Abstract :
Despite the increased use of dry active Saccharomyces cerevisiae yeast supplementation in
ruminant feeding, inconsistent results among studies hamper the prediction of its effects
on animal performance. A meta-analysis has been conducted to quantify the magnitude
of yeast supplementation effects on ruminal parameters, total tract nutrient digestibility,
growth and feed conversion across different studies with sheep. Different methodologies
and small numbers of studies necessitated the use of the classical effect size method, in
which a unitless standardised effect size (Hedges’s g) was used to calculate differences
obtained in outcomes between supplemented and non-supplemented sheep. Summary
statistics across studies were calculated with fixed and random effects models, whereas
subgroup-analysis and meta-regression were applied to identify possible interfering factors
that could be responsible for between-study variability. Possible publication bias was
evaluated with graphical and statistical tests. Effect sizes for ruminal ammonia nitrogen
(33 comparisons), pH (42 comparisons) and total volatile fatty acids (38 comparison) did
not (P > 0.10) present heterogeneity among studies, and was not affected by yeast supplementation.
No effects (P > 0.05) were detected on the stoichiometry of volatile fatty acids
or protozoa counts. Effects sizes calculated for digestibility of dry matter, organic matter,
crude protein, acid-detergent fibre and neutral-detergent fibre, which included from
17 to 28 comparisons, showed considerable (>50%) between-study variability. This variability
could not effectively be explained by the categorical variables (1) mode of yeast
application, (2) feed intake (ad lib versus restricted) or (3) faeces collection method, or
the continuous independent variables (1) adaptation period, (2) study period, (3) dietary
roughage concentration and (4) dietary crude protein concentration. According to random
effects models, digestibility of dry matter, organic matter and crude protein were
increased by yeast supplementation, with no effects found for digestibility of fibre components.
Substantial unexplained between-study variability (50–90%) was found for growth
(13 comparisons) and feed intake (9 comparisons). This meta-analysis presented evidence
that addition of dry active S. cerevisiae yeast to diets did not have any effect on growth, feed
conversion, ruminal parameters or fibre digestibility in sheep.
