Varietal effects of barley carbohydrate composition on digestibility, fermentability and microbial ecophysiology in an in vitro model of the pig gastrointestinal tract.

Carbohydrate (CHO) composition can vary markedly between barley
varieties. Their influence on digestibility, intestinal fermentation and
microbiota in pigs was studied in vitro. Ten hulless (HLB) and 6
hulled barleys (HB) differing in B-glucan, non-starch polysaccharides
(NSP), starch content, and amylose/amylopectin ratio, were hydrolyzed
enzymatically and subsequently fermented for 72h. CHO fermentation
kinetics were modeled; microbial composition and short-chain fatty
acid (SCFA) production were analyzed.
In HLB, in vitro DM digestibility was positively correlated to starch
and amylopectin content and CP digestibility to amylopectin (P<0.05),
whereas both were negatively correlated to insoluble NSP (P<0.05). Rate
of fermentation was different (P<0.01) between barley types but not
correlated to the CHO composition. However, high B-glucan contents
induced faster fermentation (P<0.05, HLB; P<0.10, HB). SCFA molar
ratios after fermentation of HLB were higher in propionate and branchedchain
fatty acids and lower in acetate compared to HB (P<0.01). With
HLB, amylose content was positively correlated to butyrate production
and negatively to propionate, which was positively correlated to soluble
NSP content (P<0.01). In HB, no correlation between SCFA production
and the carbohydrate composition was found. TRFLP analysis
revealed that Bacteroides and members of Clostridium cluster XIVa
were differentially affected in HLB compared to HB as well as by
the type and source of CHO. Microbial profiles were also correlated
(P<0.05) to SCFA and fermentation parameters but response differed
significantly between HB and HLB. The strongest correlation between
CHO structure, microbial abundance and fermentation parameters was
evident in HLB. Hulless barleys may offer the greatest opportunity to
improve gut health in pigs.