| dc.description.abstract | Six rumen-fistulated Friesian steers (Liveweight = 385 ± 76 kg) were used to
determine the effect of abrupt dietary changes and graded levels of concentrate
supplement on rumen pH, ammonia nitrogen, volatile fatty acid (VFA)
concentration, nutrient digestibility, nitrogen balance and microbial yield.
There were three treatments: Treatment 1: Napier grass offered with a medium
level of concentrate (diet N+m) during the first five days followed by barley straw
offered with a medium level of concentrate (diet S+m) in the next five days. The
ratio of Napier grass to concentrate on OM basis was 5:1 while the ratio of straw
to concentrate was 3: 1. Treatment 2 was similar to treatment 1, but Napier grass
was offered with a low level of concentrate (diet N+I, ratio 16:1), while straw was
offered with a high level of concentrate (diet S+h, ratio 2:1). Treatment 3
(Mixture) was the control and consisted of a "mixture" of Napier grass, straw and
concentrate (N+S+m) offered in a ratio of 3:2:1 daily respectively, on OM basis.
The intakes of the animals were pre-determined and calculated according to
AFRC (1993) and maintained at these levels.
The rumen total VFA concentration for treatment 1 increased significantly
(P<0.05) following a diet change from N+m on day 5 to S+m on day 6. The
mean total VFA concentration was 49.4 mmol/I and 78.9 mmol/I for N+m and
S+m respectively. Similar trends were observed for treatment 2, where significant
increases in total VFA concentration were observed following a change from diet
N+I to S+h. The mean total VFA concentration was 49.7 mmol/I and 74.0 mmol/I
for N+I and S+h respectively. The total VFA concentration for treatment 3
showed minimal variation. The mean total VFA concentration for treatment 3
was 46.1 mmol/I.
The pH trends were a reflection of the total VFA concentration, with pH
decreasing following dietary changes when concentrations of total VFA
increased. However the differences in pH as a result of dietary changes were
not significant (p>0.05) and pH levels for all diets remained above the critical
level of 6.2.
The rumen ammonia nitrogen concentration decreased significantly (p<0.05)
when animals were changed from N+m to S+m in treatment 1. The mean
concentration of rumen ammonia nitrogen was 211.0 mg/I and 71.0 mg/I
respectively, for N+m and S+m. Significant (p<0.05) drops for ammonia nitrogen
were also observed on day 6 following a diet change from N+I to S+h. The mean
concentration for diets N+I and S+h were 151.9 and 68.7 mg/I respectively. The
mean ammonia nitrogen concentration for treatment 3 showed minimal variation
and was relatively stable throughout the ten day measurement period, at 101.3
mg/I.
Increased levels of concentrate supplements had no significant effect (p>0.05) on
the OM and OM digestibility coefficients, and nitrogen balance for Napier grassbased
diets. The reverse was true for straw, with increased levels of
supplementation resulting in significant (p<0.05) increases in OM and OM
digestibility coefficients, and nitrogen balance. The OM digestibility coefficients
for the Napier diets N+I and N+m (0.63 and 0.64, respectively) were not different
(p>0.05) from that of the control (0.60). The OM digestibility of the straw-based
diets (S+m and S+h) were 0.60 and 0.64, respectively. The OM digestibility
coefficients were 0.63 (N+I), 0.63 (N+m), 0.64 (S+m), 0.67 (S+h) and
0.63(N+S+m) and the digestibility of S+h was significantly different (p<0.05) from
the other diets.
The nitrogen balance values were 32.95, 38.71, 33.70, 42.34 and 29.01 g/day for
N+I, N+m, S+m, S+h and N+S+m, respectively. Increased level of
supplementation increased the nitrogen balance for both the Napier-based diets
(p>0.05) and straw-based diets (p<0.05). The nitrogen balance value for
N+S+m was similar(p>0.05) to those of diets N+I and S+m but different (p<0.05)
to those of diets N+m and S+h.
Increased level of supplementation improved (P<0.05) nitrogen digestibility
coefficient for napier diets (0.63 versus 0.68 for N+I and N+m respectively), and
these values were also different (p<0.05) from that of the control (0.57). The
nitrogen digestibility coefficients for the straw-based diets at 0.54 and 0.57 for
S+m and S+h respectively were similar to that of the control (p>0.05).
The NDF digestibility coefficients for the Napier-based diets (0.61 versus 0.59 for
N+I and N+m respectively) were not affected (p>0.05) by increased levels of
supplementation, and they were also similar (p>0.05) to that of the control (0.60).
Increased level of supplementation led to a significant depression (p<0.05) of the
NDF digestibility coefficient of straw (0.65 versus 0.60 for S+m and S+h
respectively), but the value for S+h was similar (p>0.05) to that of the control diet.
From this study, it was concluded that abrupt dietary changes led to marked and
significant disturbances in the rumen environment. The higher variation in rumen
parameters (total VFA and ammonia nitrogen concentrations) for treatment 1
compared to treatment 2 suggested the superiority of treatment 2 over 1. Thus,
adjusting the level of supplementation to a low level with high quality forage and
a high level with low quality forage may lead to better animal performance than
feeding a constant medium level of concentrate irrespective of the quality of
forage. This is especially so when the forage that is fed to animals changes in
quality every few days. | en |
