The purpose of this study was to investigate the factors affecting somatic cell counts (SCC) and milk composition of Chinese Holstein cows in Ningxia. The correlation of 198 855 measurements of the dairy herd improvement (DHI) data of 19 020 Chinese Holstein dairy cows (from 2009 to 2018) from 7 different dairy farms was analyzed using Minitab software. The results showed that 1) SCC was significantly correlated with farm, year and parity (P < 0.05); 2) SCC fluctuated with season and calving month, and decreased first and then increased with increasing lactation length; and 3) there were significant correlations between SCC and milk yield, milk fat percentage, milk protein percentage, milk sugar percentage and dry matter percentage (P < 0.05). Moreover, SSC, milk yield and the major milk components were influenced by environmental factors (farm, year and season) and cow’s own factors (parity, calving season and lactation period). In summary, fine management is an effective avenue to improve milk production and quality.

In this study, 30 strains of S. cerevisiae were examined by in vitro gastrointestinal transit, bile salt tolerance and microbial inhibition tests to screen strains having potential probiotic properties and a symbiotic relationship with Lactobacillus plantarum IMAU10120 from them. The results showed that S. cerevisiae IMAU6Y047, IMAU7Y006, IMAU7Y007 and IMAU7Y036 were bile salt-tolerant strains and had a high survival rate over 50% during gastrointestinal transit. They were also resistant to E. coli O157, Salmonella typhimurium and Shigella flexneri. In addition, IMAU7Y006 had mutualism relationship with Lactobacillus plantarum IMAU10120. This study may provide basic data for further study on mixed-strain fermentation.

The impact of mixed starters consisting of Streptococcus thermophilus PZST4 and Lactobacillus bulgaricus PZLB5 in different proportions on the fermentation period of fermented milk and its acidity, texture, syneresis and flavor during storage was investigated. The results showed that viable cell ratios between Streptococcus thermophilus PZST4 and Lactobacillus bulgaricus PZLB5 had a striking effect on the quality of fermented milk, and a ratio of 50000:1 was the optimal combination based on the quality parameters tested.