44 F
.J. Mulligan et al. Livestock Production Science 68 2001 41 –52
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
High urea 890
839 182
51 652
385 51
17.63
F .J. Mulligan et al. Livestock Production Science 68 2001 41 –52
45 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 0.88 3 maintenance
CP g kg DM 42
103 182
using metabolisable energy 5 digestible energy 3
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 P 5 0.05. 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 P 5 0.099. 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
A and B C and D
E and F Allowance g d
600 700
800 DM allowance g d
538 627
717 DMI g d
535 614
702 DMI range
526–538 556–627
675–715 DMD
596.8 610.8
617.6 9.13
0.122 OMD
608.0 620.4
627.1 9.32
0.163 CPD
686.4 706.4
693.5 5.49
0.368 NDFD
562.9 582.0
587.2 13.25
0.208 ADFD
522.4 538.5
552.9 12.45
0.099 GED
569.0 582.0
596.7 9.38
0.050 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
DMI, Dry matter intakes. D, Denotes digestibility coefficients for that nutrient. S.E.M., Standard error of the mean. , Indicates a significant quadratic effect P
, 0.05.
46 F
.J. Mulligan et al. Livestock Production Science 68 2001 41 –52 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
CP g kg DM
a a
A, C and E 103
15.56 4.10
11.46 73.65
b b
B, D and F 182
16.65 3.87
12.78 76.76
S.E.M. 0.251
0.084 0.214
1 ab
, 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
G 42
14.80 3.82
10.98 74.19
a a
E 103
15.94 3.92
12.02 75.41
b b
F 182
17.40 3.85
13.55 77.87
S.E.M. 0.453
0.137 0.403
1 ab
, 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.
F .J. Mulligan et al. Livestock Production Science 68 2001 41 –52
47 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 80 8C 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.
291.1 g kg DM determined using the method of When the three groups fed 800 g d of hay at
Weisbjerg et al. 1990. It is possible that the dietary CP contents of 42, 103 and 182 g CP kg of
difference between both values is due to particle loss DM i.e., groups E, F and G are compared, the
from the nylon bags at washing. Smith et al. 1972 effect of increasing the protein content from 103 to
also report large soluble DM components for a range 182 g CP kg DM gave responses similar in mag-
of mature grasses ranging from 290 g kg DM for the nitude to those obtained when the 6 treatment groups
Pennlate variety of Orchardgrass to 460 g kg DM in the factorial experiment were compared. What is
for common wheat. of interest in the comparison of groups E, F and G is
the effect of giving no supplementary protein at all. 4.2. Dietary protein level and digestibility
This gave the lowest digestibility coefficients but they were not substantially lower than the other two
The results obtained in the factorial analysis treatments OMD; 639.8, 614.4 and 603.3 g kg for
Table 3, groups A to F differ somewhat from groups fed 182, 103 and 42 g CP kg DM, respective-
previous reports Satter and Roffler, 1977; NRC, ly. Given that a response to protein supplementation
1984; Boggs et al., 1987; Willms et al., 1991 which when going from 103 to 182 g CP kg DM was found
suggest that dietary CP contents in the region of 100 in the factorial experiment, one could reasonably
to 120 g kg are adequate for optimum digestibility. expect a larger response when a very low protein diet
In this experiment, digestibility increased when the was supplemented. The absence of such an effect
dietary CP content was increased from 103 to 182 indicates that ruminants fed a maintenance diet have
g kg DM. However, the magnitude of the increase is the ability to recycle sufficient nitrogen to compen-
small in biological terms 28.6 g kg in OMD and is sate for very low protein levels in the diet. Kennedy
not much greater than the normal variation associ- and Milligan 1980 stated that 23–92 of plasma
ated with carefully controlled digestibility trials urea may be recycled with the higher value associ-
which amounts to a minimum of 20 g kg Van Soest, ated with low protein diets. Thus on low protein
1994. A similarly small effect was reported by diets, recycling of urea may avoid severe depressions
Moore et al. 1997 for total digestible nutrients in digestibility provided the extent of protein under-
48 F
.J. Mulligan et al. Livestock Production Science 68 2001 41 –52
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-
P 5 0.05. 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. Owen, 1989; Zinn et al., 1994; Woods et al., 1999.
The observed trend of increasing digestibility with However level of intake effects on the digestibility of
increased feeding level was unexpected. Increases in predominantly forage diets are less common, or are
digestibility in response to increased feeding level not of the same scale as those which accrue on
have been previously reported by Ortigues et al. concentrate diets. Galyean and Owens 1991 in a
1993 who also conceded that such responses were review, stated that increasing intake has greater
contradictory to most other reports. Attempts to effects on the site and extent of digestion with
explain this phenomenon in the current work are high-concentrate and mixed diets than with all-
based on the observed pattern of consumption of roughage diets. This theory is substantiated by the
urea treated hay. The sheep fed 600 g d of hay results of Blaxter et al. 1956, Ulyatt et al. 1983
tended to eat their allowances much quicker than and Mbwile and Uden 1997 who report no di-
those fed the 700 g d or the 800 g d treatments, gestibility decline due to increased level of intake for
with the sheep fed the 800 g d treatment almost fresh grass or hay when fed to sheep in the case of
consuming some of their allowance in each hour of both Blaxter et al. 1956 and Ulyatt et al. 1983
the day. This was most likely due to a pungent and dairy cows in the case of Mbwile and Uden
ammonia-like odour, which seemed to effect the 1997. Tyrrell and Moe 1975 and Colucci et al.
palatability of the urea treated hay. This frequent
F .J. Mulligan et al. Livestock Production Science 68 2001 41 –52
49
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.
13.45 between true average of True OMD 1 and The excretion of microbial and endogenous faecal
True OMD 2 and apparent digestibility have been N averaged 5.24 g kg DMI Table 8. This was the
previously reported. Woodward and Reed 1995 average of a range of values extending from 4.72 to
reported a difference of 15 in the true and apparent 5.99 g kg DMI. These values agree well with results
digestibility of diets composed of East African previously reported by Robertson and Van Soest
browses and vetch straw when fed to sheep and 1975, Paquay et al. 1972 and Petit et al. 1985
goats. where microbial and endogenous faecal nitrogen
The significantly higher P , 0.05 level of micro-
excretion was estimated to be 5.8, 4.96 and 5.74 bial and endogenous faecal N output calculated as
g kg DMI, respectively. Mason and Frederiksen the difference in total faecal N and faecal NDF N as
1979 agree well with these estimates and report an
50 F
.J. Mulligan et al. Livestock Production Science 68 2001 41 –52
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
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5. Conclusions
