Abstract:

This study investigated the effect of difference in particle size on in vitro rumen fermentation and microbial population when Uraria crinita was used as the dietary fiber source for lambs. Six treatments were set up with differing U. crinita particle size （1.00， 2.36， 3.35， 4.75， 8.00 and 12.50 mm） as rumen fermentation substrates incubated 48 hours in vitro using Rusitec-s artificial rumen simulation. There were four replicates per treatment. The results showed that degradation rate of in vitro neutral detergent fiber degradability （IVNDFD）， the molar ratios of butyrate， isobutyrate， valerate， isovaleric， and the copy number of Butyrivibrio fibrisolvens （B.f）in the 2.36 mm particle size treatment were significantly increased （P<0.05）， while the number of total Methanogens （TMe） （P<0.05） was significantly decreased. Total bacteria number was positively correlated with ammonia nitrogen concentration and acetic acid molar ratio （R² =0.84， P=0.038； R² =0.88， P=0.021）. B.f was positively correlated with propionate molar ratio （R² =0.82， P=0.045）， and negatively correlated with pH （R² =-0.98， P=0.001）. TMe were positively correlated with in vitro crude protein degradability （IVCPD） （R² =0.91， P=0.012）， and negatively correlated with in vitro dry matter degradability and IVNDFD （R² =-0.86， P=0.027； R² =-0.83， P=0.043）. Methanobacteriaceae（Mba） was negatively correlated with pH （R² =-0.90， P=0.015）. These results describe how different particle sizes of U. crinita fermentation substrate affect the rumen microbial community structure and population numbers in lamb fermentation broth in vitro， resulting in changes in fermentation outcomes. Under the experimental conditions， the 2.36 mm particle size provided the best fermentation outcome for 3-month-old lambs.

Key words: particle size, in vitro fermentation, Uraria crinita, microbial population