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Optimisation of Nutrient Supply for Beef Cattle Fed Grass or Silage.
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2001-05-01
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Moloney, A.P., O'Kiely, P., Hickey, M.C., Adams, L.A., Optimisation of Nutrient Supply for Beef Cattle Fed Grass or Silage, End of Project Reports, Teagasc, 2001.
Abstract
Since forage forms a large part of growing ruminant rations in Ireland,
the trust of this project was to examine the effect of ensilage on
ruminal digestion of grass and to examine ruminal microbial protein
and intestinally absorbable protein supplied by grass and/or clover. A
range of in vitro and in vivo techniques were employed and strategies
used by commercial beef producers to optimise cattle growth (and
nutrient supply) were also documented.
To accomplish the aims of this project, a range of methodology developments/
modifications in vitro and in vivo was carried out. From in
vitro methodology development it was concluded that :
(i) Compared with fresh silage, drying per se may give artifically
higher rates of dry matter (DM) digestion.
(ii) Greater experimental precision can be obtained by
ensuring a greater substrate surface area to reaction volume
ratio in each reaction vessel.
(iii) For studies where the rate of digestion is of greatest
importance, pre-incubation of frozen inoculum in a nutrient
medium best simulated the cellulolytic activity of
unfrozen inoculum. In studies that require large volumes
of inoculum for extended work, freezing directly is justified.
(iv) Neutral detergent extraction altered in vitro digestion
characteristics of silage. The residue after washing with
water at 70°C has a high residual fibre concentration
and is more representative of the structural components of
silage ingested by ruminants. (v) A semi-continuous culture system developed at Grange
Research Centre can successfully model in vitro ruminal
digestion of fibre and starch-based diets in a controlled
environment.
From in vivo methodology development it was concluded that :
(i) Oven drying at 60°C and correction for loss of volatiles gives a
good estimation of DM concentration of ruminal particulate
digesta. This procedure has the added advantage that drying
at 60°C allows the residual materials to be analysed for
fibre fractions without concern for heat damage which can
occur at a higher drying temperature.
(ii) A naso-ruminal sampling device can be used to measure
the relative patterns of fermentation of contrasting
diet types when in situ for up to 7 days.
(ii) Application of a vacuum to withdraw samples had no
negative effect on ruminal fluid variables.
From in vitro studies on grass digestion, it was concluded that :
(i) Ensiling of grass decreased the apparent extent of digestion of
cell walls when in the presence of the whole
plant and that this largely reflected an increase in the lag
time before digestion commenced.
(ii) Ensiling of grass did not negatively affect the digestion of
isolated cell walls.
(iii) There is a negative impact of ensiling on microbial protein
production from the water soluble carbohydrate fraction of
grass. (iv) Supplementation with the water soluble fraction of grass
significantly improved the apparent extent of digestion for
ensiled forages when compared with the supplementation
of the post-ensiling fraction in a batch culture system.
(v) There is a negative impact of maturity on the pattern of
cell wall fermentation and that this impact can be decreased
by ensiling method.
From studies on herbage digestion in vivo it was concluded that :
(i) Grass silage type had a greater effect than the rate of
concentrate fermentation on ruminal microbal protein
synthesis.
(ii) Harvesting time had a bigger impact on nutrient supply from
herbage than sward type (grass or grass/clover).
(iii) Increasing clover content in the herbage decreased the
biological value (g nitrogen retained/kg absorbed) of dietary
protein.
Diverse stratgies were used on commercial beef farms to optimise
nutrient supply and animal growth. Average animal performance on
individual farms was not better than would be typically recorded in
a research environment. There was scope on many of the farms to
improve technical performance and to decrease the costs of production.