From NRC 36, CP and ME requirement of our animal (BW 364 kg, milk yield 8.6 kg/d) increased by about 1,190 g/d and 60.9 MJ/d, respectively. Therefore, the nutrient composition in feed, especially CP (provide 1,300-1,400 g CP/d) and ME (provide 79.05–84.14 MJ/d) values, were sufficient in our study for supporting dairy cows’ performance. Furthermore, ensiled RS with P. kudriavzevii KKU20 and C. tropicalis KKU20 (Crabtree-negative yeast) were established as having a low fiber content when compared with adding S. cerevisiae (Crabtree-positive yeast). The low fiber content can be clarified by the yeast’s ability to release cellulase enzymes and digest fiber during the fermentation process. Suntara and Cherdthong 8 confirmed that C. tropicalis KKU20 and P. kudriavzevii KKU20 were more capable to releasing cellulase enzymes than S. cerevisiae by about 0.7 to 6.8 times, respectively. Moreover, the experiment on in vitro gas production of ensiled RS at 14 days with the P. kudriavzevii KKU20 could decrease the NDF content by about 6.7% when compared with S. cerevisiae 15. Ilmén et al. 37 discovered yeast isolated from a plant named C. konsanensis species could excrete cellulase enzymes and digests fiber, and it is a new yeast strain that had not been reported previously. Similar with our study, C. tropicalis KKU20 and P. kudriavzevii KKU20 are great potential yeasts to improve feedstuffs and this study is the first report in ruminant nutrition feed research.
The fermentation quality of ensiled RS with different yeast species indicated that the silage was well preserved. The ensiled RS still maintained appropriate pH, high lactic acid content, and a low NH3-N level. Acceptable silage was defined by the pH value and the composition of their fermentation products 38. The pH is the main indicator for evaluating silage quality and our study showed ensiled RS still has a satisfactory score of about 4.1 to 4.3 39. In addition, pH is highly related with lactic acid content, which in this study showed a consistent range of about 19.8–22.1 g/kg DM. Lactic acid content in silage should range between 21 to 25 g/kg DM to be considered of high quality, according to Flieg’s score 40; therefore, it is close to the high quality of silage. In addition, our result showed lactic acid content similar to an earlier study by Suntara et al. 15 who revealed that about 20.53 to 26.14 g/kg DM of lactic acid was produced when ensiled RS with C. tropicalis KKU20 and P. kudriavzevii KKU20 at 14 day. NH3-N concentration in ensiled RS within the range of 1.80 to 2.00 g/kg DM indicated the normal standards for estimating silage. These results are similar to those of Li et al. 41, who collected information on various types of RS parameters and concluded that RS silage has a NH3-N concentration of approximately 1.61 to 2.36 g/kg DM. Other parameter such as C2 show great value for preserved silage within range 20 to 25 g/kg DM 39. Moreover, after the fermentation process, the moisture content should range from 650 to 750 g/kg to be optimum 16, which in our study showed an average of 722.1 g/kg. Therefore, our study proposes that the nutrients in ensiled RS are still well preserved.
Crabtree-negative or –positive yeast has no effect on the dry matter intake (DMI). Our results showed that the DMI (range from 2.6 to 2.8 %BW) was similar to previous experiments, which is that feeding separate ensiled RS with a concentrate diet to dairy cows creates a DMI range from 2.5 to 3.2 %BW 42,43. Generally, the amount of RS that an animal intakes daily is limited to around 2.0 % BW or less BW 44. Because RS is rich in polysaccharides and has a high lignin and silica content, and thus it limits the voluntary intake 45. However, Aquino et al. 46 reported that the amount of RS that ruminants can consume can be as high as 1.2% BW, which is similar with our result of 0.8–1.0%BW. The intake of OM, EE, NDF, and ADF was similar with previous studies of lactating crossbred dairy cows 47,48. The CP intake (CPI) in this study was also similar with Wanapat, et al. 43, which used lactating crossbred dairy cows (50% Holstein Frisian × 50% Thai native cows) and BW around 365.5 kg, and the CPI was about 1.0 to 1.2 kg/d. Typically, the CP found in tropical forage plants is often relatively low 49. Especially in RS (3%CP) when using a roughage source it can have an effect on the animal’s yield adequacy 50. However, our study showed that ensiled RS with yeast could support protein from yeast to low quality roughage as RS, and the enhanced intake of protein were high enough to meet the requirement of tropical lactation dairy cows.
The dry matter digestibility (DMD) was increased when ensiled RS with Crabtree negative yeast was offered to animals. This strain is outstanding in terms of high proliferation ability and its high yield of cellulase enzymes 15. The improved digestion may be due to the potential of how rumen microflora are promoted for better digestibility. Yeast is an important biological responder in the rumen fermentation, live yeast cells improve microorganisms in rumen 51 and stabilizes pH in the rumen 52. Habeeb 53 stated that yeast could provide rumen with biological stimulants, which is necessary for microorganisms’ growth in the rumen. Therefore, yeast contributes to establishing microbiota 54 and is why the digestibility was apparently enhanced. This is consistence with Wang et al. 6, who found that Crabtree-negative yeast as C. tropicalis could increase digestion in the in vitro technique and that it generated 3.03% more gas production than did S. cerevisiae.
However, Crabtree-negative yeast did not change the apparent digestibility of OM, CP, NDF, and ADF. The digestibility of NDF and ADF are similar among Cabtree-negative and positive yeast (601.50 vs 650.05 g/kg DM and 492.8 vs 518.15 g/kg DM, respectively). Noticeable changes occurred after the silage process was complete, but when the animal intakes the feed, its digestion was not altered. The reason for this is still not clear, but it is possible that yeast does not react directly on RS. Rather, digestion in the rumen occurred by the cooperation of microbes’ synergy until the resulting values were not statistically different. This is similar with an experiment by Suntara et al. 15, who compared the effect of Crabtree-negative and -positive yeast on ensiled RS on the in vitro gas and confirmed that in the rumen, there was no difference among yeast species in the digestibility of NDF and ADF (705.2 vs 703.6 and 464.8 vs 464.4 g/kg DM).
Ensiled RS with the P. kudriavzevii KKU20 and C. tropicalis KKU20 (Crabtree-negative yeast) could increase bacterial populations when compared to S. cerevisiae (Crabtree-positive yeast) by about 4.76%. The ruminal bacterial populations depend on sufficient nutrients or stimulants supply 53. Yeast is a great supply to stimulate bacteria because it is enriched in essential substances 55. Previous studies have confirmed that yeast could supply essential amino acids, vitamins, and minerals to increase the ruminal bacteria more than without yeast 2,56. The key explanation is that under aerobic conditions, Crabtree-negative yeast may proliferate more than Crabtree-positive yeast since the enzyme mechanism functions differently 9,57. Suntara and Cherdthong 8 found that at 72 h of incubation time, P. kudriavzevii KKU20, C. tropicalis KKU20, and S. cerevisiae had growth by about 10.02, 9.6, and 8.87 Log cells/ml, respectively. The high amount of Crabtree-negative yeast creates a greater supply of essential nutrients to the rumen bacteria 15, thus the amount of rumen bacteria is increased in response to the Crabtree-negative yeast.
The ensiled RS with Crabtree-negative yeast has more effect on the total VFAs than with Crabtree-positive yeast by about 6.1% at the mean value. The high production of total VFAs in rumen fluids is related to the amount of ruminal bacteria 58. The great bacterial population could enhance carbohydrate digestion and then the animal obtains the greater VFAs 59. This is similar to Castillo-González et al. 60, who stated that the expansion of rumen microorganisms could increase the quantity of rumen VFAs. Certainly, a high bacterial population in our experiment was related with the Crabtree-negative yeast’s effect. Nonetheless, the direct influence of the Crabtree-negative yeast on rumen bacterial populations was unclear and this hypothesis required further research to be conducted. However, expanding the Crabtree-negative yeast population (during fermentation process) may be more effective than expanding that of the Crabtree-positive yeast (S. cerevisiae). This suggests that animals have a greater chance of obtaining stimulants for activate rumen bacteria. In agreement with our results, Wang et al. 6 compared the effect between Crabtree-negative yeast (C. tropicalis) and Crabtree-positive yeast (S. cerevisiae) for in vitro gas technique and found that the inclusion of 0.25 × 107 of Crabtree-negative yeast could enhanced the total VFAs by 7.7%. Moreover, Suntara et al. 15 reported that Crabtree-negative yeast (P. kudriavzevii KKU20) increased the total VFAs by 2.3% for in vitro gas study more than Crabtree-positive yeast.
The milk yield and milk composition of ensiled RS with Crabtree-negative yeast did not have any impact. Our study showed that the actual milk yields are about 8.5 to 8.8 kg/h/d, which are slightly lower than previous trials using early to mid-lactation cows (12.6 kg/h/d according to Supapong and Cherdthong 61; 11.1 kg/h/d according to Wanapat et al. 43). To produce milk, cows must calve and split its lactation cycle into four phases (early, mid, late lactation and dry period) 62. The milk yield response was greater in the early lactation, and in the mid-lactation period, the milk yield begins to decline from its peak 63. Therefore, the lower actual milk yields in this study may be because dairy cows were in mid to late lactation (DIM 165.5 to 186.5). Our study indicated that daily protein yields in milk of the C. tropicalis KKU20 group was highest at 35.6 g/kg and lowest when applied with S. cerevisiae and P. kudriavzevii KKU20 in ensiled RS at 34.5 and 34.1 g/kg, respectively. Milk protein is associated with the feed degradation energy supply as VFAs and microbial protein (MCP) synthesis 64. High amounts of microorganisms in rumen could affect the MCP synthesis. This will be the supply protein and amino acids (AA) in the small intestine and could enhance the milk protein yields 65. Our result clearly demonstrated that C. tropicalis KKU20 was unique in the highest bacterial population (11.2 Log10 cell / ml), which is why the increase in milk protein yields occurred. Furthermore, there were no differences in milk proteins between S. cerevisiae and P. kudriavzevii KKU20. This suggests that the influence of Crabtree-negative yeast may play different roles in terms of milk quality. This thought is support by Intanoo et al. 66, who compared different yeast strains that were in the same group of Crabtree-negative, and found that P. kudriavzevii KKU20 decreased daily protein yields in milk by 14.9% when compared with Kluyveromyces marxianus in crossbred lactating cows. This yeast species could provide high biomass, which possibly supplies more amino acid sources for milk protein synthesis. This is similar to Wardrop et al. 11 who stated that K. marxianus has an outstanding ability to provide high biomass when compared with other strain. The explanation is limited in regard to P. kudriavzevii’s impact on daily protein yields in milk. A few studies have focused on applying non-S. cerevisiae to dairy cows and further research about the influence of each strain is required.
Based on this study, we conclude that Crabtree-negative yeast-treated RS, especially C. tropicalis KKU20, could enhance the RS’s nutrition value through increasing DMD, the ruminal bacterial population, and total VFAs. In addition, C. tropicalis KKU20 could increase the milk protein when compared with other groups. However, there are certain drawbacks associated with the high-producing lactating cows influenced by C. tropicalis KKU20 treated RS, which requires further investigation.