments) or with supplemental alfalfa (Me&ago saliva: late vegetative) hay at approximately 0.5% BW (BG-A and RW-A treatments; dry matter basis), ground corn at approximately 1.0% BW (BGC and RW-C treatments; dry matter basis) or alfalfa plus corn (BG-AC and RW-AC treatments). Alfalfa hay was chopped through a 0.635 cm screen. Supplements were fed once daily at 08:OQ I;. Steers receiving both supplements were offered corn after consumption of alfalfa. Unsupplemented steers were fed basal hay at OS:00 h and supplemented steers received basal hay immediately after consumption of supplement. Generally, alfalfa was consumed in less than 30 min and corn ingestion was quicker. When alfalfa was not consumed in 30 min, access to the vessel containing alfalfa was maintained but other feedstuffs were offered. Half of the al!otment of basal hay was given at OS:00 h and the other half was given at 16:OO h. All steers received 10g daily of a 3 : 1 mix of NaCl: trace mineral mix (the latter consisted of > 12.0% Zn, 10.0% Mn, 5.0% K, 2.5% Mg, 1.5% Cu, 0.3% 1, 0.1% Co, 8.02% Se). Hay samples taken on Days 9-l 4 were composited and ground through a 1 mm screen. On Day 9, 100 g of Yb-labeled hay (Goetsch and Galyean, 1983; 24 h soak) were mixed with 127 g of unlabeled hay of the OS:00 h meal, and remaining hay was offered thereafter. Fecal grab samples were taken on Days I 1-14 at 12-h intervals, advancing 3 h daily, and frozen. Later, samples were dried at ‘55°C for 48 h and ground through a 2 mm screen. Composite samples formed within steer and period were ground through a 1 mm screen. Feed and fecal composites were analyzed for dry matter (DM), ash, Kjeldahl nitrogen (N, Association of Official Analytical Chemists, 1984), neutral detergent fiber (NDF, Goering and Van Soest, 1970) and acid-insoluble ash (2 N HCI; Van Keulen and Young, 1977). Hay also was analyzed for acid detergent fiber (ADF) and acid detergent lignin (ADL; Goeringand Van Soest, 1970). Cellulose was determined as the loss in weight upon sulfuric acid treatment; ADF subtracted from NDF yielded hemicellulose. For NDF analysis, grain was treated with amylase (Robertson and Van Soest, 1977). Acid-insoluble ash was used as an internal marker to estimate digestion of organic matter (OM), NDF and N. Individual fecal samples were ashed, solubilized with acid (Ellis et al., 1982) and analyzed for Yb by atomic absorption spectrophotometry. Particulate passage rate (PPR) was estimated by regressing the natural logarithm of Yb concentration against time post-dosing. The mean of squared correlation coefficients was 0.9907. Hay intake data for Days IO-14 first were analyzed as a split-plot in time. Because the effect of day of the period and day x forage and day x supplement interactions were not significant (P> O.lO), intakes over the last 5 days were averaged. Data first were analyzed with the following sources of variation: forage or square (I d.f.), steer within square (6 d.f.), period within square (6 d.f.), supplement (3 d.f.) and foragexsupplement interaction (3 d.f.).

244

w. SUN ETAL.

When the foragexsupplement interaction was not significant (P>O.lO), independent contrasts were made for supplementation (BG and RW vs. other treatments), supplement type (alfalfa vs. corn) and supplementation method (supplementation with alfalfa and corn alone vs. together). But when the interaction was significant, the two squares were analyzed separately, and supplement treatment means within squares (forage type) were separated by least significance difference procedures with a protected Ftest (PC 0.05; Snedecor and Cochran, 1967). The significance levei of 0.05 was used throughout unless otherwise stated. Simple correlation coefficients were determined. The Statistical Analysis System (SAS, 1985) was used for all analyses. Experiment 2 Four Holstein steers (average initial and final BW of 179 kg and 277 kg, respectively) were used in a 4x4 Latin square study with a 2x2 factorial arrangement of treatments. Prior to the experiment, steers were managed as above. Trial periods lasted 2 I days. Steers consumed long-stemmed bermudagrass (vegetative growth stage) or orchardgrass (Ductless glomerafa; vegetative growth stage) hay given ad libitum (105-l 10% of previous days consumption) at 0800 h and 16:OO h. Before offering hay, 0.5% BW daily (DM basis) of ground corn was offered alone (BG and OG treatments) or with RumensinO (Elanco Products Co., Indianapolis, IN) (200 mg monensin sodium daily; BG-M and OG-M treatments) mixed in a portion of corn. Dicalcium phosphate at I7 g day- ’ and trace mineralized salt at 7 g day-r (consisting of more than 95% NaCI, 0.25% Mn, 0.2% Fe, 0.03% S, 0.033% Cu, 0.0025% Co, 0.007% I and 0.005% Zn) were also given. On Day IS, 75 g of Yb-labeled hay were mixed with I52 g of unlabeled hay from the 08:OO h meal, and remaining hay was offered thereafter. Fecal grab samples were taken on Days 18-2 1 at 12-h intervals advancing 3 h daily, and frozen. Later, two wet composites were formed by subsampling. One was used to measure DM and particle characteristics. Wet feces ( 15 g) were wet-sieved through screens with 2.34, 1.18, 0.60, 0.30,O. 15and 0.075 mm openings. Particles retained on screens were washed onto fiber paper and dried. The percentage of DM retained on each screen was calculated and mean particle size (MPS) was derived by probit analysis (Jaster and Murphy, 1983). The other composite was lyophilized and ground through a 1 mm screen. Individual fecal samples were ground through a 2 mm screen. Hay intake during the last 7 days of each period was not affected by day of the period, so values were averaged for expression of DM intakes and estimation of digestion. The mean of squared correlation coefftcients for the regression estimating PPR was 0.9732. Digestibilities and PPR were calcu‘-1_J -- ‘lDLC”(13 11. C-ra*:-on+ ti*y*&..llr... 1, Da!? were statistically analyzed as a 4x4 Latin

FEED INruE

AND

D,GESTlON

BY HOLsTElN

245

CALVES

as in Experiment 1, except that orthogonal contrasts were made for effects of forage type, supplementation with monensin and the interaction between forage type and monensin supplementation. ripdie

RESULTS

Experiment I Feed composition is given in Table I. Alfalfa and corn were consumed at levels similar to those intended, except that, in one period, a RW-C steer did not completely ingest corn. The body weight of steers consuming ryegrasswheat increased more during the trial compared with steers fed bermudagrass; hence, differences among treatments in DM intake as a percentage of BW and OM intake in grams per day were not identical. Intake of ryegrass-wheat OM without supplementation was 37% greater than bermudagrass intake (Table 2). Total OM intake increased more when bermudagrass was supplemented with corn (36% increase) than when ryegrass-wheat was supplemented with corn (5% increase: P
Ash Crude protein Neulral detergent fiber Acid dciergent fiber Acid detergent lignin Cellulore Hemicellulose

Experiment 2’

Experiment I’ BG

RW

8.5 13.9 71.7 35.2 4.7 29.4 36.5

8.3 13.1 58.9 33.9 3.8 28.3 25.0

A

I I.2 21.3 41.8 31.4 6.2 24.3 10.3

C

BG

OG

1.4 10.0 10.0

7.4 10.3 75.4 37.5 5.8 30.2 38.0

8.2 15.2 64.4 35.6 5.1 29. I 28 R

‘BG. bermudagrass;RW. ryegrass-wheat mix; A, alfaIr% C, corn: OG. orchardgrass.

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TABLE 2 DailyinrakeoFdrymatter.intakeanddigestionaforganicmnlter (OM).neutraldctcr%entfiber (NDF) and nitrogen (N) and paniculateparsage fate (PPR) for Holstein steercalvesfed bermudagrass !BG) orryegrass-wheal (RW) hays with or without supplemcnlal alfalfa (A) orcorn (C) (Enpcnmem

I)

BG Dry maIIw

Alfalfa Corn Basal bay TOtal

intake (I

BG-A

EC-C

BG-AC

body we@) 0.52 2.50h 2.29” 2.50” 2.81”

0.97 2.29b 3.26b

0.52 0.98 1.82’ 3.30”

2.9P

4.05”

4.080

3.35”

58.3” 60.00-b 64.4b,’ 66.8’ 1.728 2.01’ 2.w 2.74” ; :.
62.3 1.52

61.1 1.38

72.4”

90.lb

61.3 44.4 4.57

64.3 58.0 4.97

I

RW

3.09d 3.09

4.07” 66.4 2.71 2.61”

RW-A

0.50 2.621 3.12

4.07” 66.9 2.11 2.50b

0.87

2.2Sb 3.13

4.28” 65.1 2.77 2.05”

0.50 0.95 1.83” 3.28

0.080 0.080

4.54’

0.106

FI

1.83 0.127

FI FI

0.078

SI sM Si

64.7 3.J: 1.OV

FI FI

54.2

2.97

103.2”

89.3”

104.7”

2.57

FI

60.6 62.4 4.7,

48.6 42.7 5.06

53.6 58.0 5.37

1.95 3.48

Fr STM

0.200

SM

68.9

67.3

88.1’

97.1”

93.1”

54.9

62.4 61.8 5.50

55.3 51.8 4.92

5.20

-

RW-AC

60.9

59.7 1.29

48.5

RW-C

I.SObI.68” 1.23” 1.1P0.090

‘Effccl: F. forage type (PcO.05); S and s, supplementation (PcO.05 and P
t

Intake of NDF (Table 2) was similar among bermudagrass diets but was lower with than without corn for ryegrass-wheat. The supplement treatment affected total tract NDF digestion similarly for both forages: NDF digestion was decreased by supplementation and by offering alfalfa and corn together vs. alone. Though forage type and supplement treatment did not significantly interact (Pt0.26), numerically it appeared that alfalfa had less effect than corn, and the depression with corn was greater for ryegrass-wheat (8.0 percentage units) than bermudagrass (2.1 percentage units). Likewise, the depression in NDF digestion by adding alfalfa to supplemental corn tended to be greater for ryegrass-wheat (6.7 percentage units) than for bermudagrass (1.4 percentage units). No differences in the amount of NDF digested daily were noted among bermudagrass diets, but with ryegrass-wheat, means were lower with than without corn.

FEED lNTAKEAN0

247

DlCESTlON BY HOLSTElN CALYES

Total tract N digestion was higher for bermudagrass than for ryegrass-wheat and affected by supplement treatment similarly for both forages, being greater for alfalfa than corn when given alone (Table 2). Digestible N intake was affected by supplement treatment similarly for both forages, being higher with than without supplements, for alfalfa than corn and for alfalfa and corn given together vs. alone. Particulate passage rate was similar for both grasses and was affected by supplement treatment similarly for both forages (Table 2). Particulate passage rate was higher with than without supplements and for supplementation with alfalfa and corn together vs. alone. Experiment2

Without monensin, total and hay DM and OM intakes were similar for bermudagrass and orchardgrass diets (Table 3). Total DM and OM intakes were numerically slightly lower with than without monensin for bermudagrass but were similar for both monensin levels with orchardgrass. Intake of NDF was similar among treatments, tending to be slightly lower with than without monensin. This difference tended to be slightly greater for bermudagrass than for orchardgrass. TABLE 3 Daily intake of dry matter and intake and digestion of organic matter (OM), neutral detergent fiber (NDF) and nitrogen (N) for Holstein steer calves fed bermudagrass (BG) or orchardgrass (OG) hays without or with monensin ( M 1 (Experiment 2) Item

Treatment BG

SE BG-M

OG

Effect’

OG-M .-

Dry matter intake (% body weight) Grain 0.49 2.84 Hay Total 333 OM intake (kg) 6.65 OM digestion 52.5 3.57 kg 4.98 NDFintake (kg) NDFdigestion 53.6 % 2.72 kg 0.120 N intake (kg) N digestion 41.0 n 0 053 kg

%

0.50 2.41 2.91 6.0

I

0.49 2.86 3.36 6.70

0.49 2.77 3.25 6.42

48.6 2.98 4.46

59.7 3.98 4.36

60.4 3.88 4.17

2.03 0.305 0.291

F f

48.2 2.18 0.108

65.9 2.85 0.170

65.4 2.72 0.163

2.37 0.250 0.0141

F

45.9 0.053

56.1 0.097

60.2 0.098

4.44 0.0108

F F

‘Fand f, forage type (P
0.186 0.182 0.379

F

w SUN ET 4L.

248 TABLE

4

Feces mean panicle sine (MFS), sieve retention and panizulate passage rate (PPR) steers fed bermudagrass (BG) or orchardgrass (OG) without or with monensin 2)

for Holstern

(M) (Experiment

ttem

SE

Treatment BG

dry ,nauer MPS (mm!

BG-M

OG

OG-M

Flmv

Sievv~te;tion 1.18mm 0.60 mm 0.30 mm O.lSmm 0.075 mm PaIlL /‘,“R (% h-‘)

(%)

0.223 2.8 6.6 14.; 18.7 20.5 12.6 24.6 4.04

0.199 2.9 6.6 13.3 17.2 19.1 12.2 28.7 3.32

IF. forage type (PcO.05): M, monensin ‘Dry matter passing through all sicws.

0.138 2.3 3.6 8.3 14.5 15.8 10.6 45.0 4.51

Effect’

0. IO6 2.3 3.4 8.5 14.5 15.5 10.4 45.0 4.39

0.024 0.69 1.03 .06 0.77 1.37 1.21 4.00 0.163

!

I

F

F F F F F FM

(PcO.05).

Total tract OM, NDF and N digestibilities were greater for orchardgrass than bermudagrass but were not affected by monensin or the foragexmonensin interaction (Table 3). Organic matter digestion was higher for the cool season grass than for the warm season grass. Digestible OM (PcO.08) and N intakes were greater for diets with orchardgrass than bermudagrass, although digestibla NDF intake was similar among treatments. The forage typexmonensin interaction in digestible OM intake was not significant, but numerically monensin elicited a greater depression with hermudagrass than with orchardgrass (0.59 vs. 0.10 kg). Particulate passage rate (Table 4) was greater for orchardgrass than for bermudagrass and without than with monensin. Feces MPS and feces DM retained on 2.36, I. 18,0.60, 0.30 and 0. IS mm screens were greater for bermndagrass than for orchardgrass, and DM passing through all sieves was greater for orchardgrass than for bermudagrass. DISCUSSION Experiment I Treatment effects of digestibilities of OM and NDF were not parallel because of greater post-ruminal digestion of OM than NDF and greater proportions of non cell wall materials in supplements than in grasses. Supplementing with alfalfa and corn alone and together affected NDF digestion similarly for

FEED G-WAKE AND DLOESTDN BY HOLSrElN CALVES

249

both grasses, although changes were greater for ryegrass-wheat than for bermudagrass. A small portion of the difference between forages was probably because of higher digestion of ryegrass-wheat than of bermudagrass, and differences in digestibilities between supplemental feeds and grass hay were greatest for ryegrass-wheat. Part of the difference also may be attributed to ruminal c.:nditions. The greater amount of OM fermented with ryegrasswheat would have led to low ruminai pH and high availability of readily fermentable substrate relative to conditions with bermudagrass. These factors and higher levels of cell contents and non-structural carbohydrate in the cool season grass than in the warm season grass probably created a microbial population more adapted to fermentation of readily available substrate and perhaps more subject to an increase in number of amyloiytic microbes with an increased dietary fraction of rapid!y degradab? substrates accompanying aifalfa and/or corn addition. The additive nature of change in digestion when aifaifa and corn were supplemented together supports this possibility. The increase in PPR when alfalfa was given with vs. without corn may have correspcnded to accelerated ruminal outflow of undigested NDF; PPR and NDF digestibility were correlated (r= -0.54). The higher ingestibility of legumes than of grasses has been attributed to the little amount of space in the gut taken up by legume particles because of their physical nature, which also may facilitate rapid ruminal outflow (Minson, 1990). Further, legume OM is generally digested in the mmen more completely than grass OM with ad libitum intake (Beever and Siddons, 1986). Minson ( 1990) concluded that mixing legumes and grasses does not increase voluntary intake or digestibility above that expected with additivity unless grass is deficient in an essential nutrient, typically N. Nitrogen did not appear to restrict ruminai digestion in this study, but other forage substances may have been present in limiting quantities. Bermudagrass and other warm season grasses are high in slowly degradable cell walls and low in rapidly degradable substrates (Akin, 1986), charac;eristics which restrict rumen microbial growth and can limit fiber digestion (Demeyer, 1981). Small increases in dietary corn level, raising non-structural carbohydrate concentration, have stimulated fiber digestion, or affected it less deleteriously than higher levels (Uden, 1984). Thus, the low level of readily fermentable substrate in bermudagrass relative to that in ryegrass-wheat may have minimized the effect of supplementation on digestion of bermudagrass. Increased total intake when alfalfa or corn alone supplemented bermudagrass may have occulred because animal energy and/or nutrient status was improved. Nutrients whose supplies can regulate feed intake by ruminants with high growth potential include amino acids and glucose precursors (Weston and Poppi, 1987; McCoiium and Horn, 1990). Becawse of differences between grasses in basal forage intake and digestibility without suppiementation, effects of supplements on fermentable OM supply and, thus, on micro-

250

W.SUNET*L

bial protein and volatile fatty acid synthesis, would have been proportionally greater when alfalfa was added to bermudagrass than when added to ryegrass-wheat. This may explain why alfalfa increased total intake with bermudagrass but not with ryegrass-wheat. Jarrige et al. ( 1986) stated that low nutrient or energy status corresponds to relatively small decreases in consumption oflow quality forage with supplementation relative to high nutrient and energy status with high quality forage. Supplementing bermudagrass with corn would have lessened potential alterations ofenergy and nutrient supplies in relation to totals by adding alfalfa, minimizing energy and/or nutrient deficits, so that feed intake remained steady when alfalfa was supplemented. However, higher PPR for BG-AC than for BG-A and BG-C suggest that microbial protein synthesis may have been increased when alfalfa and corn were offered together. PatticuHte passage rates indicate that the large decrease in ryegrass-wheat consumpbon when corn was added alone was not because acceptability of forage digesta for ruminal exit declined with reduced fiber digestion. Rather, it may relate to similar digestible OM intake or energy status resulting from compensating alterations oftotal OM intake (trend for an increase) and total tract NDF digestion (decrease) and the difference in total tract OM digestion between corn and ryegrass-wheat OM (that for corn being greater than that for rye.grass--wheat). Conversely though, for increased total feed intake when corn was added alone to ryegrass-wheat, increased amino acid absorption may have been necessary (McCollum and Horn, 1990). Differences in supplementation level and in digestion between supplement and grass most likely led to unique changes in feed intake with each supplemental feed given alone, an example of which is the trend for slight increase in OM intake with ryegrass-wheat for singly supplementing with corn but not alfalfa. Though corn addition did not affect the amount of OM digested daily with ryegrass-wheat, efficiency of energy metabolism could have been improved through increased glucose precursors from elevated propionate production and intestinal digestion of corn starch and protein escaping ruminal breakdown. Dietary legume inclusion can cause a shift from acetate to propionate (Varga et al., 1990). but the level of supplementation in this study was probably inadequate for a large change. The mrmerical increase in OM intake with ryegrass-wheat when alfalfa was added to corn could have bsen because changes in supplies of aforementioned digestion products were sufficiently greater than with corn alone. Alternatively, perhaps feed intake varied in such a way as to avoid transient and deleterious increases in peripheral blood ammonia owing to high ruminal absorption (Symonds et al., 1981; Beever and Siddons, 1986). Adding corn to supplemental alfalfa would have increased fermentable OM to increase rumen microbial capture of nitrogenous compounds and lower ruminal ammonia absorption.

FEED INTWE

Experimenf

4NO DlCESTlON

BY

HO!_STElN CALVES

2.51

2

Numerically, monensin depressed feed intake more with bermudagrass than with orchardgrass. Deswysen et al. ( 1987) concluded that effects of monensin vary considerably among animals, and high animal numbers are necessary to detect significant differences. The level of monensin used (200 mg) in the present study probably affected results and may have affected variability (Faulkner et al., 19S5); greater performance responses with medium to low quality forages have been seen with monensin levels lower than 200 mg (Oliver, 1975; Faulkner et al., 1985). Ellis et al. ( 1983) noted that monensin depresses consumption of low and high quality forages and increases intake of medium quality forage, but such changes are not always observed (Pond et al., 1980). Organic matter digestibilities of 49-60% in the present study suggest that bermudagrass and orchardgrass should be classed as medium quality (Ellis et al., 1983). Pond et al. ( 1980) recorded a 2 1% decrease in intake of ryegrass by BrahmanxJersey cows by supplementing the daily diet with 200 mg monensin. Brahmanx Jersey heifers, whose diet was supplemented with 100 mg day-’ monensin, consumed less bermudagrass than heifers not receiving monensin when forage quality was lowest (after a frost ), whereas changes earlier in the growing season with higher quality forage were small (Pond and Ellis, 1979). Ellis et al. ( 1983) suggested that increased gut Iill with monensin supplementation of medium quality forages allows feed intake to rise. Depressions in intake of low quality diets by monensin could involve lowered microbial

protein synthesis (Poos et al., 1979) if the intestinal protein supply is low and limits feed intake ( McCollum and Horn, 1990). Changes in digestion and particulate passage rate with monensin in this experiment suggest a greater possible change in microbial protein synthesis with bermudagrass than with orchardgrass, and it is more likely that nutritional conditions such as amino

acid status limited the intake of bermudagrass than of orchardgrass. Faulkner et al. ( 1985 ) suggested that improvements in forage digestion occur more frequently with low than high levels of monensin (e.g. low, 100 mg; high, 200 mg), and adverse effects of monensin rise with declining forage quality (Ellis et al., 1983; Zorrilla-Rios et al., 1985). Ruminal fitngi appear to be more important in the digestion of low than medium or high quality forage and of tropical than temperate grasses (Akin, 1986). Ruminal fungi can be sensitive to monensin (Elliott et al., 1987); hence, adverse effects of monensin on ruminal fungi might impair the digestion of bermudagrass more than that of orchardgrass. Duff et al. ( 1989), however, did not attribute changes in in vitro DM disappearance with monensin additions to effects on ruminal fungi. As well as the variable contributions of fungi to the digestion of different forages, forage characteristics also modulate specific types of fibrolytic bacteria present in the mmen (Akin, 1986), and influences of mo-

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nensin on fiber digestion vary with fibrous substrate (Henderson et al., 1981). Perhaps the types of microbes particularly critical to the rapid and thorough digestion of bermudagrass are highly susceptible to monensin compared with those important to the digestion of orchardgrass. When monensin increases ruminal digestion, lengthened ruminal digesta retention is usually credited. The difference in particulate passage rate between BG-M and BG in this study tended to be greater than between OG and OG-M, suggesting a greater potential with bermudagrass than orchardgrass forcompensation in digestion for the depression by monensin. However, based on total tract NDF digestion, this was not realized, PPR correlated positively with total tract NDF digestion (r-0.52). Similar to the results of Experimeni 2, Brake et al. ( 1990) observed larger particles in the feces of dairy steers fed bermudagrass than of those fed orchardgrass. Pond et al. ( 1980) found that monensin depressed PPR with highforage diets, as in this experiment. Changes in PPR can be functions of feed intake or vice versa. Deswysen et al. ( 1987) depressed the number of ruminal contractions of 290 kg crossbred heifers fed corn silage by supplementing with 100 mg of monensin. The authors stated that this effect of monensin, coupled with possibly weakened ruminal contractions, may slow ruminal digesta outflow. The presumed higher ruminal concentrations of monensin in the present study relative to those found by Deswysen et al. ( 1987) may indicate greater depressions in the number and strength of ruminal contractions in this trial. The lack of effect of monensin on particle characteristics of feces DM in this trial indicates that if monensin affected the prevalence or actions of ruminal fungi, such changes were inadequate to modify particle disintegration during remastication. Alternatively, elicited differences in ruminal fungi action were of little consequence to particle breakdown. The results of Experiment 2 indicate that the performance ofgrowing cattle consuming bermudagrass would not increase with monensin. However, Rouquette et al. ( 1980) supplemented growing beef calves grazing bermudagrass with 200 mg of monensin and improved daily BW gain, Levels of NDF in forage during the grazing period averaged only slightly lower than for bermudagrass hay used in the present study, but the CP content was considerably higher than for hay used here. Oliver ( 1975) elevated gains of beef steers grazing bermudagrass by supplementingwith monensin; gains were higher for 100 than for 2OfJ mg day-’ of monensin. Similar to the findings of Experiment 2, Sun et al. ( I99 I ) found no difference between the daily gain of growing beef calves consuming bermudagrass hay without supplementation and those whose diets were supplemented with a low amount of ground corn given alone or with a mix of protein meals. It was also found in limit-fed beef cows fed similar diets that lasalocid depressed ruminal fiber digestion while causing only a small volatile fatty acid shift and no change in the intestinal protein SUPPlY.

in conclusion, there appears to be more potential and likelihood of increasing the energy and nutrient status of growing cattle by supplementing warm season grass than cool season grass with legume or grain alone, primarily through increased feed intake. Supplementing with grain and legume together may increase the opportunity to raise digestible OM intake with cool season grass above that with either feed alone. Trends for decreased feed intake and fiber digestion with monensin addition to a warm season grass diet indicate that animal performance would not necessarily be enhanced. Responses to different supplements vary with forage characteristics, such as those unique to cool and warm season grasses.

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