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The effect of dietary protein content and hay intake level on
the true and apparent digestibility of hay
*
F.J. Mulligan , P.J. Caffrey, M. Rath, M.J. Kenny, F.P. O’Mara
University College Dublin, Department of Animal Science and Production, Lyons Research Farm, Newcastle, Co. Dublin, Ireland Received 14 September 1999; received in revised form 25 January 2000; accepted 8 June 2000
Abstract
The digestibility of concentrate feeds has been determined by difference using hay as a basal forage in many institutions. The objective of this experiment was to examine the effect of hay intake level and dietary crude protein (CP) content on hay digestibility. Groups of wether lambs (1 year old) were fed three levels (600, 700 and 800 g / day) of hay each at two CP contents (103 and 182 g CP/ kg DM), achieved by using a urea solution. An extra group of lambs were fed hay unsupplemented with urea (42 g CP/ kg DM) at a level of 800 g / day. The true organic matter digestibility (True OMD, calculated using two methods: True OMD 1 and 2) and apparent OMD of all treatments fed the 182 g CP/ kg DM diets were significantly higher (P,0.05) than treatments fed the 103 g CP/ kg DM diet (OMD: 632.8 g / kg vs. 604.2 g / kg; True OMD 1: 720.6 vs. 694.1 g / kg; True OMD 2: 711.8 vs. 681.0 g / kg). Apparent and true digestibility also increased as hay allowance increased and a significant linear effect was observed for GED. For the three groups fed 800 g / day of hay, significant differences in digestibility due to CP content only occurred between the 182 and the 42 g CP/ kg DM diets (with the exception of CP digestibility). Although the effect of CP content was significant in some cases, the magnitude of the observed digestibility response is of little consequence for concentrate feed evaluation. 2001 Elsevier Science B.V. All rights reserved.
Keywords: Hay digestibility; Intake level; Protein content; Sheep
1. Introduction concentrate feeds by difference. As part of this
procedure, it is necessary to assume that the di-Ruminants require some source of forage to ensure gestibility of the hay is unchanged when it is fed normal rumen function. This being the case, diets with the concentrate being evaluated. If the diges-used to evaluate the digestibility of concentrate feeds tibility of the hay were to change markedly, then the often include hay as a small proportion of the overall digestibility of the concentrate when calculated by diet. The digestibility of this hay must be pre-de- difference would be distorted. Two factors which termined to allow calculation of the digestibility of may affect the digestibility of hay are dietary protein
content and hay intake level.
Although several reports exist in the literature on
*Corresponding author. Tel.: 1353-1-6012-167; fax:1
353-1-6288-421. the effect of dietary protein content on the
tibility of feeds, considerable ambiguity still exists 2. Materials and methods with regard to the level of dietary protein required
for optimum digestibility. Several authors (Satter and 2.1. Animals and treatments Roffler, 1977; National Research Council (NRC),
1984; Boggs et al., 1987; Willms et al., 1991) have Twenty-eight Suffolk or Texel cross wether lambs suggested that a crude protein (CP) content of 100– (1 year old) of mean weight 41.6 kg (C.V. 8.54%), 120 g / kg DM is optimum for digestibility. However were used to determine the true and apparent di-Cronin (1996) reported large increases in the di- gestibility of hay. The experiment had a 332 gestibility of hay as the CP content of a mixed factorial design (groups A to F) with three feeding hay / soyabean meal ration was increased up to 202 levels of hay (600, 700 and 800 g / d) at two dietary g / kg DM. Krishna Mohan et al. (1987) also reported CP contents (110 and 200 g CP/ kg DM). As it was increases in the apparent digestibility coefficients of desired to effect a change in the CP content of the isocaloric, mixed, rations with a roughage (sun- diet without markedly changing the DM intake or nhemp hay and paddy straw) to concentrate (maize introducing a concentrate into the diet, urea solution and de-oiled groundnut-cake) ratio of 1:1 as CP was used as the supplementary protein source. content was increased up to 160 g / kg DM. Another treatment group (G) which was not part of Most of the evidence on intake effects and di- the factorial, was fed 800 g / d of hay (42 g CP/ kg gestibility is concerned with mixed diets, or the DM) unsupplemented with urea. The inclusion of 10 concentrate proportion of the diet. Several authors g of a mineral / vitamin mix (14% Ca, 7% P) was (Tyrrell and Moe, 1975; Colucci et al., 1982; Robin- common to all diets. Details of the experimental son et al., 1987; Edionwe and Owen, 1989; Zinn et groups are given in Table 1.
al., 1994; Woods et al., 1999) report depressions in
digestibility for various diets with increasing level of 2.2. Experimental procedure intake. However other researchers (Blaxter et al.,
1956; Ulyatt et al., 1983; Mbwile and Uden, 1997) A period of 18 days was allowed for acclimatisa-have reported no significant effect of increasing tion to both the diets fed and the experimental intake on digestibility. In all cases, where no differ- environment. This included a period of 7 days on full ence in digestibility was reported as intake increases, dietary allowance, then movement to metabolism mostly forage diets were fed. crates for 4 days before total faecal collections began The digestibility of large numbers of concentrate for a period of 8 days. During this period, account ingredients has been determined at this institute in was taken of any feed refusals. Each days faecal recent years (Coyle et al., 1996; Cronin et al., 1997; collection was dried at 558C for 3 days and a Mulligan et al., 1998; O’Mara et al., 1999). Hay was constant proportion of the daily faecal DM output used as the basal forage and the level of hay fed was used to make a composite sample for the 8 days. would have differed when hay was fed as the basal The protein source (urea solution) was sprayed on forage (typically 15% of dietary DM) than when its the previously weighed hay allowances (which were digestibility was measured separately (typically 90%
of dietary DM). The CP content of the diet would
Table 1
also have varied depending on the CP content of the
Details of treatments and urea supplementation (daily allowances)
concentrate ingredient being evaluated. It was
gener-digestibility of the hay was being determined
separ-C 700 14.8 103
ately. In that context, this study was important to
D 700 34.1 182
validate these measurements of concentrate diges- E 800 17.0 103 tibility. The objectives of the experiment were to F 800 39.0 182
a
G 800 0 42
determine the effect of hay intake level and dietary
a
protein content on the true and apparent digestibility Group G received 30 ml of water sprayed on hay twice daily
fed daily in equal halves) using manual pump-action meal and 100 g / d of a mineral–vitamin mix over a spray bottles, at the time of feeding. This took period of 10 days. Degradability constants a, b and c approximately 1 h, animals being fed at 8–9:00 a.m. for hay DM were determined according to the
2ct
and 4–5:00 p.m. Two urea solutions were used, equation P5a1b(12e ) (Orskov and which consisted of either 264 g (110 g CP/ kg DM McDonald, 1979) where ‘a’ is the rapidly soluble treatments) or 700 g (200 g CP/ kg DM treatments) DM, ‘b’ is the potentially degradable DM not of granular urea (46% N) added to 1 l of distilled including a and ‘c’ is the fractional rate of degra-water and heated to approximately 808C. The daily dation per hour of the ‘b’ fraction with time ‘t’. Dry allowance of minerals and vitamins was sprinkled matter solubility was also determined using the over the dampened hay. method of Weisbjerg et al. (1990) using Whatman
No. 1 filter paper. 2.3. Chemical analysis
2.5. True digestibility calculations A sample of the hay was collected over the 8-day
faecal collection period and sprayed with both urea Two separate estimates of true OMD (true organic solutions at the end of the experiment. This sample matter digestibility) were made. The first method was used for determination of the actual CP content called True OMD 1 (Mason, 1968) estimates true of the diets. Crude protein was determined as digestibility as 12(faecal NDF / total OM intake). Kjeldahl nitrogen (N)36.25 using a Buchi 435 This method attempts to quantify undigested diet digestion unit and a Buchi 323 distillation unit (faecal NDF) exclusive of other faecal products by (Buchi, Postfach, Flawil / Schweiz, Switzerland) ac- assuming that the converse of NDF (i.e., cell con-cording to the Association of Official Analytical tents) are 100% digestible and that animal and Chemists (AOAC) (1980). Neutral detergent fibre microbial cells are soluble in neutral detergent (NDF) and acid detergent fibre (ADF) were de- reagent. The second method of estimating true termined using a Fibertec extraction unit (Tecator, digestibility, True OMD 2, estimates it as 12[(total Hoganas, Sweden) according to the methods of Van faecal OM2microbial and endogenous faecal OM) / Soest et al. (1991) and Van Soest (1973), respective- total OM intake]. This method for estimating true ly. Acid detergent lignin (ADL) was determined after digestibility assumes that the difference in total re-suspension of ADF residue in 10% acid detergent faecal nitrogen and faecal NDF nitrogen represents reagent. The re-suspension was boiled and simmered microbial and endogenous faecal nitrogen (Mason for 5 min, then filtered and washed. The crucibles and Frederiksen, 1979). Faecal NDF N was de-were then transferred to a Fibertec cold extraction termined on the residue of faecal samples subjected unit where 25 ml of sulfuric acid was added. After 3 to neutral detergent extraction using the filter bag h the crucibles were filtered and washed until the technique and the Ankom Fiber Analyser (Ankom, acid had disappeared and ADL was then determined Fairport, NY, USA). Microbial and endogenous as the residue. The gross energy (GE) content of faecal organic matter may then be determined by both feed and faecal samples was determined using a assuming that microbial and endogenous faecal Parr 1201 oxygen bomb calorimeter (Parr, Moline, nitrogen is 7% of microbial and endogenous faecal IL, USA). Ash was determined after ignition of feed organic matter (Van Soest, 1994).
or faeces at 5508C for 4 h and used to calculate
organic matter (OM). Dry matter (DM) was de- 2.6. Statistical analysis termined in an oven at 1048C for a minimum of 16 h.
model included feeding level and CP content and their interaction as sources of variation. This was carried out on the data of groups A to F (i.e., all the supplemented groups). This data was also subjected to regression analysis using the PROC GLM state-ment of the Statistical Analysis Systems (SAS) Institute to test the linear and quadratic effects of hay allowance on all digestibility coefficients. Data from the seventh group (G) who were fed 800 g / d of hay unsupplemented with any protein (42 g CP/ kg DM)
Fig. 1. In-sacco analysis of untreated hay.
were then combined with the other two groups who received 800 g / d of hay, group E (110 g CP/ kg DM)
and group F (200 g CP/ kg DM). These data were is potentially degradable in the rumen (Fig. 1). Using analysed as a completely randomised design using the method of Weisbjerg et al. (1990) 291.1 g / kg the PROC GLM statement of the SAS Institute DM of the hay was soluble in water.
(1985). The model included CP content (i.e., treat- Where digestibility coefficients for groups A to F ment) as the source of variation. The mean di- were compared in the factorial analysis, no signifi-gestibility coefficient for OMD, True OMD 1 and cant interaction of protein content and hay intake True OMD 2 for all animals were also compared level was found. With regard to the effect of protein using the PROC GLM statement of SAS in order to content, the digestibility of the hay in the 182 g compare the two methods of measuring true di- CP/ kg DM diets was found to be significantly higher gestibility and the estimates of apparent OMD. For (P,0.05) than the digestibility of hay in the 103 g the in-sacco analysis, parameters were estimated CP/ kg DM diets for all digestibility coefficients using the PROC NLIN statement of the SAS Institute except NDFD and GED (Table 3).
(1985).
Table 3
a Effect of dietary CP content on digestibility coefficients (g / kg) 3. Results
Groups S.E.M. P
The urea supplementation resulted in dietary CP A, C and E B, D and F
concentrations of 103 and 182 g / kg DM for the low CP (g / kg DM) 103 182
and high levels of urea, respectively (Table 2). DMD 594.6 622.2 7.84 0.026
OMD 604.2 632.8 8.01 0.021
Furthermore the use of the urea solutions did not
CPD 610.4 780.5 4.60 0.001
affect the other chemical components of the hay to
NDFD 563.6 591.1 11.4 0.106
any great extent (Table 2.) From the in-sacco
analy-ADFD 520.3 555.5 10.70 0.032
sis, the rapidly soluble DM or the so called ‘a’ GED 571.5 593.6 8.07 0.069 fraction of the hay equates to 358.2 g / kg DM, while True OMD 1 694.1 720.6 7.90 0.030 True OMD 2 681.0 711.8 7.90 0.010
the slowly degradable DM or the ‘b’ fraction is
a
452.4 g / kg DM and this fraction degrades at a rate (c D, Denotes digestibility coefficients for that nutrient. S.E.M.,
value) of 3.19% / h. Thus 810.6 g / kg DM of the hay Standard error of the mean.
Table 2
Chemical composition of the supplemented and unsupplemented hay (g / kg DM, except DM, g / kg and GE, MJ / kg DM)
Hay treatment DM OM CP Ash NDF ADF ADL GE
Hay only 896 850 42 46 678 406 50 17.64
Low urea 884 830 103 54 668 390 50 17.89
Table 5
The pattern of actual DM intakes (DMI) closely
Effect of dietary CP content on digestibility coefficients (g / kg) for
reflected that of the fresh hay allowances because 1 all groups fed 800 g / d of hay
there was little feed refusal. The dry matter intakes
Group S.E.M. P
obtained, together with the GE content and the GED
for the hay used means that the feeding levels of the G E F
animals were 0.64, 0.76 and 0.883maintenance CP (g / kg DM) 42 103 182 (using metabolisable energy5digestible energy3 DMI (g) 711.8 697.5 705.8
a ab b
DMD 589.5 605.9 629.3 7.71 0.081
0.82) for groups, A and B, C and D and E, F and G,
a ab b
OMD 603.3 614.4 639.8 8.03 0.119
respectively (Robinson et al., 1980). No significant a b c
CPD 8.1 611.6 775.4 6.48 0.001
effect of hay allowance on digestibility was observed
NDFD 563.2 574.9 599.5 12.12 0.353
in the factorial analysis. In general, digestibility ADFD 542.8 537.5 568.2 10.84 0.361 a ab b
tended to increase as intake level increased and a GED 562.6 587.5 605.9 8.14 0.072 True OMD 1 689.3 702.0 726.3 8.51 0.141
significant linear effect of hay allowance was
ob-a ab b
True OMD 2 676.8 692.1 721.4 8.17 0.062
served on GED (P50.05). In addition the linear
1 abc
effect of hay allowance on ADFD approached sig- , Values within a row with different superscripts differ significantly (P,0.05). S.E.M., Standard error of the mean. DMI,
nificance (P50.099). With the exception of CPD,
Dry matter intakes. D, Denotes digestibility coefficients for that
there was no significant quadratic effect of hay
nutrient.
allowance (Table 4).
Where groups E and F were re-analysed with average digestibility coefficients over the seven group G as a completely randomised experiment treatment groups were 704.8, 693.6 and 616.3 g / kg (i.e., all groups fed 800 g / d of hay), dietary protein (S.E.M. 5.81) for True OMD 1, True OMD 2 and content significantly affected (P,0.05) the diges- apparent OMD, respectively. The difference between tibility coefficients determined for the hay in the case the two estimates of true digestibility was not of DMD, OMD, CPD, GED and True OMD 2 (Table significant with both estimates of true digestibility 5). However, the differences were only significant being significantly higher (P,0.05) than the appar-between the lowest (42 g CP/ kg DM, group G) and ent OMD. The mean difference between the average the highest CP content diets (182 g CP/ kg DM, of both true digestibility estimates and the apparent group F) for DMD, OMD, GED and True OMD 2. OMD was 82.9 g / kg.
When the data for all animals was analysed, the Significant differences (P,0.05) were apparent in
Table 4
a Effect of hay allowance on DM intakes (g / d) and digestibility coefficients (g / kg)
Groups S.E.M. Linear
True OMD 1 697.3 710.5 714.2 9.19 0.209
True OMD 2 687.7 694.8 706.8 8.84 0.143
a
Table 6
1 Effect of feeding level and dietary CP content on the quantitative classification of faecal N as a proportion of faecal DM (g / kg DM)
Groups DMI Total N Feed N Microbial and % Microbial and
2
(g) endogenous N endogenous
ab a
A and B 534.5 16.01 4.04 11.97 74.77
a a
C and D 613.8 15.63 4.03 11.60 74.22
b b
E and F 701.8 16.60 3.88 12.78 76.99
S.E.M. 0.307 0.103 0.262
, Values within a column with different superscripts differ significantly (P,0.05). 2
Indicates the % of total faecal N that microbial and endogenous faecal N is.
the output of microbial and endogenous faecal 4. Discussion nitrogen as a proportion of faecal DM due to dietary
CP content (using groups A to F and groups E, F and 4.1. Hay quality G) (Tables 6 and 7). However, for the comparison of
groups E, F and G, the differences only reached In terms of the quality of the hay used, the low CP significance (P,0.05) between group F (182 g CP/ content (42 g / kg DM) and the high fibre content kg DM) and the other two groups with no significant (NDF: 678 and ADF: 406 g / kg DM) indicate that difference arising between groups G and E. The the hay used was mature, low-quality material (Table output of microbial and endogenous faecal N was 2). Jarrige (1989) reports a range of CP values for also significantly higher for the highest hay allow- perennial ryegrass hays extending from 81 to 119 ance groups (groups E and F: 800 g / d) than for the g / kg DM. The CP content of the hay used in this two lower hay allowance groups (groups A and B: experiment is well below the minimum of this range. 600 g / d; and groups C and D: 700 g / d). The ADF content of the hay used (406 g / kg DM) is The mean output of microbial and endogenous also higher than the maximum ADF value of 397 faecal N for all groups was 5.24 g / kg DMI, while g / kg DM reported by Jarrige (1989) for perennial the output of microbial and endogenous faecal OM ryegrass hay.
using methods True OMD 1 and True OMD 2 was With regard to the degradability characteristics of 7.33 and 8.24 g / 100 g DMI, respectively (Table 8). the hay, the observed ‘a’ value (358.2 g / kg DM) is
Table 7
Effect of dietary protein content on the quantitative classification of faecal N as a proportion of faecal DM (g / kg DM) for all groups fed 800 1
g / d of hay
Group CP Total N Feed N Microbial and % Microbial
2
(g / kg DM) endogenous N and endogenous
a a
, Values within a column with different superscripts differ significantly (P,0.05). 2
Table 8 (TDN) in digitigrass hay fed to sheep after supple-Estimates of faecal microbial and endogenous OM excretion mentation with soyabean meal at levels of 0 and (g / 100 g DMI) using methods True OMD 1 and True OMD 2 and
0.17% of body weight and by Krishna Mohan et al.
of microbial and endogenous faecal N excretion (g / kg DMI) for
(1987). However, larger responses have been
re-all treatment groups
ported. Cronin (1996) reported particularly large
Group True OMD 1 True OMD 2 Microbial and
increases of 135 g / kg for OMD and 157 g / kg for
endogenous N
NDFD for the digestibility of hay fed to sheep by
A 7.47 8.44 5.34
increasing CP contents (using soyabean meal) from
B 7.65 8.14 5.46
56 g / kg DM up to 202 g / kg DM. This report is at
C 7.00 8.36 5.00
D 7.11 8.39 5.07 odds with all others cited and the results presented
E 7.36 8.08 5.26 here.
F 7.74 8.10 5.53 It could be argued that a certain degree of
G 7.01 8.20 5.01
solubilisation of the hay cell wall had occurred by using urea, thus leading to the observed increase in
Mean 7.33 8.24 5.24
C.V. (%) 4.133 1.803 4.153 digestibility. The hay being fed however was sprayed with urea just prior to feeding. For solubilisation of the cell wall to result in digestibility advantages, a
O
period of 4 days at 808C or 3 weeks at 30 C would quite high when compared to the maximum ‘a’ value be appropriate using 30 g NH / kg DM (Ballet et al.,3 of 247 g / kg DM reported for six hays by ADAS 1997). Thus if the hay was quite readily consumed, (1989). However, the relatively large ‘a’ value is the time available for any cell wall solubilisation was supported by the large soluble DM component probably too short.
under-nutrition with regard to the microbes is not greater (1982) also noticed a lesser effect of feeding level on than the recycling capacity. The small difference in the digestibility of both forages and concentrates on digestibility coefficients between group G (42 g high forage diets than on low forage diets.
CP/ kg DM) and group F (182 g CP/ kg DM) may Digestibility depressions in response to increased indicate that the recycling of nitrogen did decrease level of intake have largely been implicated with the protein under-nutrition of the rumen microbes to increased rates of passage (Owens and Goetsch, a large extent, although not eliminating it entirely. 1986). However, in comparison to concentrate feed It should be noted that where the digestibility of particles, forage feed particles are relatively insensi-the exact same batch of hay was measured (when fed tive to feeding level in terms of rumen outflow rate. with soyabean meal) at CP contents of 100 g / kg DM This is clearly demonstrated in the data of Colucci et in two separate digestibility trials (Mulligan, 1997), al. (1982) where the fractional rumen outflow rate the digestibility coefficients (OMD: 590 and 606 for concentrates increased by a much greater amount g / kg) were similar to those obtained in this experi- than the fractional rumen outflow rate for forages ment. Thus, there appears to be no effect of protein due to increasing feeding level for dairy cows fed source on digestibility, and no detrimental effect of both high (83%) and low (32%) forage diets. Varga the urea levels used on rumen fermentation. and Prigge (1982) and Blaxter et al. (1956) also observed no significant effect of level of intake on 4.3. Level of intake and digestibility the rumen outflow rate of alfalfa and orchardgrass, and long hay, respectively with increasing levels of It has been reported (Tyrrell and Moe, 1975; intake when fed to sheep. It may be the case Colucci et al., 1982; Robinson et al., 1987; Edionwe therefore that forage will remain in the rumen until it and Owen, 1989; Zinn et al., 1994; Woods et al., has been sufficiently degraded to pass out. Bruinning 1999) that as intake increases, the digestibility of et al. (1998) clearly demonstrated that rumen outflow feeds decreases. However increasing hay intake level rate increases for grass silage, maize silage and in this trial had no detrimental effect on the di- alfalfa silage particles after particle size reduction. gestibility (both true and apparent) of hay. Indeed This theory was also supported by Van Soest (1985) hay digestibility increased with increasing hay allow- who stated that retention time in the rumen is ance and this effect was significantly linear for GED regulated by rumination that is required to commi-(P50.05). Many of the reports which exist con- nute lignified fibrous particles (i.e., digestibility and cerning digestibility depressions as level of intake particle breakdown control the intake and rate of increases relate to diets containing large amounts of passage of forages to a greater extent than vice concentrates (Robinson et al., 1987; Edionwe and versa).
feeding behaviour of livestock fed non protein a proportion of faecal DM (Table 6) for animals fed nitrogen has previously been reported by Owens and the highest hay allowance (i.e., groups E and F) is Bergen (1983) who suggest that such behaviour may possibly due to a more viable rumen microbial improve the energetic efficiency of rumen-microbes. population associated with the observed diet con-It has also been reported (Ulyatt et al., 1983) that sumption pattern. With regard to the effect of CP frequent feeding behaviour results in greater N content on this parameter, the level of microbial and retention. Both these factors may have combined to endogenous faecal N as a proportion of faecal DM result in a more viable rumen microbial population, excreted by the animals on the 182 g CP/ kg DM diet with a resulting trend of higher digestibility values at was significantly higher than the amount excreted on the higher feeding levels. Other possible explana- the 103 g CP/ kg DM diet (Table 6). This observa-tions are (a) that some solubilisation of the hay cell tion was also noted in the comparison of treatments wall fraction occurred in the extended period prior to E, F and G (Table 7) where the proportion of faecal consumption in the case of the higher feeding level DM that is microbial and endogenous faecal N is treatments and (b) it could possibly be argued that significantly lower for the 42 and 103 g CP/ kg DM the frequent feeding behaviour decreased rates of diets than for the 182 g CP/ kg DM diets. The higher passage and thus increased digestibility. microbial and endogenous faecal N output on the The significant quadratic effect (P,0.05) of hay 182 g CP/ kg DM diets may have been due to allowance on CPD is unusual. Of particular interest enhanced microbial production in the rumen, as is is the lower digestibility coefficient for animals fed suggested by the higher digestibility coefficient for 800 g / d of hay compared to animals fed 700 g / d of this group.
hay. The significantly higher amount of microbial The proportion of total faecal N that is microbial and endogenous faecal nitrogen excreted by the and endogenous is in the region of 73–78%. Mason animals fed 800 g / d of hay may be the only cause of (1968) demonstrated that in the case of diets which this anomaly. had highly available sources of protein, that almost all of the faecal N was extracted by NDR (i.e., was 4.4. Estimation of true digestibility mostly microbial and endogenous). Van Soest (1994) stated that there was no evidence of potentially Both methods for estimating true digestibility are digestible feed protein in normal faeces. However a based on the use of neutral detergent reagent (NDR). certain amount of N in forages (ca. 7%) has been Neutral detergent reagent has also been used for this implicated as being unavailable due to its association purpose by Robertson and Van Soest (1975), Deinum with lignin (Van Soest, 1994). This N may account et al. (1984), Uden (1984) and Woodward and Reed for some of the feed N present in the faeces not (1995). digested by the animal and not soluble in NDR in the When used as suggested by Van Soest (1994), the faeces. It is also possible that bacterial cell walls in two estimates of true digestibility are quite compar- the faeces posses a certain resistance to NDR able, with no significant difference (P.0.05) occur- (Mason, 1968), and thus the proportion of total ring between the means of both estimates. Similar faecal N that is microbial and endogenous may have differences to that observed in this experiment been underestimated.
overall mean of 6.0 g of microbial and endogenous consumption patterns have been cited as two possible faecal nitrogen per kg DMI, but their actual values reasons for this largely unexplained effect. The ranged from 3.6 to 8.0 g per kg DMI. The similarity determination of true digestibility values provided of the results of this trial and those reported by other little extra information on the effect of both vari-authors suggests that quite an accurate estimation of ables. However the true digestibility estimates pro-microbial and endogenous faecal N excretion was duced differences in true and apparent digestibility, made. which were consistent with other reports. The pro-Microbial and endogenous OM excretion has been cedures used for true digestibility determination were reported to be 12% of DMI by Robertson and Van quite easy to apply and may be useful in further Soest (1975). The mean percentage of DMI that investigation of the inherent inaccuracies of apparent microbial and endogenous OM excretion comprises digestibility values. It is important to note that these for the two methods is 7.33% and 8.24% for the True inaccuracies may not be of similar magnitude in all OMD 1 and the True OMD 2 methods, respectively cases.
(Table 8). The discrepancy between the observed values and those of Robertson and Van Soest (1975)
may indicate that microbial and endogenous organic Acknowledgements matter excretion is not a constant across all feeding
situations. However, the small range in values (Table This project was co-funded by the European 8) obtained by both methods of true digestibility Union from structural funds. The technical assistance determination indicate that microbial and endogen- of Mr. J. Callan and Ms. B. Flynn was essential to ous organic matter excretion is quite similar regard- the completion of this work and is greatly ap-less of intake level or dietary protein content, at least preciated.
when a similar basal diet (in this case low-quality hay) is fed.
The difference in true and apparent digestibility References however, is unlikely to be constant for all feeding
situations as the range in microbial and endogenous ADAS, 1989. ADAS Feed Evaluation Unit Tables of Rumen
faecal N excretion (3.6 to 8 g / kg DMI) reported by Degradability Values For Ruminant Feedstuffs. ADAS Feed Evaluation Unit, Stratford On Avon.
Mason and Frederiksen (1979) for 47 feeds or feed
AOAC, 1980. Official Methods of Analysis, 13th Edition.
As-mixes suggests. Such a range will result in large
sociation of Official Analytical Chemists, Washington, DC.
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factor (in this case 1 / 0.07 was used). Thus a case dehydrated lucerne. Anim. Feed Sci. Technol. 67, 69–82. Blaxter, K.L., Graham, M.C., Wainman, F.W., 1956. Some
ob-exists for further investigation of the microbial and
servations on the digestibility of food by sheep and on related
endogenous distortion of apparent digestibility values
problems. Br. J. Nutr. 10, 69–91.
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supplementation and protein withdrawal on rumen fermen-tation, microbial synthesis and site of digestion. J. Anim. Sci. 64, 907–914.
5. Conclusions
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comminu-but by a biologically small amount, which would not tion and passage of particles. Anim. Feed Sci. Technol. 73, 37–58.
cause important associative effects in digestibility
Colucci, P.E., Chase, L.E., Van Soest, P.J., 1982. Feed intake,
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