Rumen digesta kinetics in dairy cows fed grass, maize and alfalfa silage.

Animal Feed Science and Technology 73 (1998) 37±58

Rumen digesta kinetics in dairy cows fed grass, maize and alfalfa silage. 1. Comparison of conventional, steady-state and dynamic methods to estimate microbial degradation, comminution and passage of particles. Marianne Bruining*, Roel Bakker, Jaap van Bruchem, Seerp Tamminga Wageningen Institute of Animal Sciences (WIAS), Animal Nutrition Group, Wageningen Agricultural University, Marijkeweg 40, 6709 PG Wageningen, The Netherlands Accepted 30 December 1997

Abstract The rate constants of dry matter in rumen digesta in lactating rumen-fistulated Holstein±Frisian cows were determined using a four pool model with linear kinetics. The rate constants were derived from either arbitrary named `conventional' methods (i.e. in sacco incubation, marker passage), or average pool sizes (steady-state method), or changes in pool sizes with time (dynamic method). According to a 33 Latin square design, the cows were fed grass (GS), maize (MS) and alfalfa (AS) silages ad libitum twice daily for a limited period of time, supplemented with mixed concentrates (7 kg dÿ1). The ingested feed and rumen contents (evacuation) were subdivided by wet sieving into large (LP>1.25 mm) and small (0.04 mm< SP<1.25 mm) particles, and secondly, by 2-week rumen incubation into potentially degradable and truly undegradable fractions. Dry matter intake and total particulate rumen contents did not differ significantly between rations. Degradation rates of large (kdLP) and small (kdSP) rumen particles estimated by in sacco rumen incubation were significantly lower than those derived with the steady-state and dynamic methods and are likely to have been a result of inadequate recognition of a fast degradable fraction. Passage rates of SP (kpSP) from the rumen estimated by CrNDF disappearance were similar to those derived from the steady-state and dynamic methods. The steady-state method predicted non-soluble dry matter intake was most sensitive to changes in kpSP and the rate of comminution of LP (kc). The non-soluble rumen dry

* Corresponding author. Tel.: 0031 317 483442; fax: 0031 317 484260; e-mail: [email protected] 0377-8401/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved PII S 0 3 7 7 - 8 4 0 1 ( 9 8 ) 0 0 1 3 3 - 3

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M. Bruining et al. / Animal Feed Science and Technology 73 (1998) 37±58

matter degradation was most sensitive to changes in kc and kpSP for rations AS, to changes in kdSP for ration MS and to changes in kdLP, kdSP and kc for ration GS. # 1998 Elsevier Science B.V. Keywords: Rumen model; Digesta kinetics; Silage; Elasticity

1. Introduction The ad libitum intake of roughage in dairy cattle is insufficient to meet the requirements in early lactation. It is still not completely clear to what extent the intake of roughage is limited by rumen processes such as fermentative degradation, comminution and passage. These processes can be investigated in vivo as described by Bosch et al. (1992a, b). This type of research yields arbitrarily named `conventional' (i.e. in sacco, marker) estimates for the rate constants of the rumen processes. However, these conventional methods may have limitations in estimating digesta kinetic parameters. For instance, the particle size of the marker Cr-NDF can have a great influence on the calculated rates of passage (Bruining and Bosch, 1992) and estimating passage rates with indigestible external markers may not be representative for digestible material (Tamminga et al., 1989). Degradation rates estimated by in sacco rumen incubation are influenced by many factors, such as bag pore size, sample size and sample particle size. Besides, the in sacco incubated feed is not subjected to mastication, rumination and passage (Nocek, 1988). To evaluate these conventional rate constants they need to be investigated in an integrated way using a rumen model. Estimation of the model parameters can be done by using either a steady-state or a dynamic method. The first method is time-independent and assumes that the rumen pools are in steady-state, i.e. that the inflow into a pool equals the outflow. The second method is time-dependent and can account for the temporal variation in pool size due to feeding. Rumen models, incorporating different particle sizes, with steady-state method-derived estimates were described by Poppi et al. (1981) and Oosting et al. (1993). Models for ruminants incorporating different particle sizes, with dynamic method-derived estimates were described by Mertens and Ely (1979) and Baldwin et al. (1987). Aitchison et al. (1986) and Oosting et al. (1993) compared rumen model rate constants derived from a dynamic method and from a steady-state method, respectively, with those obtained by conventional methods. In previous experiments (Bosch et al., 1992a, b; Bosch and Bruining, 1995) several grass silages were investigated to determine possible forage differences on rumen kinetics. However, in sustainable animal production systems an important role has been suggested for maize silage and alfalfa silage. By partial replacement of grass by a low protein feed, like maize silage, urinary N excretion can be lowered (Van Vuuren et al., 1993) while alfalfa as a leguminous plant is able to fix elementary N2 from the air. To determine the role of rumen processes on feed intake of different roughages, in the present study kinetic parameters were estimated for grass silage, maize silage and alfalfa silage. Three methods of estimating digesta kinetics were used: the conventional, the steady-state and the dynamic method. The objective of the study was to compare kinetic parameters of three forages as derived by three methods. A sensitivity analysis was done to determine the importance of the rumen parameters.

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2. Material and methods 2.1. Animals and diet Three lactating 3-year old HF cows were used. The cows were equipped with a large cannula (internal diameter 10 cm, Bar Diamond, Idaho) in the dorsal rumen sac. At the beginning of the experiment they were lactating for 2 months. According to a 33 Latin square design, the cows were fed grass silage (GS), maize silage (MS) and alfalfa silage (AS). GS and AS were harvested in July (2nd cut) and MS in September. From 7.00 till 10.00 h and from 19.00 till 6.45 h 60% and 50%, respectively, of the ad libitum silage intake was supplied. Ten minutes prior to silage feeding 3.5 kg of mixed concentrates were offered, with maize silage including 0.5 kg as soybean meal (solvent extracted). Feed residues were collected and weighed at 6.45 and 10.00 h and samples were proportionally pooled per week for dry matter (DM) and ash determination. The silages and concentrates were analysed for DM, ash, N and Neutral detergent fibre (NDF) per experimental period. Mean values are given in Table 1. NH3±N, as a percentage of total N, in GS and AS was 6 and 31%, respectively. In the third experimental period feed intake was recorded continuously by means of a computer-controlled weighing scale. Drinking water was freely available from an automatic drinking device. The animals were milked twice daily and the milk yield per day was recorded. During the experimental weeks milk samples were taken at four successive milking times. The cows were weighed before and after each experimental period. 2.2. Experimental procedures Each of the three experimental periods had a duration of six weeks. The first three weeks were a preliminary period to allow cows to adapt to the experimental diet and to determine the ad libitum silage intake in the third week. Table 2 summarizes the scheme of measurements done during the experimental periods. In the fourth week of the experimental periods samples were collected to determine liquid passage rates, faecal particulate passage rates, overall digestibility, fermentation characteristics and rumination activity. These results will be reported in a subsequent paper. Faeces were quantitatively collected and sampled on Tuesday, Wednesday and Thursday at 8.00, 12.00, 16.00, 20.00 and 24.00 h. Faecal samples were proportionally pooled over 24 h and analysed for DM, OM, N and NDF. In a fresh part of these samples wet sieve analysis was done after storage at ÿ208C. Table 1 Chemical composition (g/kg) of grass silage (GS), maize silage (MS), alfalfa silage (AS), concentrates (C) and soybean meal (solvent extracted, SBM). Between brackets standard deviation GS DM Ash1 N1 NDF1 1

In DM

500 97 29 548

MS (10.4) (0.3) (0.1) (0.4)

257 68 15 437

AS (0.1) (1.4) (0.1) (1.1)

253 117 27 512

C (2.3) (0.4) (0.3) (1.9)

880 93 29 312

SBM (1.0) (0.3) (0.1) (2.3)

860 113 77 113

(0.7) (0.2) (0.0) (0.4)

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Table 2 Scheme of measurements done during the three experimental periods of each six weeks and measurements done beside these experimental periods Period (I, II or III)

Experimental cows number 1, 2 and 3 (fed grass, maize and alfalfa silages plus mixed concentrates)

Week 1, 2, 3 Week 3 Week 4

Adaptation Ad libitum silage intake Faecal particulate (Cr-NDF) and liquid passage rates (Co-EDTA) Overall digestibility Rumen fermentation characteristics (pH, VFA, NH3) Rumination activity Microbial degradation rates by nylon bag incubation in the rumen (silages, concentrates) Rumen evacuations Ruminal particulate passage rates Chewed silages

Week 5 Week 6

Extra cows number 4, 5 and 6 (fed grass silage plus mixed concentrates) Truly undegradable fractions by 2-weeks ruminal nylon bag incubations (LP and SP fractions of rumen contents and chewed silages) Microbial degradation rates by nylon bag incubation in the rumen (LP and SP fractions and non-sieved rumen contents, silages)

In the fifth week of the experimental periods, nylon bag (918 cm, pore size 41 mm, monofil-p41, Nybolt, Switzerland) incubations in the rumen were done. Frozen silage samples (cut 2 min in a 45 l Rowher cutter) were incubated in nylon bags in the rumen of cows fed the corresponding silage. Concentrates ground to pass a 3 mm sieve were also incubated in the rumen of all three cows. In the maize silage-fed cow a mixture of concentrates and soybean meal (solvent extracted) was incubated in the same ratio as fed. Fresh samples (approximately 5 g dry matter) were weighed into the bags, which were stored frozen (silages) or at 38C (concentrates) until 24 h prior to rumen incubation. The silages were incubated in the rumen for 3, 6, 12, 24, 48, 72 and 336 h. Concentrates had the same incubation times, except for 72 h. Bags were put loosely at a maximum of 25 bags in a fish net (ca. 3575 cm), which was made heavier with a steel weight of about 600 g. After incubation the bags were put into ice water and then washed in a washing machine for 30 min with cold water. After drying at 658C, bags were weighed and the samples were ground through a 1 mm screen and stored pending analysis for DM, OM, N and NDF. Only 336 h incubated concentrates were analysed in samples pooled per silage over periods. The results for OM, N and NDF will be reported in a subsequent paper. In the sixth week of the experimental periods, on Monday and Thursday at 6.30 h pulse doses of Cr-NDF (300 g, 5% Cr, ground through a 1 mm screen, from wheat straw prepared according to UdeÂn et al., 1980) were given through the rumen cannula. Rumen fill was measured once a day during six successive days (Monday±Saturday) at 10.15, 14.15 and 18.15 h by manual emptying. Times of rumen evacuation were randomised over the cows according to a double 33 Latin Square (Table 3). From each rumen evacuation a 1% sample was dried at 658C and ground to pass a 1 mm screen for DM,

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Table 3 Scheme of rumen evacuations for cows 1, 2 and 3 Time (h)

Monday

Tuesday

Wednesday

Thursday

Friday

Saturday

10.15 14.15 18.15

1 2 3

3 1 2

2 3 1

2 3 1

3 1 2

1 2 3

OM, N, NDF and Cr analysis. A 2% sample was stored at ÿ208C for wet sieve analysis. After sampling the remaining reticulorumen contents, which were kept warm in two insulated containers, were returned into the reticulorumen. After each rumen evacuation, when the reticulorumen was empty, animals were allowed to eat and the chewed feed samples were collected at the cardia and stored at ÿ208C until wet sieve analysis on samples pooled over the week. The chewed feed samples and the rumen samples pooled over evacuation times were fractionated by wet sieving into large (LP >1.25 mm sieve aperture) and small particles (0.04 mm < SP <1.25 mm sieve aperture). Three cows were additionally used to determine in these LP and SP fractions the potentially degradable (LPD, SPD) and truly undegradable (LPU, SPU) fractions by incubation for 336 h. Per rumen evacuation time and particle fraction, one bag (approximately 5 g dry matter) was incubated in each of these cows. These cows were thus additional to the experimental group and were fed wilted grass silage ad libitum and 3.5 kg mixed concentrates two times per day (GS ration cows). The residues were analysed for DM and ash. On the last Friday of the third week rumen evacuations were done for each cow (between 10.15 and 11.15 h) and 10% samples were stored at ÿ208C for wet sieve analysis and nylon bag incubations. From these rumen samples LP and SP fractions and non-sieved rumen contents were rumen-incubated for 3, 6, 12, 24, 48, 72 and 336 h in the additional three GS ration cows. For comparison with microbial degradation in the experimental cows also the cut silages were rumen-incubated for the same times in the additional three GS ration cows. One bag (approximately 5 g dry matter) per incubation time and rumen contents particle fraction was incubated in each cow and subsequently analysed for DM and ash. 2.3. Sieve analysis Rumen, chewed and faecal samples were divided into LP and SP by wet sieving (Fritsch Analysette 3). The lid was equipped with a shower while an intermediate water supply ring was placed above the lowest sieve. Six sieves were used of 2500, 1250, 630, 315, 160 and 71 mm pore size and at the bottom a water outlet tube was fixed to a dried and weighed nylon bag with 41 mm pore size. With faecal samples the 2500 mm pore size sieve was excluded. Conforming the soluble fraction with nylon bag degradation, it was assumed that material smaller than 41 mm pore size was soluble. Fresh samples of 50 g (faeces 100 g) were dispersed in water by stirring and then spread quantitatively on the top sieve. The tap water was turned on and the apparatus switched on with continuous maximum vibration. After 5 min the water outlet tube was lifted till above the top sieve

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Fig. 1. Model of the intake and disappearance from the rumen of truly undegradable (U) and potentially degradable (D) large (LP>1.25 mm) and small (0.04 mm< SP <1.25 mm) particles. (i, intake; r, rumen; kc, fractional rate of comminution; kp, fractional rate of passage; kd, fractional rate of degradation).

and after the water level had risen to 2 cm above the top sieve, tap water supply was turned off and the outlet tube lowered allowing the water to flow out. This procedure was repeated three times. Finally, the sieves were rinsed with tap water for 5 min. The fractions retained on the sieves were quantitatively collected in dried and pre-weighed glass filter crucibles. The crucibles and the nylon bag were dried overnight for DM determination. Pooling of the particle size fractions retained on the sieves, resulted in LP (2500 and 1250 mm pore size) and SP (630, 315, 160, 71 and 41 mm pore size). 2.4. Chemical analyses DM was determined by drying at 1038C and ash in an oven at 5508C. N was determined according to the Kjeldahl method and NDF according to Goering and van Soest (1970). Cr was determined in wet destructed rumen samples by atomic absorption spectrophotometry (Varian, SpectrAA-300) at a wavelength of 357.9 nm. 2.5. Model and calculations Rate constants were calculated according to a four pool model of the rumen (Fig. 1). The four pools represent fractions in the rumen with large truly undegradable, large potentially degradable, small truly undegradable and small potentially degradable particles (LPU, LPD, SPU and SPD, respectively). Changes in rumen pools were described by the differential equations: dLPUr ˆ LPUi ÿ …kc ‡ kpLP †  LPUr dt dSPUr ˆ SPUi ‡ kc  LPUr ÿ kpSP  SPUr dt dLPDr ˆ LPDi ÿ …kc ‡ kpLP ‡ kdLP †  LPDr dt dSPDr ˆ SPDi ‡ kc  LPDr ÿ …kpSP ‡ kdSP †  SPDr dt

(1) (2) (3) (4)

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where LPUr, SPUr, LPDr and SPDr are the truly undegradable and potentially degradable large and small particle rumen pools; LPUi, SPUi, LPDi and SPDi represent the mean intake of truly undegradable and potentially degradable, large and small particles; kc, kpLP, kpSP, kdLP and kdSP are the rate constants of: comminution, passage of large and small particles and microbial degradation of large and small particles, respectively; t is time. It was assumed that kc and kp are similar for indigestible and digestible fractions. Though there is some evidence that passage rates differ between both fractions, results are not consistent. Tamminga et al. (1989) found with diets of grass hay and concentrates a higher passage rate for rumen indigestible NDF (INDF) than for potentially rumen digestible NDF (DNDF). However, Stensig and Robinson (1997) found for alfalfa silage a higher passage rate for DNDF than for INDF and for timothy silage mixture similar passage rates for DNDF and INDF. The potentially degradable and truly undegradable fractions are not physically separate entities in the rumen, but are combined in particles. So, it seems reasonable to assume that these fractions reduce in size and pass to the omasum at a comparable rate. Three methods were used to determine the rate constants of the rumen model. (1) The conventional method, based on time series from nylon bag rumen incubation studies and marker and undegradable large particle disappearance from the rumen. (2) The steady-state method, based on measured average rumen pool sizes and the assumption that the pools are in steady-state. 3. The dynamic method, also based on measured rumen pool sizes, but taking into account the temporal variation in rumen pool sizes due to feeding. According to the conventional method (c), rumen rate constants were derived as follows. The rates of microbial degradation of large and small rumen particles (kdLP and kdSP, respectively) were based on time series of in sacco rumen-incubated large and small rumen particles. Degradation curves were determined using the residues of 6 h up to and including 336 h incubation to avoid rates of degradation to be confounded by a lag time. The degradation curves were fitted according to the equation: ft ˆ fu ‡ fd  eÿkd :t

(5)

where ft is residue at time t; fu is the truly undegradable fraction; fd is the potentially degradable but water insoluble fraction; kd is the rate of degradation of fd and t is incubation time. The passage rate of small particles (kpSP) was derived from the exponential decline in rumen Cr pool size, corrected for the amount of Cr removed in rumen samples. The passage rate of large particles (kpLP) was derived from the fraction of LP in the nonsoluble (>0.04 mm) faeces, the passage rate of SP and the rumen pools according to the equation: kpLP ˆ

kpSP  fLP  …SPUr ‡ SPDr† …1 ÿ fLP †  …LPUr ‡ LPDr†

(6)

where fLP is the fraction LP in the non-soluble faeces; kpSP is the passage rate of small particles; SPUr, SPDr, LPUr and LPDr are the truly undegradable and potentially degradable small and large particle rumen pools.

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The rate of comminution (kc) was estimated from the exponential decline in pool size of LPU by subtraction of kpLP. The rates of degradation of the ingested feeds (silages ‡ concentrates; kd(feed)) were determined in order to compare them with the rates of degradation of rumen particles. Based on time series (6±336 h) of separately in sacco-incubated silage and concentrates the rates of degradation of silages and concentrates were calculated (Eq. (5)). The kd(feed) value was calculated as a weighted average based on the ingested potentially degradable silage and concentrates fractions multiplied by their respective kd values. Degradation curves of the in sacco rumen-incubated large and small rumen particles were also fitted using the sum of two exponentials according to the equation: ft ˆ fu ‡ fds  eÿkds :t ‡ fdr  eÿkdr :t

(7)

where ft is residue at time t; fu is the truly undegradable fraction; fds and fdr are the potentially slowly and rapidly degradable though water insoluble fractions, respectively; kds and kdr are the slow and rapid rates of degradation, respectively and t is incubation time. These DM degradation curves for LP and SP ruminal samples were determined using at the same time the residues of 6 h up to and including 72 h incubation of the three cows fed the same ration. The mean residue of these cows at 336 h incubation was used as fixed fu value. The steady state method (ssm), is based on the assumption that daily feed intake and rumen pools are constant and equal to their average values. In steady-state the derivatives of the rumen pools with respect to time (Eqs. (1)±(4)) can be substituted by zero, so that the following equations for the rate constants can be derived: LPUi ÿ kpLP LPUr LPDi kdLP ˆ ÿ kc ÿ kpLP LPDr SPUi ‡ LPUr  kc kpSP ˆ SPUr SPDi ‡ LPDr  kc kdSP ˆ ÿ kpSP SPDr kc ˆ

(8) (9) (10) (11)

where LPUi, SPUi, LPDi and SPDi represent the mean intake of truly undegradable and potentially degradable, large and small particles; LPUr, SPUr, LPDr and SPDr are the truly undegradable and potentially degradable large and small particle rumen pools; kc, kpLP, kpSP, kdLP and kdSP are the rate constants of: comminution, passage of large and small particles and microbial degradation of large and small particles, respectively. The passage rate of large particles (kpLP) was calculated from Eq. (6). The dynamic method (dm) takes the time behaviour of the rumen pools due to discontinuous feed intake into account. Solving the differential equations Eqs. (1)±(4) yields the equations in Table 4, which describe the time behaviour of the rumen pools. These equations were used to determine by nonlinear regression the values of kdLP, kdSP,

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Table 4 Equations used in the dynamic model   LPUit LPUit  eÿk0 :t LPUrt ˆ ‡ LPUr0 ÿ k0 k0   SPUit ‡ kc =k0  LPUit kc LPUit  eÿk0 :t SPUrt ˆ ‡ LPUr0 ÿ kpSP kpSP ÿ k0 k0    SPUit kc LPUit  eÿkpSP :t ‡ SPUr0 ÿ ÿ LPUr0 ÿ kpSP kpSP ÿ k0 kpSP   LPDit LPDit  eÿk1 :t LPDrt ˆ ‡ LPDr0 ÿ k1 k1   SPDit ‡ kc =k1  LPDit kc LPDit  eÿk1 :t SPDrt ˆ ‡ LPDr0 ÿ k2 k 2 ÿ k1 k1    SPDit kc LPDit  eÿk2 :t ‡ SPDr0 ÿ ÿ LPDr0 ÿ k2 k 2 ÿ k1 k2 where k0 ˆ kc ‡ kpLP; k1 ˆ kc ‡ kpLP ‡ kdLP; k2 ˆ kdSP ‡ kpSP; kc, kpLP, kpSP, kdLP and kdSP are the fractional rate constants of: comminution, passage of large and small particles and microbial degradation of large and small particles, respectively; LPUrt, SPUrt, LPDrt and SPDrt are the truly undegradable and potentially degradable large and small particle rumen pools at time t; LPUr0, SPUr0, LPDr0 and SPDr0 are the rumen pools before the first feed intake at Monday 7.00 h and are estimated by the model; LPUit, SPUit, LPDit and SPDit represent the intake of truly undegradable and potentially degradable, large and small particles. LPUit, SPUit, LPDit and SPDit were zero outside the feeding hours. Within the periods of feed intake in experimental periods one and two the values of LPUit, SPUit, LPDit and SPDit were adjusted to the average intake per hour during every morning and evening period of feed intake. In experimental period three they were adjusted every 15 min to the average intake during that 15 min period, which was based on the computer recorded decline in weight of presented feed.

kpSP and kc which yielded the best fit between the measured and calculated pool sizes at the sampling times. A fixed value of kpLP was taken from the steady-state method. 2.6. Sensitivity analyses To investigate to what extent the processes in the rumen may limit feed intake, the effects of changing the rumen parameters on feed intake and rumen degradation were determined. Under steady-state conditions, the time derivatives of the rumen pools are zero, and Eqs. (1)±(4) can be rewritten as: f LPU  nsDMI ÿ …kc ‡ kpLP †  LPUr ˆ 0

(12)

f LPD  nsDMI ÿ …kc ‡ kpLP ‡ kdLP †  LPDr ˆ 0

(13)

f SPU  nsDMI ‡ kc  LPUr ÿ kpSP  SPUr ˆ 0

(14)

f SPD  nsDMI ‡ kc  LPDr ÿ …kpSP ‡ kdSP †  SPDr ˆ 0

(15)

where nsDMI denotes non-soluble dry matter intake and fLPU, fLPD, fSPU and fSPD denote fixed feed fractions of truly undegradable and potentially degradable large (LPU and LPD) and small (SPU and SPD) particles. A fifth equation representing a constant total rumen pool size C can be added: LPUr ‡ LPDr ‡ SPUr ‡ SPDr ˆ C

(16)

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With known values for the rate constants, the feed fractions and the rumen size, these five equations represent a linear system that can be solved using standard methods for the four unknown rumen pools LPUr, LPDr, SPUr, SPDr and the feed intake nsDMI. From the pools and the rate constants, the rumen degradation nsDMD (non-soluble dry matter degradation) can be derived: nsDMD ˆ kdLP  LPDr ‡ kdSP  SPDr

(17)

The relative rumen degradation nsRDMD is defined as: nsRDMD ˆ

nsDMD nsDMI

(18)

The effect of rumen parameters on feed intake and degradation is quantified by calculating the elasticities (E) of nsDMI, nsDMD and nsRDMD with respect to the various rate constants. The elasticity of A with respect to b is defined as the ratio of the relative change in A to the relative change in b:  dA db (19) Eˆ A b The elasticities were determined numerically by calculating the changes in nsDMI, nsDMD and nsRDMD due to a relative increase of the rate constants of 0.1%. 2.7. Statistics All data within the Latin square design were statistically analysed using the GLM procedure of SAS (SAS, 1989) with the following model: Yijk ˆ  ‡ Ci ‡ Pj ‡ Fk ‡ eijk

(20)

where Yijk is dependent variable;  is overall mean; Ci is effect of cow (i ˆ1 to 3); Pj is effect of period (j ˆ1 to 3); Fk is effect of feed (k ˆ1 to 3) and eijk is residual error. The significance of differences between the means was tested for feeds by the test of Tukey using SAS-GLM (SAS, 1989). Statistical significance was declared at P <0.05. Comparison between the three methods of estimating parameters was made using the same model (Eq. (20)) and SAS procedures. In this the dependent variable Yijk is always the difference between two methods of estimating parameters. To compare elasticities for changes in the five rate constants the same procedures were used as mentioned above for comparing methods of estimation.

3. Results 3.1. Intake, particle sizes and rumen pools For GS the standard deviation of DM content was high, caused by differences in DM contents between periods (Table 1). For MS and AS these standard deviations were low. No significant effect of period on DM intake was found.

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Table 5 Intake of feed and water, fat corrected milk (FCM) production and average weight of the cows GS1 Feed intake (kg DM/day) Silage Concentrates Water intake (kg/day) Drinking water Total water FCM production (kg/day) Average weight (kg) 1 2

MS

9.5 6.2 78.3a 88.7 23.1 526

AS

RMSE2

11.2 6.2

10.5 6.2

1.0

56.8b 89.9 25.4 527

75.8a 108.1 22.6 518

1.3 4.4 0.8 4.4

GS: grass silage; MS: maize silage; AS: alfalfa silage. RMSE: Root mean square error. Different superscripts in a row indicate significant differences (P <0.05).

a,b

No significant differences between rations in the intake of DM and total water, FCM production and average weight were found (Table 5). Intake of drinking water, however, was lower for ration MS than for rations GS and AS (P <0.05). In Table 6 the distributions of particle sizes of chewed silage, ruminal and faecal samples and the U fractions of the chewed silage and ruminal samples are given. The LP and SP fractions of the chewed silage and ruminal samples were subdivided into U and D fractions, because the soluble fraction had disappeared during wet sieving. It was assumed that all concentrate particles were smaller than 1.25 mm sieve pore size. SPU, Table 6 Particle size distribution (g/kg DM) of the chewed silage, ruminal and faecal samples and the truly undegradable large (LPU) and small (SPU) particle fractions (g/kg DM) of chewed silage and ruminal samples

Chewed silage >1.25 mm (LP) >0.04 mm and <1.25 mm (SP) <0.04 mm LPU SPU Rumen particles >1.25 mm (LP) >0.04 mm and <1.25 mm (SP) <0.04 mm LPU SPU Faecal particles >1.25 mm (LP) >0.04 mm and <1.25 mm (SP) <0.04 mm 1

GS1

MS

AS

RMSE2

711 66 223 277a 373

576 104 320 229a 301

621 94 285 565b 500

45 17 60 25 61

375 250 375a 509a 614a

362 318 320b 341b 406b

368 340 292b 881c 816c

20 19 8 28 40

111 648 241

140 710 150

121 663 216

38 50 36

GS: grass silage; MS: maize silage; AS: alfalfa silage. RMSE: Root mean square error. a,b,c Different superscripts in a row indicate significant differences (P<0.05). 2

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Table 7 Mean rumen pools (LPUr, LPDr, SPUr, SPDr, <0.04 mm, Total r) and mean ingested feed (LPUi, LPDi, SPUi, SPDi, <0.04 mm, Total i) per 24 h (kg DM)

LPUr LPDr SPUr SPDr Total r >0.04 mm <0.04 mm Total r LPUi LPDi SPUi SPDi Total i>0.04 mm <0.04 mm Total i 1

Grass silage

Maize silage

Alfalfa silage

RMSE1

1.94 1.94ab 1.53a 0.99ab 6.40 3.79a 10.19a 1.80a 4.88 0.66 3.07 10.41 5.24 15.64

1.17 2.32a 1.20b 1.78a 6.47 3.04b 9.51ab 1.54a 5.03 0.77 3.55 10.89 6.48 17.37

2.71 0.40b 2.21c 0.51b 5.83 2.39c 8.22b 3.69b 2.86 0.95 3.16 10.66 6.04 16.70

0.38 0.34 0.06 0.24 0.27 0.09 0.36 0.22 0.71 0.21 0.14 1.08 0.80 1.02

Root mean square error. Different superscripts in a row indicate significant differences (P <0.05).

a,b,c

SPD and soluble fractions of concentrates were 70, 438 and 492 g/kg DM, respectively. Chewed AS had a significantly higher LPU fraction than chewed GS and MS. Rumen samples for ration GS had a significantly higher soluble (<0.04 mm) fraction than rumen samples for the rations MS and AS. The rumen U fractions for the rations GS, MS and AS differed significantly and were lowest for the ration MS and highest for the ration AS. The high fractions of faecal DM that were retained on the 1.25 mm pore size sieve were remarkable. Kennedy and Poppi (1984) proposed for cattle a critical particle size of 1.18 mm sieve pore size. The value of 1.18 mm was based on the sieve aperture retaining the top 50 g/kg of faecal particulate DM. Total particulate rumen contents and total ingested feed did not differ significantly between rations (Table 7). However, including the rumen pool <0.04 mm, ration GS had a significantly higher total rumen pool than ration AS. Remarkable for ration AS was the high ruminal amount of U, which was on average 840 g/kg of ruminal particulate DM. The rations MS and GS had on average 370 and 540 g/kg U in ruminal particulate DM, respectively. The amount of SPUr differed significantly between the three rations and was highest for ration AS and lowest for ration MS. The amounts of LPDr and SPDr were significantly higher for ration MS than for ration AS. In ruminal particulate DM the fraction LP was highest and was on average 600, 540 and 530 g/kg for the rations GS, MS and AS, respectively. The ruminal pool size <0.04 mm differed significantly between the three rations and was highest for ration GS and lowest for ration AS. Between rations there were no significant differences in material entering the rumen after ingesting feed (rumen input) except for LPU, which was higher for ration AS. The rumen inputs of LPD and SPD were higher than those of LPU and SPU, except for ration AS which had an input of LPU higher than LPD.

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49

3.2. Rumen digesta kinetics DM rate constants derived from the conventional, steady-state and dynamic methods are given in Table 8. Between rations there were significant differences in degradation rates. For rumen LP fractions the conventionally derived kdLP estimates differed significantly between the three rations and were highest for ration AS and lowest for ration MS. The conventionally derived kdfeed was significantly lower for ration MS than for rations GS and AS. For the steady-state (ssm) and dynamic (dm) methods, kdLP was significantly higher for ration AS than for rations MS and GS. For ssm kdSP differed significantly between the three rations and was highest for AS and lowest for MS. For dm kdSP was also significantly lowest for ration MS, but rations GS and AS did not differ significantly. The high kdLP and kdSP values for ration AS by the ssm and dm are remarkable. The conventionally derived kpLP value was significantly lower for ration GS than for rations MS and AS. For all three rations kpSP was not estimated higher with CrNDF than with the ssm and dm. Comparing methods of estimation, for the mean over rations the c method resulted in significantly lower estimates for kdLP and kdSP than did the ssm and dm, and the dm resulted in significantly lower estimates for kdSP than did ssm (Table 8). Table 9 presents the rapid and slow degradation rates and the potentially degradable though water-insoluble fractions. In most cases a better fit could be obtained by using the sum of two exponentials, but standard deviations were high. Both conventionally derived kdLP and kdSP appeared to be particularly determined by the slowly degradable fractions, except for the LP fraction in ration AS, for which no rapidly and slowly degradable fractions could be distinguished. 3.3. Sensitivity analyses For ration MS the elasticity of non-soluble DM intake (nsDMI) for changes in kdSP was significantly higher than for ration AS (Table 10). NsDMI for rations MS and AS was significantly most sensitive to changes in kpSP , though for ration AS not significantly different from the sensitivity to changes in kc. For ration GS the nsDMI was most sensitive to changes in kpSP (though not significantly different from the sensitivity to changes in kdLP) and kc (though not significantly different from the sensitivity to changes in kdLP and kpLP). The elasticities of non-soluble rumen DM degradation (nsDMD) for changes in kpSP for ration AS and for changes in kdSP for ration MS, were significantly higher than for the other rations (Table 11). For ration MS the elasticity for changes in kdLP was significantly higher than for ration AS. NsDMD for ration MS was significantly most sensitive to changes in kdSP and kdLP. NsDMD for ration AS was most sensitive to changes in kc and kpSP, though the sensitivities to changes in kpSP and kdSP did not differ significantly. NsDMD for ration GS was most sensitive to changes in kdLP , though not significantly different from the sensitivity to changes in kdSP and kc. Elasticities of non-soluble relative rumen DM degradation (nsRDMD) for changes in rate constants were negative for kc, kpLP and kpSP, and positive for kdLP and kdSP (Table 12). In absolute terms, for ration MS elasticities were higher than for the other rations, though not significantly higher for the elasticity to changes in kc and, for ration

4.9 1.8b 1.7 1.1b 6.6 3.7b

1.2 2.5a 2.4 0.5a 4.5 5.8a

4.5 3.2c 6.3 1.2b 7.2 6.9a

AS

2.6 0.1 1.8 0.1 0.6 0.3

RMSE

1

3.4 6.4p 14.4p 0.6 6.0

GS 4.3 3.6p 7.1q 1.2 6.8

MS 4.9 24.5q 22.4r 1.2 7.4

AS 1.3 2.4 1.3 0.2 1.1

RMSE

Steady-state method (ssm)

3.6 6.1x 11.5x 0.6 6.1

GS 4.2 3.1x 5.9y 1.2 6.7

MS 5.1 21.7y 14.9x 1.2 7.7

AS

Dynamic method (dm)

1.6 1.1 0.9 0.2 1.4

RMSE 0.7 9.0 11.2 0.1 0.6

mean

2

ssm-c

2

Root mean square error. Mean difference between two methods of estimation. Different superscripts per estimation method within rows indicate significant differences (P <0.05) between rations: a,b,c conventional method; p,q,r steady-state method; x,y dynamic method. Superscripts * (P <0.05), ** (P <0.01) and *** (P <0.001) indicate significance from zero.

1

kc kdLP kdSP kpLP kpSP kd(feed)

MS

GS

Conventional method (c)

2.7 2.5* 3.1*** 0.1 1.2

RMSE

Method comparison

0.7 7.8 7.3 0.1 0.7

mean

dm-c

2.7 1.2* 2.7*** 0.1 1.4

RMSE

ÿ0.1 1.2 3.9 ÿ ÿ0.1

mean

ssm-dm

0.4 1.3 0.4** ÿ 0.4

RMSE

Table 8 Dry matter rate constants (%/h) of comminution (kc), microbial degradation (kd) and passage (kp) for large (LP >1.25 mm) and small (0.04 mm < SP<1.25 mm) rumen particles estimated conventionally (c) and as derived from the steady-state (ssm) and dynamic (dm) method, and comparisons between the three methods of estimation

50 M. Bruining et al. / Animal Feed Science and Technology 73 (1998) 37±58

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51

Table 9 Dry matter rapid and slow degradation rates (kd, %/h) of large and small rumen particles and the potentially rapidly and slowly degradable though water insoluble fractions (%). Between brackets standard deviation Grass silage kd Large, Large, Small, Small, 1)

rapid slow rapid slow

11.3 2.0 11.3 2.2

Maize silage fraction

(21.2) (0.1) (50.6) (1.2)

23.0 43.4 10.6 38.4

kd

Alfalfa silage fraction

(16.5) 11.0 (23.7) 1.6 (19.0) 11.6 (28.0) 1.3

(22.0) (0.1) (15.4) (0.1)

18.4 57.4 26.4 44.7

kd

fraction

1)

(14.0) 1) (17.8) (13.6) 21.4 (14.4) 5.4

1) 1)

(368) (8.2)

6.9 20.9

(55.4) (58.9)

No rapidly and slowly degradable fractions could be distinguished.

Table 10 Elasticity, and comparisons between elasticities, of non-soluble DM intake (nsDMI) for changes in rate constants predicted from ssm Grass silage kc 0.29 0.21 kdLP 0.10ab kdSP 0.10 kpLP 0.29 kpSP Difference: mean2 and significance 0.08 kcÿkdLP kcÿkdSP 0.19* kcÿkpLP 0.19 kpSPÿkc 0.00 0.08 kpSPÿkdLP kpSPÿkdSP 0.19** kpSPÿkpLP 0.19* kdLPÿkdSP 0.11 0.11 kdLPÿkpLP kdSPÿkpLP 0.00

Maize silage 0.23 0.19 0.14b 0.12 0.32 0.04 0.09* 0.11* 0.09* 0.13* 0.18** 0.20*** 0.05 0.07 0.02

Alfalfa silage 0.30 0.06 0.07a 0.16 0.40

RMSE1 0.03 0.04 0.01 0.04 0.03

0.24** 0.23** 0.14* 0.10 0.34** 0.33** 0.24* 0.01 ÿ0.10* ÿ0.09

1

Root mean square error. Mean difference between two elasticities. Different superscripts in a row indicate significant differences (P <0.05). Superscripts * (P <0.05), ** (P <0.01) and *** (P <0.001) indicate significance from zero. 2

a,b

GS, kdSP and kpSP. The elasticities of nsRDMD were significantly most positive and negative for changes in kdSP and kpSP, respectively, except for ration GS by which elasticities between kdLP and kdSP and between kpSP and kpLP did not differ significantly. 4. Discussion 4.1. Intake, particle sizes and rumen pools The net energy content of GS and MS did not differ much from the values given by CVB (1995): for GS 5285 (CVB: 5693) and for MS 6314 (CVB: 6272) kJ/kg DM. However, the AS had a low net energy content: 4071 (CVB: 4878) kJ/kg DM, as it was

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M. Bruining et al. / Animal Feed Science and Technology 73 (1998) 37±58

Table 11 Elasticity, and comparisons between elasticities, of non-soluble rumen DM degradation (nsDMD) for changes in rate constants predicted from ssm Grass silage kc 0.25 0.29ab kdLP 0.26a kdSP 0.06 kpLP 0.14a kpSP 2 Difference: mean and significance ÿ0.04 kcÿkdLP kcÿkdSP ÿ0.01 kcÿkpLP 0.19* kcÿkpSP 0.11 ÿ0.15* kpSPÿkdLP kpSPÿkdSP ÿ0.12 0.08* kpSPÿkpLP 0.03 kdLPÿkdSP kdLPÿkpLP 0.23* 0.20* kdSPÿkpLP

Maize silage

Alfalfa silage

0.17 0.33a 0.44b 0.04 0.03b

0.29 0.10b 0.20a 0.14 0.27c

ÿ0.16* ÿ0.27** 0.13** 0.14* ÿ0.30** ÿ0.41** ÿ0.01 ÿ0.11 0.29** 0.40**

0.19** 0.09* 0.15* 0.02 0.17* 0.07 0.13 ÿ0.10* ÿ0.04 0.06

RMSE1 0.03 0.04 0.03 0.03 0.02

1

Root mean square error. Mean difference between two elasticities. Different superscripts in a row indicate significant differences (P <0.05). Superscripts * (P <0.05) and ** (P <0.01) indicate significance from zero. 2

a,b,c

Table 12 Elasticity, and comparisons between elasticities, of non-soluble relative rumen DM degradation (nsRDMD) for changes in rate constants predicted from ssm Grass silage kc ÿ0.03 0.07a kdLP 0.16ab kdSP ÿ0.04a kpLP ÿ0.16ab kpSP Difference: mean2 and significance ÿ0.10* kcÿkdLP kcÿkdSP ÿ0.19* kcÿkpLP 0.01 0.13* kcÿkpSP kpSPÿkdLP ÿ0.23** kpSPÿkdSP ÿ0.32* kpSPÿkpLP ÿ0.12 kdLPÿkdSP ÿ0.09 kdLPÿkpLP 0.11* kdSPÿkpLP 0.20** 1

Maize silage

Alfalfa silage

RMSE1

ÿ0.06 0.14b 0.30b ÿ0.08b ÿ0.30b

ÿ0.02 0.04c 0.14a ÿ0.02a ÿ0.14a

0.01 0.01 0.03 0.01 0.03

ÿ0.20** ÿ0.36** 0.02* 0.24** ÿ0.44** ÿ0.60** ÿ0.22** ÿ0.16** 0.22** 0.38**

ÿ0.06*** ÿ0.16** 0.00 0.12** ÿ0.18** ÿ0.28** ÿ0.12* ÿ0.10* 0.06** 0.16**

Root mean square error. Mean difference between two elasticities. a,b,c Different superscripts in a row indicate significant differences (P <0.05). Superscripts * (P <0.05), ** (P <0.01) and *** (P <0.001) indicate significance from zero. 2

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53

harvested at a late stage of maturity due to bad weather. This might be a reason for the relatively low DM intake of AS. Shaver et al. (1988a) found a significantly lower DM intake for midbloom and full bloom alfalfa hay than for prebloom alfalfa hay. Also the high NH3±N content in AS could have affected AS intake as a result of a bad palatability. Total water intake was highest, though not significantly, for ration AS. This can hardly be explained by the factors DM intake, temperature, physiological stage and feed composition (Forbes, 1986). Possibly the higher ash fraction in AS resulted in a higher urine excretion; potassium intake amounted 41.0, 27.6 and 14.9 g per kg DM for AS, GS and MS, respectively, while sodium was comparable between the three silages. MS was coarsely chopped during harvesting, while GS and AS were not chopped. However, this did not result in significant differences between silages in particle size fractions of ingested samples. This conforms to results of Shaver et al. (1988b). They found similar particle distributions in masticated samples collected at the cardia for long and chopped alfalfa hay and suggested little influence of chopping forage on particle size reduction when high quality forage is fed. Also, Brouk and Belyea (1993) found coarsely chopped alfalfa hay as effective as long alfalfa hay in eliciting chewing activity. The ingested AS contained higher LPU and SPU fractions than GS and MS. This cannot be explained by the NDF content of AS, which was comparable with GS, but might be caused by a higher lignin content of AS (Shaver et al., 1988a). It appears to be a general trend that leguminous forages have a higher U fraction than grasses (Tamminga, 1993). Rumen samples of all three rations had a higher LP than SP fraction, with a LP/SP ratio of 1.5, 1.1 and 1.1 for rations GS, MS and AS, respectively. This does not correspond with the higher rumen SP (>0.071 and <1.25 mm) than LP (>1.25 mm) fractions found by Bosch et al. (1992b) and Bosch and Bruining (1995) for grass silage rations and by Oosting et al. (1994) for wheat straw rations. However, they used different sieving methods for rumen samples compared with this study. Wet sieve analyses may give different results due to several variations in sieving equipment and methods (Kennedy, 1984). Only for ration MS there was a higher D than U fraction in rumen particles. The very high rumen LPU and SPU fractions for ration AS can be explained by the late harvesting stage of alfalfa resulting in a high lignin content, but might also be caused by a limited disappearance of these fractions from the rumen. However, kc and kpSP values were not lower for ration AS than for rations GS and MS. Moreover, Tamminga et al. (1989) found a higher fractional passage rate for the U than for the D rumen NDF pool. Another explanation can be the much higher kdLP and kdSP values for rumen AS ingesta, which caused the D fraction to be small soon after feed intake was completed. In faecal samples considerable (11±14%) LP was found. Therefore, it was decided to also include kpLP in the model. The higher faecal LP compared with the 5% faecal LP according to the critical particle size theory (Poppi et al., 1980; Kennedy and Poppi, 1984) might well be explained by differences in feed intake, feed composition, particle density and separation of particles in the reticulorumen as explained by Lechner-Doll et al. (1991). So, the critical sieve size of 1.18 mm for cattle found by Kennedy and Poppi (1984) cannot be generalised.

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M. Bruining et al. / Animal Feed Science and Technology 73 (1998) 37±58

4.2. Rumen digesta kinetics The in sacco-derived kd(feed) values were calculated as a weighed average of the kd values of the silages and concentrates. Their separate kd values were 5.7, 2.5, 8.3, 5.8 and 5.9% hÿ1 for GS, MS, AS, concentrates and concentrates ‡ soybean meal (solvent extracted), respectively. For GS and AS these kd values were comparable with values found for grass silages by Bosch et al. (1992a) and for midbloom and full bloom alfalfa hay by Shaver et al. (1988a). For MS the kd value was considerably lower than the measured 5.8% hÿ1 by Shaver et al. (1988a), but was slightly lower than 3.4% hÿ1 found by De Visser et al. (1991) and was comparable with 2.7 and 2.0% hÿ1 found by Susmel et al. (1990) and Mir et al. (1991), respectively. In sacco-derived kdLP and kdSP values were lower than kd(feed) values, which means that rumen-derived feed particles had lower dagradation rates than the feed itself. This can be explained by a different chemical composition of rumen particles, which contains relatively more fiber, compared with the feed. In sacco kdLP and kdSP were very similar in GS and MS. Only for ration AS a higher in sacco-derived kdSP than kdLP was found, which might be caused by a high fiber AS stem fraction in rumen LP. Estimates of kpSP were not higher with CrNDF than by modelling. This is different from results of Aitchison et al. (1986) and Oosting et al. (1993), where model-derived rates of passage were lower than those estimated by using CrNDF. The present study however, suggests that CrNDF is a good marker for determining kp of indigestible SP. This was concluded from the comparable estimates of kpSP derived with CrNDF and by modelling using undegradable DM. Estimates of kdLP and kdSP were consistently higher by modelling than by conventional methods, which corresponds with results of Aitchison et al. (1986) for the total rumen pool and with Oosting et al. (1993) for rumen SP. In sacco-derived values of kd ranged between 13±58% of the values derived by both modelling methods. These differences in kd between modelling and in sacco might be at least partly the result from lower microbial activity in nylon bags than in rumen ingesta (Huhtanen and Khalili, 1992). Using the steady-state equations to calculate the pool sizes from the known feed intake and the rate constants derived with the c method, impossible values for the pool sizes are estimated: e.g. for ration GS 4.4, 4.9, 1.8 and 2.7 kg DM for LPUr, LPDr SPUr and SPDr, respectively. After reexamining the method for determining kds from in sacco incubations, it was concluded that the kd values to a large extent are determined by the slow decrease in residue between 24 and 336 h. Since feeding was every 12 h, the first 12 h of in sacco incubation are very important. However, a fast degrading component in the in sacco measurements might easily be overlooked due to a good single exponential fit of the later samples. By examining the effect of including a second degradable fraction in the formula for estimating nylon bag degradation rates, we concluded that the conventionally derived kdLP and kdSP are likely to have been determined particularly by the slowly degradable fractions. The large difference in kd between the rapidly and slowly degradable LP and SP fraction would indicate that a six pool model is useful. However, not all rate constants could be determined in a six pool model as that requires more experimental data. The rapid degradation rates reflect the fast degradation of recently added potentially degradable rumen contents. The slow degradation rates, however, cause

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55

an accumulation of poorly degradable rumen contents. At 12 h post-feeding, on average 78 and 84% of the slowly degradable fractions was not yet degraded for rations GS and MS, respectively. Therefore, in steady state there has to be a higher passage to the omasum of the slowly degradable fractions than of the rapidly degradable fractions. An explanation for the higher passage rate of the slowly degradable fractions might be that a lower digestion rate gives less gas spaces and so a higher functional specific gravity, compared with the rapidly degradable fractions. In general, the rate of passage increases with increasing specific gravity (Lechner-Doll et al., 1991). These results suggests that the rapidly degradable fractions are particularly important for rumen degradation and that the slowly degradable fractions behave in the rumen more or less like the truly undegradable fractions. A higher passage rate for the undegradable than for the degradable rumen NDF pool was reported by Tamminga et al. (1989). By comparing the rate constants derived by ssm and dm almost similar estimates were found for kc as well as for kpSP. Ssm seems to give slightly higher kd values than dm. As dm might be more in line with reality, this probably means that ssm slightly overestimates kd. In general, however, the differences between ssm and dm are very small compared to the differences with conventional methods. 4.3. Sensitivity analyses The highest sensitivities of nsDMI to kpSP and kc may suggest that physical processes are a limiting factor for nsDMI. However, it should be noted that metabolic feedback at the systemic level, not included in the models, may override mechanisms at the gastrointestinal level. The high sensitivity of intake (wheat straw NDF) to kp was also found by Oosting et al. (1993), but in this case the sensitivity to kc was less with elasticities of 0.40 and 0.14, respectively. No correlation was found between elasticities of nsDMI and contribution by the rate constants kdLP, kdSP, kpLP and kpSP to total nsDM disappearance from the rumen. The highest elasticities of nsDMD to changes in kc and kpSP for ration AS can be explained by the high elasticities of nsDMI to changes in kc and kpSP and the rather small negative influence of the increased rate of size reduction or passage on nsRDMD. The highest elasticity of nsDMD to changes in kdSP for ration MS can be explained by the high sensitivity of nsRDMD to changes in kdSP combined with a positive influence of extra SP degradation on nsDMI. The highest elasticities of nsDMD to changes in kdLP, kdSP and kc for ration GS are also explained by the high elasticities of either nsDMI or nsRDMD to changes in these rate constants. The high sensitivities to changes in kc for rations AS, GS and also, though somewhat lower, for ration MS, mean that cows fed these rations can substantially increase their rumen DM degradation by ruminating more or more intensively. So, the important roughages grass, maize and alfalfa silages have high elasticities of nsDMI and nsDMD to changes in kc, which means that a high kc is an interesting breeding goal for dairy cattle. Besides this it is interesting to look for possibilities to improve kd in MS and GS as nsDMD is most sensitive to changes in kdSP and kdLP in these rations. NsRDMD is most sensitive to changes in kdSP and kpSP. The negative elasticities to changes in kpLP and kpSP can be easily understood as a higher passage rate increases

56

M. Bruining et al. / Animal Feed Science and Technology 73 (1998) 37±58

escape of degradable material. An increase of kc can also cause a larger escape of degradable material, but can also result in a higher degradation. In this case the first effect seems to be stronger, as the elasticities for changes in kc have negative values. Also Oosting et al. (1993) found a slightly negative elasticity of NDF rumen degradability for changes in kc. However, it should be noted that the sensitivities of nsDMI, nsDMD and nsRDMD were tested with the total rumen pool assumed to stay constant. Only the proportions of LPUr, LPDr, SPUr and SPDr could change and only one rate constant was changed each time. Changes in total rumen pool (as observed in reality) and simultaneous changes could not be tested. 5. Conclusions The kinetics of microbial degradation, comminution and passage of DM in three rations were studied as determined by conventional, steady-state and dynamic methods. The importance of kinetic parameters was determined in a sensitivity analysis. The study showed that with regards to the methods: 1. Estimates of passage rates of small particles (kpSP) based on CrNDF were similar to those derived from the steady-state and dynamic methods. 2. Nylon bag derived estimates of degradation rates of large (kdLP) and small (kdSP) particles were lower than those derived with the steady-state and dynamic methods. This is likely to be so because of inadequate recognition of a fast degradable fraction with nylon bag derived degradation rates. With regards to the sensitivity analysis the study showed that: 1. The non-soluble dry matter intake was most sensitive to changes in kpSP and rate of comminution (kc). 2. The non-soluble rumen dry matter degradation was most sensitive to changes in kdLP, kdSP and kc for ration GS, to changes in kdSP for ration MS and to changes in kc and kpSP for ration AS. Acknowledgements The authors gratefully acknowledge Germ Bangma, Martin Los, Dirk Vink and Anneke Fleurke for taking care of the fistulated animals and Toos Lammers-Wienhoven and Tino Leffering for analytical assistance. They further thank Jan Wijma, Gert Schoterman, Jos Bakker, Caroline van Brakel, Jan Willem Muller and Willem van de Wetering who contributed to this experiment as MSc students and Johan Roerink who contributed as Agricultural College apprentice. References Aitchison, E., Gill, M., France, J., Dhanoa, M.S., 1986. Comparison of methods to describe the kinetics of digestion and passage of fibre in sheep. J. Sci. Food Agric. 37, 1065±1072.

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