Rumen and blood variables in steers fed grass
silage or whole-crop fodder beet silage
A.P. Moloney
*, P. O'Kiely
Teagasc, Grange Research Centre, Dunsany, Co. Meath, Ireland
Received 19 November 1998; received in revised form 12 May 1999; accepted 15 June 1999
Abstract
The objectives of this study were (i) to examine the effects on rumen and blood variables when grass silage (GS) was replaced with whole crop fodder beet silage (FBS) in the diet of beef cattle, and (ii) an abrupt change from GS to FBS. Six rumen-fistulated Friesian steers (initial bodyweight (BW) 416 kg, SD 43) were offered GS ad libitum for four weeks. The dry matter (DM) consumed daily by each animal in that period (14.3 g/kg BW) was used as the daily allowance for that animal for three consecutive two-week periods, during which the animals were offered diets consisting of increasing proportions (420, 880 and 1000 g/kg) of FBS DM in the total DM. FBS was then offered ad libitum for 14 days. Cattle were then offered GS ad libitum for 14 days, after which GS was substituted with FBS for 14 days. The DM content (g/kg) and contents (g/kg DM) of organic matter, crude protein and total volatile fatty acids (VFA) were 220 and 174, 905 and 749, 153 and 115, and 26 and 75 for GS and FBS, respectively. Animals fed the diets of 0, 420, 880 and 1000 g FBS DM/kg DM had rumen pH, concentrations of ammonia (mg/l),l-lactic acid (mmol/l),d-lactic acid (mmol/l) and VFA (mmol/l) of 6.44, 6.18, 6.61 and 6.75 (linearp< 0.001, quadraticp< 0.001), 149, 104, 65 and 50 (linearp< 0.001), 0.76, 1.69, 1.15 and 3.98 (linearp< 0.01, cubicp< 0.01), 1.65, 2.67, 2.83 and 5.93 (linearp< 0.001, cubicp< 0.001) and 82.7, 81.8, 72.8 and 73.7 (linearp< 0.01), respectively. The corresponding plasma concentrations of urea (mmol/l) were 3.74, 2.32, 1.95 and 1.51 (linearp< 0.001), glucose (mmol/l) were 3.81, 3.51, 3.70 and 3.70 (quadraticp< 0.05) and insulin (mIU/ml) were 19.2, 31.0, 17.4 and 20.4 (quadratic;p< 0.05).
Animals offered unsupplemented FBS ad libitum had no obvious symptoms of ill-health and rumen fermentation was qualitatively similar to when offered at a restricted level. When animals were abruptly offered FBS, they had adapted in terms of feed consumption and rumen pH after six days. It is concluded that (i) GS and FBS had different patterns of fermentation in the silo and in the rumen, and (ii) cattle adapted quickly to an abrupt change from GS to FBS.#1999 Elsevier Science B.V. All rights reserved.
Keywords: Cattle; Silage; Fodder beet; Rumen fermentation; Blood metabolites 81 (1999) 221±235
*Corresponding author. Tel.: +353-46-25214; fax: +353-46-26154 E-mail address: amoloney@grange.teagasc.ie (A.P. Moloney)
1. Introduction
Fodder beet (beta vulgaris) when grown under suitable conditions, can produce almost 20 t dry matter (DM)/ha (DAF, 1998) compared with 13±15 t DM/ha from four harvests of grass. Approximately 75% of fodder beet DM is in the root component (DAF, 1998). The roots are a high energy, low crude protein (CP) feedstuff (in vitro digestible organic matter in DM value of 850 g/kg; Givens, 1990). They are usually stored in large clamps, and may be subsequently washed or dry cleaned on a batch basis prior to chopping or slicing and feeding. The leaf portion of beet, together with the upper part of the root, can have a high CP concentration (Givens, 1990). Beet leaves are normally wilted prior to feeding to avoid the reported toxicity, attributed to oxalic acid and/or saponins (Clarke and Clarke, 1975), if they are consumed fresh. Leaves may then be left in the field to be grazed in situ, used as a green manure, harvested and zero-grazed or harvested and ensiled for subsequent feeding.
Both sugar beet pulp (Courtin and Spoelstra, 1989) and tops (leaves + crown; Thomas, 1989) have each been successfully ensiled. Moreover, the fact that 6 kg of ensiled unwilted sugarbeet leaves were consumed by dairy cows without an adverse reaction (Engling and Rohr, 1988) suggests that ensiling also decreases the risk of toxicity from beet leaves. Ensiling the whole-crop of fodder beet provides the opportunity to streamline the harvesting and storage of both the root and leaf components of the crop to produce a feedstuff that can be incorporated into cattle diets without further processing. Furthermore, the relatively high CP concentration in the leaf component of the crop can partially compensate for the relatively low CP concentration in the roots. Thus, high growth rates were recorded in finishing beef cattle offered whole crop fodder beet silage (FBS) (1.12±1.05 kg/day) relative to an ad libitum concentrate ration (1.26 kg/day), and no response was observed to supplementary CP in addition to that supplied by barley (O'Kiely and Moloney, 1999).
Little information is available on the end-products of digestion of FBS or the ruminal consequences of consumption of large amounts of FBS. The objectives of this study were to examine the impact on rumen fermentation in steers of (i) an increase in the ratio of FBS to grass silage in the diet, (ii) offering unsupplemented FBS ad libitum, and (iii) an abrupt change from grass silage to FBS.
2. Materials and methods
2.1. Silage/animals
Whole-crop fodder beet (cv. Magnum) was harvested adjacent to Oak Park, Carlow, Ireland, between October 4 and 11. A prototype harvester was used (Suicre Eireann, Carlow, Ireland) that separated the leaf plus stem component from the roots, vibrated adhering soil from the roots before crushing them and then mixing them back with the leaves plus stems. The crop was transported (2 h) to Grange Research Centre and ensiled without an additive in a horizontal, concrete, walled silo that allowed effluent release.
The ensiled crop was sealed with two layers of black 0.125 mm polythene which were then covered with tyres. The silo was opened and feeding commenced after 315 days ensilage. The grass silage used was a primary growth of perennial ryegrass. It was cut with a rotary mower and harvested within 15±30 min with a precision-chop harvester. Grass was ensiled with 2.3 l sulphuric acid (450 g/kg)/tonne in a horizontal walled silo and sealed as described above. This silo was opened and feeding commenced after 150 days ensilage.
Six Friesian steers (liveweight 416 (SD 43) kg), each fitted with a permanent rumen cannula (internal diameter 32 mm), were used. The animals were individually tethered in concrete-slatted pens with free access to fresh water.
2.2. Experimental procedures
2.2.1. Phase 1
Animals were offered unsupplemented grass silage (Table 1) ad libitum for 28 days and their daily DM consumption was monitored for the final 21 days. In the remainder of this phase, each animal was offered this amount of DM daily. The animals were then offered for three consecutive 14-day periods, a diet consisting of increasing proportions (420, 880 and 1000 g/kg) of FBS DM in the total DM. The remainder of the diet was the grass silage used in the introductory period. The appropriate daily allowance of FBS and grass silage was weighed into each individual feed box and manually mixed. The daily allowance was offered at 0800 h.
Table 1
Chemical compositionaof grass and whole crop fodder beet silage (FBS)
Grass silage FBS
Dry matter (g/kg) 220 (21.6) 174 (6.9)
pH 3.68 (0.075) 3.67 (0.076)
Crude proteinb 153 (4.4) 115 (3.8)
In vitro dry matter digestibility (g/kg) 718 (18.9) 700 (26.6) In vitro organic matter digestibility (g/kg) 702(16.6) 731(22.7)
Ammonia (g/kg N) 73 (17.2) 59 (15.5)
Ashb 95 (8.2) 251 (20.1)
Organic matterb 905 (8.2) 749 (20.1)
Neutral detergent fibreb 480 (15.1) 264 (4.0)
Acid detergent fibreb 290 (7.4) 171 (8.1)
Lactic acidb 141 (17.1) 170 (18.7)
Ethanolb 10 (1.1) 59 (8.9)
Acetic acidb 22 (5.8) 67 (6.0)
Propionic acidb 1.6 (1.51) 4.0 (1.8)
Butyric acidb 2.6 (1.60) 3.6 (0.58)
Total volatile acidsb 26 (8.4) 75 (7.8)
Water soluble carbohydrateb 27 (14.2) 54 (5.2)
On days 27 and 28 of the introductory period and on days 13 and 14 of each subsequent period, rumen fluid samples were withdrawn immediately before, and 1, 2, 4, 8 and 12 h after, offering fresh silage, as previously described (Moloney et al., 1996). The pH was measured immediately following collection and a 20 ml sub-sample acidified with 0.5 ml
9 M sulphuric acid and stored at ÿ208C. A blood sample was obtained by jugular
venipuncture from each animal prior to offering fresh silage on Day 28 of the introductory period and prior to feeding on Day 14 of each subsequent period. Sodium ethylenediamine tetra-acetic acid was used as anti-coagulant. Further blood samples were
collected 2 and 4 h thereafter. Blood samples were centrifuged at 2000g for 20 min
and plasma stored atÿ208C. Animals were weighed at the end of the introductory period and at the end of Phase 1.
2.2.2. Phase 2
Following completion of Phase 1, all animals were offered unsupplemented FBS ad libitum at 0800 h for 30 days. Daily DM consumption was monitored. Rumen fluid samples were collected on days 13 and 14 and blood samples on Day 14 of this phase as described above (in vivo digestibility of FBS was measured between Day 18 and Day 30 of this phase and is reported by O'Kiely and Moloney (1999)). Animals were weighed at the end of this phase.
2.2.3. Phase 3
Following completion of Phase 2, all animals were offered ad libitum the same grass silage as used in Phase 1, at 0800 h, for 14 days. Daily DM consumption was monitored and rumen fluid samples were collected on days 13 and 14 of this phase as described above.
2.2.4. Phase 4
Following completion of Phase 3, FBS was offered ad libitum at 0800 h to four fistulated steers. Rumen fluid samples were collected as described above on days 2, 5, 6, 10 and 15 of FBS feeding. Only the pH of rumen fluid was measured in this phase. Daily DM consumption was monitored.
Throughout the experiment, daily silage samples were collected and stored atÿ208C. Upon thawing, samples were composited and sub-sampled on a weekly basis prior to chemical analysis.
2.3. Chemical analyses
The DM concentration of silage was determined by drying at 408C (48 h) in an oven
with forced air circulation. Other chemical analyses of silage (Table 1) were carried out as described by O'Kiely and Moloney (1995). Neutral detergent and acid detergent fibre concentrations were analysed as described by Van Soest et al. (1991) and Van Soest
(1973), respectively. Rumen fluid samples were centrifuged at 2500gfor 20 min and
supernatants pooled within time across both days within each period for each animal. Samples taken immediately before, and 2 and 8 h after feeding were analysed for ammonia and volatile fatty acid (VFA) concentrations. These samples, together with
samples taken at 1 h after feeding, were also analysed for d- and l-lactic acid concentrations. The concentration of VFA was determined by gas chromatography (Shimadzu gas chromatography GC-8A) as described by Supelco Inc. (1975) and of
ammonia and lactic acid (both d and l-isomers) using a Ciba Corning Diagnostics
530 express clinical chemistry analyzer with appropriate reagent kits. Plasma concentrations of urea and glucose were determined using the above analyser with appropriate reagents kits. Plasma insulin concentration was determined by radio-immunoassay as described by Chikhou et al. (1991) using bovine insulin (Novobiolabs, Bagsvaerd, Denmark) as reference standard, guinea-pig antiporcine insulin antisera as first antibody (1 : 30 000 working dilution; Scottish Antibody Production Unit, Lanarkshire, Scotland) and cellulose coated antibody (SacCell, Wellcome Ireland, Dublin) as second antibody.
2.4. Statistical analyses
Data were subjected to analysis of variance. The experimental design was a randomised block (animals) with repeated measures. For Phase 1 metabolite data, the model used had animals and diet in the main plot and sampling time within a diet and the time by diet interaction in the sub-plot. For ruminal fluid pH data, the mean of both sampling days within FBS level for each sampling time for each animal was used. Feed intake data were analysed using a model that had animal and diet as main effects. The linear, quadratic and cubic effects of inclusion of FBS in the diet were partitioned using orthogonal polynomials. Similar models were used for Phase 2 and Phase 3 data. For Phase 4, ruminal fluid pH data were analysed using a model that had animal and day in the main plot, and sampling time within a day and time by day interaction in the sub-plot. Feed intake was analysed similarly but with the effect of time omitted.
3. Results
Unless otherwise stated, only significant (p< 0.05 or greater) treatment effects and interactions are presented.
3.1. Silage composition
The chemical composition of grass silage and FBS is summarised in Table 1. Both silages were well preserved. Silage made from whole crop fodder beet underwent an extensive lactic acid-dominant fermentation, and was characterised by lower CP and organic matter concentrations, but higher organic matter digestibility than grass silage.
3.2. Phase 1
Plasma and rumen fluid variables in steers offered an increasing proportion of FBS in the diet are summarised in Table 2. There was a linear decrease in DM intake (5.9, 5.7, 5.6 and 5.6 kg/day), organic matter intake (5.3, 4.8, 4.3 and 4.2 kg/day) and CP intake (0.9, 0.8, 0.7 and 0.6 kg/day), with an increase in the level of inclusion of FBS in the diet. Plasma urea concentration was linearly decreased as the level of consumption of FBS increased and the pattern of response of plasma insulin and glucose concentrations was quadratic with the point of inflection at 420 g FBS/kg DM.
An increase in FBS inclusion in the diet increased rumen fluid pH (linear and quadratic) and concentrations of both isomers of lactic acid (linear and cubic), the molar proportion of acetate (quadratic) and the acetate to propionate ratio (linear and quadratic) in rumen fluid. An increase in FBS inclusion in the diet decreased the concentration of ammonia (linear) and VFA (linear) and the molar proportion of propionate (linear and quadratic) in rumen fluid. There were minor effects on the molar proportions of valerate
and caproate in rumen fluid. There were sampling timediet interactions for rumen
fluid pH (Fig. 1(a)), the molar proportions of acetate (Fig. 1(b)) and butyrate (Fig. 1(c)), the concentration of ammonia in rumen fluid (Fig. 2(a)) and urea in plasma (Fig. 2(b)) and both isomers of lactic acid (Fig. 3).
Table 2
Mean rumen fluid and plasma variables in steers fed differing ratios of grass silage to whole-crop fodder beet silage (FBS) in Phase 1
FBS (g/kg DM)
0 420 880 1000 SED significancea
Rumen fluid
pH 6.44 6.18 6.61 6.75 0.07 L***,Q***
l-lactic acid (mmol/l) 0.76 1.69 1.15 3.98 0.663 L**,C** d-lactic acid (mmol/l) 1.65 2.67 2.83 5.93 0.524 L***,C***
Ammonia (mmol/l) 8.8 6.1 3.8 2.9 0.56 L***
Volatile fatty acids (mmol/l) 82.7 81.8 72.8 73.7 3.6 L**
Acetateb 628 652 662 638 12.0 Q*
Propionateb 238 196 194 193 8.0 L***,Q**
Butyrateb 105 115 104 113 8.0 NS
Valerateb 30 30 35 27 3.0 C*
Caproateb 1 8 12 29 3.0 L***,Q*,C***
Acetate : propionate 2.70 3.45 3.55 3.51 0.19 L***,Q*
Plasma
Glucose (mmol/l) 3.81 3.51 3.70 3.78 0.134 Q*
Urea (mmol/l) 3.73 2.32 1.95 1.51 0.320 L***
Insulin (mIU/ml) 19.2 31.0 17.4 20.4 4.40 Q*
aL, Q and C, respectively, are linear, quadratic and cubic effects of proportion of FBS in the diet DM. bIn mmol/mol.
3.3. Phase 2
Feed consumption, plasma and rumen fluid variables in steers offered grass silage or FBS ad libitum are summarised in Table 3. Dry matter intake was lower and CP intake was higher in animals offered grass silage. On average, animals consuming FBS had lower ammonia and VFA concentrations in rumen fluid, higher pH, lactic acid
concentration and acetate (p= 0.1) proportion of VFA but lower propionate proportion
of VFA. This resulted in a higher acetate-to-propionate ratio in the rumen of animals consuming FBS rather than the grass silage. There were no interactions between silage type and sampling time for these variables.
On average, animals consuming FBS ad libitum had lower plasma urea concentration than those consuming grass silage. There was a significant interaction between silage type and time of sampling for plasma urea concentration (mean 3.31, 3.78 and
4.08 mmol/l for samples collected before, and at 2 and 4 h after, feeding for grass silage. Corresponding values for FBS were 1.23, 1.28 and 1.38, respectively (SED 0.34)). There were no signs of ill-health in cattle consuming unsupplemented FBS ad libitum for 30 days.
3.4. Phase 3
When animals were re-offered grass silage ad libitum, 10 weeks after the
introductory period, DM intake (on a bodyweight basis) was similar to that observed earlier (Table 4). Other than rumen ammonia concentration and pH, the mean values and the pattern of rumen fermentation variables were also similar to those observed earlier. There were no period by sampling time interactions for any variable examined.
Fig. 2. Ammonia concentration in rumen fluid (a, sampling timediet SED 1.25) and urea concentration in plasma (b, sampling timediet SED 0.33) (^), 0 g FBS; (~), 420 g FBS; (*), 880 g FBS; and (&),1000 g FBS/kg DM.
3.5. Phase 4
Mean DM and organic matter intakes were 6.36 and 5.76 kg, respectively, for animals offered grass silage ad libitum on the final day of Phase 3. Mean DM intakes were 6.80, 7.24, 7.67, 7.74 and 8.04 (SED 0.893;p> 0.05) on days 2, 3, 6, 10 and 15 after the grass silage was substituted totally with FBS. The corresponding organic matter intakes were
5.10, 5.43, 5.74, 5.80 and 6.02 (SED 0.677;p> 0.05) kg/animal. Mean rumen fluid pH
was 6.56, 6.22, 6.81, 6.65, 6.75 and 6.66 (SED 0.134; (p< 0.05) for animals offered grass silage ad libitum, and on day 2, 3, 6, 10 and 15 after the grass silage was substituted with FBS. Daily pH profiles are shown in Fig. 4. There was a day-of-sampling by sampling time interaction such that rumen fluid pH tended to be lowest at all times on the second day after the introduction to FBS ad libitum.
Table 3
Feed intake and mean rumen fluid and plasma variables in steers fed either grass silage or whole crop fodder beet silage (FBS) ad libitum in Phase 2
Grass silage FBS SED Significance
Feed intake(g/kg bodyweight)
Dry matter 14.3 15.9 0.63 *
Organic matter 13.0 11.9 0.48 +a
Crude protein 2.2 1.9 0.08 *
Rumen fluid
pH 6.44 6.97 0.06 ***
l-Lactic acid (mmol/l) 0.71 1.43 0.19 *
d-Lactic acid (mmol/l) 1.60 2.75 0.29 *
Ammonia (mmol/l) 8.8 2.4 0.66 ***
Volatile fatty acids (mmol/l) 82.7 68.3 5.8 +a
Acetateb 628 647 10.0 +a
Dry matter intake and rumen fluid variables in steers fed grass silage ad libitum at the beginning of the study and between Week 11 and Week 13 thereafter, in Phase 3
Week SED Significance
ÿ3 to 0 11±13
Dry matter intake (g/kg bodyweight) 14.3 13.4 0.59 NS
Rumen fluid
pH 6.44 6.67 0.066 *
l-Lactic acid (mmol/l) 0.71 0.84 0.361 NS
d-Lactic acid (mmol/l) 1.60 1.33 0.315 NS
Ammonia (mmol/ml) 8.8 6.4 0.56 **
Volatile fatty acids (mmol/l) 82.7 85.7 2.51 NS
Acetatea 628 622 3.8 NS
Propionatea 238 236 3.1 NS
Butyratea 105 104 4.5 NS
Valeratea 30 31 1.7 NS
Acetate : propionate 2.70 2.71 0.045 NS
aIn mmol/mol.
4. Discussion
4.1. Methodology
The primary objective of this study was to define rumen fermentation in steers fed whole crop FBS. There was no a priori information on the effect of feeding FBS per se on rumen physiology. However, O'Kiely and Moloney (1999) reported that, in the preparatory phase of their study, cattle abruptly offered FBS ad libitum vomited the silage. Because of this and because of the time likely to be required for pre-measurement adaptation to high levels of FBS, a sequential design was employed rather than a Latin square/changeover design. The effect of FBS on permanent rumen function was tested by offering the original grass silage ad libitum upon completion of the ad libitum FBS phase. The general similarity in feed intake and rumen fermentation before, and after, FBS feeding indicates that the animals were not deleteriously affected by FBS. Moreover, it gives confidence that the procedures for sample collection and analysis used throughout the study were consistent. The difference in rumen ammonia concentration before, and after, FBS feeding most likely reflected the small variability in CP concentration in the grass silage (155 g/kg DM during the early phase and 145 g/kg DM during the later phases) and the non-significant difference in DM intake than any physiological consequence of FBS consumption in the interval between these phases.
4.2. Influence of FBS inclusion in the diet of steers
The grass silage used was of good quality with respect to in vitro digestibility and preservation as indicated by the pH and concentration of ammonia (Haigh and Parker, 1985). In general, when compared to other studies where animals were offered unwilted, unsupplemented grass silage, pH and concentrations of metabolites in rumen fluid were within the range previously reported (Moloney and O'Kiely, 1994; McKee et al., 1996; Thiago et al., 1992). In Phase 1, to ensure complete consumption by all animals and to reduce variation, the initial intention was to offer animals an allowance of DM on an
individual animal basis equal to that consumed when grass silage was offered ad libitum. The linear decrease in DM intake was due to the lower DM concentration of FBS as fed compared to that measured at the outset of the study. Nevertheless, the difference was only 5%. Despite the higher DM intake for FBS compared to grass silage when both were offered ad libitum, organic matter intake was lower. This reflected the high ash concentration of FBS. Of the total ash concentration, there were 138 g acid insoluble ash/ kg DM (O'Kiely and Moloney, 1999), indicative of considerable soil contamination during harvesting. Decreases in organic matter and CP consumption, when FBS was offered as a proportion of the diet or ad libitum, were reflected in decreases in VFA and ammonia concentrations in ruminal fluid (particularly 8 h after feeding), respectively. The decrease in CP intake was also reflected in a decrease in plasma urea concentration which was more pronounced 4 h after feeding. The mean rumen ammonia concentration in animals offered the grass silage, 420 g FBS or 880 g FBS/kg DM diets exceeded the minimum level of 2.9 mmol/l recommended to maintain maximum microbial protein synthesis (Satter and Slyter, 1974), and was close to this value in the rumen of animals receiving the 1000 g FBS/kg DM diet. Reports on the ammonia concentration necessary for optimum ruminal cellulose digestion are, however, inconsistent. Hoover (1986)
concluded that when the CP concentration of the diet was 60 g/kg, the ruminal
ammonia concentration required for optimum microbial growth and digestion was 12.6 mmol/l. When the CP concentration of the diet was >60 g/kg, an ammonia concentration of 4.7 mmol/l was necessary. It is clear, however, that for several hours during the day, rumen ammonia concentration may have been below that required for optimum cellulose digestion and microbial protein synthesis in the 880 g, 1000 g FBS/kg DM diets and when FBS was offered ad libitum. The lack of any symptoms of illness in animals offered FBS ad libitum indicates that no additional roughage was required.
The pattern of pH decline post-feeding in animals fed grass silage is similar to that observed previously (Moloney and O'Kiely, 1999). In contrast, the pH pattern in animals offered FBS, on a restricted basis, had a more rapid initial pH fall. This may reflect the higher concentrations of water soluble carbohydrate (WSC) and lactic acid in FBS than grass silage. It may also be influenced by the pattern of intake, i.e. animals offered FBS may have had a greater first meal when offered the fresh diet. That the minimum pH is lower for animals fed 420 g FBS/kg DM than 880 g or 1000 g FBS/kg DM most likely reflects total digestibile organic matter intake. Since cellulolytic activity is impaired when pH falls below pH 6.2±6.3 (Stewart, 1977), it is possible that fibre digestion was impaired for at least a portion of the day in animals fed these diets. The similarity in the pattern of pH between animals offered FBS and grass silage ad libitum may again reflect the meal pattern of the animals, and was never <6.2.
As with pH, the pattern of molar proportions of VFA in rumen fluid tended to be similar for all levels of FBS offered on a restricted basis and to differ from grass silage offered ad libitum. The alterations in VFA proportions, i.e. an increase in acetate and a decrease in propionate without an effect on butyrate, differ from the alterations observed when molasses (primarily sucrose) was substituted for grass silage at a fixed DM allowance (an increase in butyrate, and a decrease in acetate proportions (Moloney, 1997)). Daily intake of WSC averaged 158, 220, 284, 301 g/animal offered 0, 420, 880
and 1000 g FBS/kg DM. Since the WSC component of fodder beet roots is primarily sucrose (Jarrige and Fauconneau, 1973), changes in VFA similar to that reported by Moloney (1997) would be expected. Indeed, such a response was reported by Izumi (1976) when increasing amounts of fodder beet roots were offered to hay-fed cattle and by Verite and Journet (1973) in cows offered increasing amounts of fodder beet meal and a grass silage-based diet. In the present study, therefore, fermentation of non-WSC substrates such as the cell wall fraction or lactic acid and ethanol may have contributed to different end-products between the two silages. Thus, FBS was characterised by a fivefold higher concentration of ethanol than grass silage. Durix et al. (1991) demonstrated in vitro that inclusion of ethanol increased the acetate and caproate and decreased the propionate proportions in rumen fluid. These alterations are consistent with the observations of the present study.
The increase in l-lactic acid concentration in rumen fluid with increasing FBS
inclusion in the diet is consistent with that observed when steers were offered increasing amounts of molasses (Moloney, unpublished). The pattern of lactic acid concentration reflects the rapid fermentation of components of the feed (sucrose is believed to be metabolised to lactate (Khalili and Huhtanen, 1991)) and the contribution of lactic acid from the silage. Kulasek et al. (1976) also reported a large increase in ruminal lactic acid concentration when bulls were offered sugar beet silage compared to corn silage-based rations.
Based on the alterations in VFA proportions, alterations in plasma glucose and insulin concentrations were unexpected. The decrease in mean glucose concentration in animals offered 420 g FBS/kg DM most likely reflects the increased insulin secretion. Both propionate (Bines and Hart, 1984) and butyrate (Itoh et al., 1998) can stimulate insulin secretion in different circumstances, but the concentrations of neither were increased relative to the grass silage diet.
4.3. Effects of an abrupt change from grass silage to FBS
In a preliminary study, O'Kiely and Moloney (1999) observed that cattle offered FBS ad libitum together with 1 kg supplementary concentrates vomited considerable amounts of feedstuff. The fact that additional grass silage rectified this behaviour suggested that its etiology had a ruminal component. Accordingly, the effect of an abrupt dietary change from grass silage to FBS ad libitum on rumen fluid pH was examined. Both the intake and rumen data indicate that animals adapted fully to FBS after six days consumption of unsupplemented FBS. The observation of O'Kiely and Moloney (1999) thus reflects either an interaction with the supplementary concentrates used and/or some undetected differences between the animals used in both the studies.
It is concluded that:
(i) the FBS used in this experiment resulted in a different pattern and end-products of fermentation in the rumen compared to animals offered the grass silage used; (ii) the FBS could be offered ad libitum, unsupplemented without apparent deleterious effects on rumen or whole animal health; and
Acknowledgements
The technical assistance of V. McHugh, A. McArthur and J. Hamill and the secretarial assistance of M. Smith are acknowledged. The assistance of the staff of Grange Laboratories with chemical analyses and Brendan Lynch with the care of the animals is also acknowledged. The authors thank Greencore plc, Carlow, Ireland, for the whole crop fodder beet used and P. Rogers for insertion of ruminal cannulae.
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