198 M
.J. Ranilla et al. Livestock Production Science 62 2000 193 –204
Fig. 2. Changes in ammonia-N concentrations in the rumen fluid Fig. 1. Changes in the rumen fluid pH of Churra and Merino
of Churra and Merino sheep grazing a grass white clover pasture. sheep grazing a grass white clover pasture. Significance level: †
P ,0.10; P ,0.05.
4. Discussion
period, rate of degradation, potential degradability Changes in chemical composition of grazed her-
and ED of both DM and NDF were lowest P ,0.05 bage through the experimental periods were similar
during June for both breeds. for both breeds. Similar increases in CP content and
M .J. Ranilla et al. Livestock Production Science 62 2000 193 –204
199
position of the diet are the consequence of changes in both the quality of the herbage on offer and in the
selection made by the animals. Thus, in a grazing study carried out on the same plots used here,
´ Mantecon et al. 1995 reported an increase of the
proportion of white clover and in the herbage regrowth produced as grazing period advanced, both
of them being consequent with the changes observed in the chemical composition.
Regrowth of pasture swards is influenced by the frequency
and intensity
of previous
grazing Brougham, 1955 and, generally, its digestibility is
reasonably constant, with little decline as the season progresses. In our experiment, the herbage grazed
during July and October had a higher DMED than that grazed during June, thus reflecting the pasture
regrowth produced during those grazing periods
´ Mantecon et al., 1995. The lower DM fractional
rate of degradation observed for the herbage grazed in June almost half of the estimated rate in July and
October is probably related to its higher cell wall content. Lignin is the major cell wall component
limiting digestion of the cell wall polysaccharides in the rumen Jung and Allen, 1995. The lower
potential degradability of the NDF from the forage grazed during June observed for the two breeds of
sheep was probably due to its higher cell wall lignification mean value of 83.8 g kg NDF com-
pared to that observed in July and October 69.2 and 61.0 g kg NDF, respectively. In the same way, the
rate of NDF degradation and its ED in grazed herbage were lower in June than in the later grazing
periods.
Ruminal environment, as defined by its pH and fermentation end-products concentrations, affects the
rate and extent of food rumen degradation. Previous studies in Churra and Merino sheep Ranilla et al.,
1997 suggest this could differ between the two breeds. When animals from both breeds were offered
Fig. 3. Changes in total volatile fatty acids VFA concentrations in the rumen fluid of Churra and Merino sheep grazing a grass
alfalfa hay at maintenance level, the Churras ex-
white clover pasture. Significance level: † P ,0.10; P ,0.05.
hibited a higher level of rumen microbial fibrolytic activity than the Merinos, which was related to their
ability to maintain pH within a range of values more favourable to fibre degradation Ranilla et al., 1997.
decreases in NDF content have been reported by Some of the differences in the in situ degradation of
´ Valdes et al. 1995 for a grass-white clover pasture
grazed herbage observed in our experiment could continuously stocked by Churra sheep from May to
also be related to differences between breeds in their November. These changes in the chemical com-
ruminal pH. Stewart 1977 showed that a reduction
200 M
.J. Ranilla et al. Livestock Production Science 62 2000 193 –204 Table 4
Influence of breed Churra CH and Merino ME and advancing season on molar proportions mmol mmol of acetate, propionate, butyrate and isoacids calculated as the sum of isobutyrate, isovalerate and valerate and on the ratio acetate propionate Ac Pr in the
rumen of sheep grazing grass-white clover pasture data are the mean value for the day
Sampling period Middle June
Late July Early October
s.e.d. Acetate
c b
a
CH 63.2
61.3 59.3
0.52
b b
a
ME 62.6
63.7 58.2
0.63 s.e.d.
1.84 0.91
0.72 –
Sig. NS
† NS
– Propionate
CH 18.2
18.3 17.5
0.45 ME
19.9 19.2
19.2 0.68
s.e.d. 0.70
0.26 0.45
– Sig.
† †
– Butyrate
a b
c
CH 11.8
13.6 15.5
0.44
a a
b
ME 12.1
11.8 14.8
0.77 s.e.d.
0.67 1.32
0.72 –
Sig. NS
NS NS
– Isoacids
a b
c
CH 5.3
6.5 7.7
0.22
a a
b
ME 5.2
5.2 7.5
0.34 s.e.d.
0.28 0.25
0.40 –
Sig. NS
NS –
Ac Pr CH
3.46 3.35
3.37 0.113
a,b b
a
ME 3.14
3.31 3.02
0.101 s.e.d.
0.13 0.036
0.05 –
Sig. †
NS –
a,b,c
Means in a row with different superscript differ significantly P ,0.05. Sig.: significance level: NS5non-significant; † P ,0.10; P ,0.05; P ,0.01.
of the rumen pH from 6.8 to 6.0 results in a presented shorter periods of low pH values. In
moderate depression in fibre digestion, but a further addition, the greater Ac Pr ratio found in the rumen
decrease in pH to below 6.0 causes severe inhibition. of Churras compared to Merinos in October would
The lower ED of the grazed herbage observed in also indicate increased fibre digestion.
October for Merinos compared to Churras could be However, it should be noted that when the grass
related to the low rumen pH values below 6.0 hay was incubated in the rumen of the grazing sheep,
found at 17.00 and 23.00 h in Merino sheep Fig. 1. no differences between breeds were detected in
On the contrary, rumen pH in Churra did not drop either DMED or NDFED. Grazing ruminants rely
below 6.0 at any sampling time. It has to be almost entirely on mastication to disrupt plant tissues
remembered that it is not just the extent of pH that create physical barriers to digestion, whereas
depression but also the period over this occurs what mechanical processing of feeds i.e., ground facili-
reduces fibre degradation. With sampling times of tates microbial attack McAllister et al., 1994.
17.00 and 23.00 h it is difficult to interpret which has Grazed herbage was incubated into the rumen of
occurred in the interim, but it is possible that Churras sheep unprocessed, but the grass hay was ground
M .J. Ranilla et al. Livestock Production Science 62 2000 193 –204
201 Table 5
Influence of breed Churra CH and Merino ME and advancing season on dry matter DM and neutral-detergent fibre NDF fractional rate of degradation c, potential degradability a 1b, lag time lag and effective degradability DMED and NDFED from grazed herbage
in sheep grazing grass-white clover pasture
Sampling period Middle June
Late July Early October
s.e.d.
21
DM c h
a b
b
CH 0.055
0.095 0.109
0.0071
a b
b
ME 0.046
0.091 0.108
0.0096 s.e.d.
0.0041 0.0069
0.0122 –
Sig. †
NS NS
– a
1 b g kg
a b
c
CH 656
881 927
0.0
a b
b
ME 754
883 904
11.2 s.e.d.
10.6 13.1
6.4 –
Sig. NS
– DMED g kg
a b
c
CH 541
765 857
2.5
a b
c
ME 588
762 821
13.1 s.e.d.
13.3 9.5
9.3 –
Sig. NS
-
21
NDF c h
a b
b
CH 0.059
0.117 0.115
0.0055
a b
c
ME 0.048
0.105 0.131
0.0083 s.e.d.
0.0074 0.0081
0.0109 –
Sig. NS
NS NS
– a
1 b g kg
a b
c
CH 671
821 887
11.0
a b
b
ME 730
846 849
12.5 s.e.d.
9.4 18.1
7.5 –
Sig. NS
– lag h
CH 2.3
2.8 1.8
0.49 ME
3.7 2.4
2.7 0.53
s.e.d. 0.84
0.16 0.36
– Sig.
NS NS
NS –
NDFED g kg
a b
c
CH 532
668 775
13.6
a b
c
ME 528
674 739
18.6 s.e.d.
16.4 13.6
13.9 –
Sig. NS
NS †
–
a,b,c
Means in a row with different superscript differ significantly P ,0.05. Sig.: significance level: NS5non-significant; † P ,0.10; P ,0.05; P ,0.01; P ,0.001.
through a 2 mm screen, thus, reducing particle size The reasons why DMED and potential de-
and increasing the surface area available for micro- gradability of the grazed herbage during June were
bial attachment and enzymatic attack. This could higher in Merinos than in Churras are unclear. In
explain the lack of differences between breeds when fact, rumen pH values during June were higher in
grass hay was incubated, in spite of the differences Churra animals, although it must be stressed that at
observed in rumen pH values. all sampling times both breeds showed values higher
202 M
.J. Ranilla et al. Livestock Production Science 62 2000 193 –204
than those expected to impair fibre degradation. The rate of OM degradation. Therefore, it can be as-
in situ degradation depends on many factors, being sumed that ammonia-N concentrations were adequate
one of them the characteristics of the feed incubated. for an optimal rumen fermentation in both Churra
As suggested by Mertens 1977, the morphology of and Merino sheep over the three grazing periods.
the plant, the crystalline structure of the fibre and However, the high concentrations observed for all
other factors not related to the chemical composition periods and specially during October could be
could affect the in situ degradability of the substrate indicative of substantial losses of dietary N before
incubated. Herbage grazed by each breed could have the small intestine, as suggested by Beever et al.
presented different characteristics which were not 1986. These authors reported that the high levels of
reflected in its chemical composition, but affected its ammonia-N found in the rumen of cattle grazing
in situ degradation. The fact that both breeds may white clover values similar to the ones found in our
have consumed a diet of different botanical com- study in October were due to the readily soluble
position, due to potential breed differences in their nature of the N constituents of the herbage, which
ability for diet selection Revesado et al., 1994, and gave rise to a supply of degraded N ammonia-N in
that the extent of mastication, may also differ particular, in excess of the capacity of the rumen
between breeds, should also be considered. micro-organisms to assimilate the N into microbial
Daily variations in rumen parameters pH, VFA mass. Under these rumen conditions, the supply of a
and ammonia concentrations are mainly related to complementary feed with a high energy content, but
the pattern of intake. In grazing animals, the primary low in rumen degradable N would help to maximise
determinant of when animals graze is day length, microbial synthesis in both breeds.
with major grazing periods around dawn and dusk In relation to VFA values, both total VFA con-
Arnold, 1981. However, temperature and humidity centrations and molar proportions of the main VFA
may alter when grazing periods begin and end. observed in our study were in the range of those
´ Grazing behaviour was not recorded in our experi-
reported for other pastures Garcıa et al., 1994: ment, but the daily evolution of the rumen parame-
Olson et al., 1994. ters agrees with the observations reported by other
authors Berggren-Thomas and Hohenboken, 1986. During July, when the temperatures were high during
5. Conclusions
