In vitro experiment
This in vitro experiment was conducted to select optimal dosage of PO and SBO which could be used in in vivo experiment by considering the effect of the oils on production of methane, CLA and influence on ruminal fermentation.
In this experiment, ruminal pH was not affected by PO or SBO (Table 1). Some studies have reported that ruminal pH could be variously changed by high-concentrate diets in feed and the addition of fat source. High-concentrate diets supplemented with sunflower oil can promote the fermentation of rumen microbes and consequently decrease ruminal pH 27. Doreau and Ferlay 28 have reported that ruminal pH does not change even in high-energy diets such as SBO. As mentioned before, the variety of ruminal pH could be affected by changes of ruminal fermentation patterns. Considering that neither SBO nor PO affected ruminal pH value, these sources might not directly affect the environment of ruminal fermentation.
Results of our experiment showed that there was no significant difference in the amount of ammonia concentrate in the group treated with 0% PO at 48 h (Table 1). However, ammonia was affected by SBO (increase of ammonia in SBO supplementation group with PO at 0.1%). The synthesis of ruminal ammonia nitrogen is also known to be influenced by the composition of fat source. Beauchemin et al. 29 reported that the supplemented fat sources can be used as an energy source for rumen microbes, affecting ruminal fermentation patterns and the synthesis of microbial protein. However, it is also known that fat additives, which can be used as an energy source, have various effects on ruminal pH and microbial activity, depending on the fat composition 28. In this study, when considering that the ammonia-N concentrate was numerically higher in 0% than in 0.1% PO treatments and the ammonia was increased by the SBO at 0.1% PO treatments, it is speculated that PO and SBO affected the synthesis of microbial protein and ammonia-N differently.
Generally, total gas production and methane levels in rumen are negatively correlated in in vitro experiments with fat supplement. This is because the fat source can facilitate ruminal lipolysis, resulting in decreased methane and increased total gas production by affecting ruminal microbes 12,22. Wettstein et al. 30 have reported that fat supplement in diet can reduce total gas production, which means exhausted energy. Thus, they suggested that fat supplement could improve the energy availability of microorganisms by reducing the exhausted energy in rumen. On the other hand, it has been reported that the addition of unsaturated fatty acids in feeds can act as a hydrogen sink in the rumen besides lipolysis, inducing hydrogen ions that could be used for methane production, to the process of converting unsaturated fatty acid into saturated fatty acid 22.
In addition, as with PO, the essential oil has antimicrobial effects in the rumen, and results in lowering gas production during ruminal fermentation 31. Considering all these phenomena, the results of this study indicate that the reduced total gas and methane production were affected by the hydrogen sink effect and especially accelerated the lipolysis effect of the oil; the effect could be confirmed more severely at 0.1% PO and 2% SBO addition (Table 1).
Factors affecting VFA compositions in association with the rumen fermentation environment are various, including the ratio of forage to concentrate, high protein, carbohydrate and fat content in feed, supplemented fat composition, and so on 30,32. Depending on the fermentation rate of feed in rumen, ruminal pH and VFA composition changes are associated with each other. For example, the rate of fermentation in the rumen in the case of overfeeding concentrate could be faster than that in the case of feeding low concentrate, resulting in lower ruminal pH, lower levels of acetate and butyrate, and increased propionate ratio in the VFA composition. In particular, it has been reported that oil supplement in feed can be fermented by ruminal microbes and used as an energy source, thereby modifying the VFA composition by reducing the pH in the rumen 33. As previously mentioned, the addition of unsaturated fatty acids in the rumen can increase the production of propionate, act as a hydrogen sink, and reduce methane production. Castillejos et al. 34 have reported that antimicrobial materials such as essential oil can increase the ratio of acetate to propionate in calves fed a 6: 4 alfalfa hay: concentrate diet. In contrast, Cardozo et al. 35 have observed that the ratio of acetate to propionate is decreased in calves fed a 1: 9 straw: concentrate (based on corn, barley, and soybean meal) diet supplemented with essential oil. It has been reported that effects of essential oil on ruminal pH and feed composition are very diverse 31. Considering these results, further research is needed to determine the effect of plant-derived essential oil on ruminal VFAs.
In results of long chain fatty acids in rumen, consecutive changes of linoleic acid were observed (Table 3). In the early stage of fermentation, TVA from linoleic acid was increased by the SBO supplement. CLA was increased by SBO and PO. Linoleic acid, which is found in SBO, is known to produce CLA and TVA through isomerization and biohydrogenation by microorganisms in the rumen 36. According to Kairenius et al. 37, fatty acid metabolism is affected by the composition of the added fat. Higher degree of unsaturation of the added fat causes more extensive metabolism of fatty acids. In addition, Kim et al. 16 have reported that the addition of not only fat, but also antimicrobial essential oil such as PO, can induce an increase of CLA in the milk of ruminants. Taking all these studies into account, the increase of CLA in the present study might be due to effects of SBO containing a large amount of unsaturated fatty acids and PO. Furthermore, it was speculated that the most activation of inducing hydrogen sink might be occurred by 0.1% PO and 2% SBO addition when considering most decreases in methane and VFAs and increase in CLA production simultaneously.
In vivo experiment
This in vivo experiment was carried to confirm the effect of selected supplementation (0.1% PO and 2% SBO) in in vitro experiment on increasing milk CLA and depressing methane emission by using dairy cows.
As expected from results of in vitro experiment (Table 6), CLA percentage and CLA yield were higher in the PSO group. According to Wang et al. 36, CLA in milk could be increased when linoleic acids, the same as SBO, are added to the diet. The problem is that increased t10, c12 CLA often decreases milk fat as a result of increased total CLA. In the current study, milk fat percentage was decreased. However, milk yield was higher in PSO group than control thus, amount of milk fat (kg/d) was not difference between two groups. In addition, total CLA yield was not decreased, although t10, c12 CLA was higher in the treatment group than that in the control group. Considering all these phenomena, PO might have induced more c9, t11 CLA isomerization than t10, c12 CLA isomerization by affecting related microbes (e.g., Butyrivibrio fibrisolvens). In addition, increased UFA and PUFA levels were observed at the same time in the treatment group. All these differences between the two groups could be explained by the amount of linoleic acid in added SBO and PO inducing hydrogen ion to isomerization of linoleic acid from what might be methanogenesis results of in vitro experiment. Thus, induced isomerization in the treatment group might have increase UFA, PUFA, and CLA compared to the control group. In addition, stearic acid was higher in the PSO group at 35 d. This higher stearic acid could be explained by greater microbe biohydrogenation of linoleic acids in the PSO group fed more linoleic acid than the control group.
In the methane production results, one lack was the measuring methane per pen which was not individual animal, even though we tried to redress this limitation by measuring the methane 3 times during the experiment. However, interestingly, Figure 2 shows the adaptability of cows to the PSO treatment, as it shows considerable increase in 7 d, and gradual increase after 7 d to the end of the experiment. Generally, the production of methane in ruminant is decreased when dietary fat is added, and it is reported that the effect is varies greatly, depending on the constituents or unsaturation of added fat 12,22. As mentioned earlier, the addition of unsaturated fatty acids is known to induce fatty acid metabolism and inhibit methane production in rumen microbes. In the result of gas production in in vitro experiment, it is observed that the addition of SBO, which contains a large amount of unsaturated fatty acid, decreased methane production, and PO addition resulted in more decrease in methane. Considering the results of fatty acid in rumen and milk, the CLA enhancement effect of the two oil sources confirmed in this study may ultimately lead to a decrease in methane production, by competition of hydrogen ion between biohydrogenation and methane production.
Complete blood cell counting (CBC) is a representative physiological index of animals. It was used in this study for the purpose of verifying the safety of experimental additives through animals. Some researchers have shown that oil supplements can affect animal disease because of the negative effect of unsaturated fatty acid in rumen 38. However, in this study, there were no significant differences in results of CBC (white blood cell, lymphocyte, monocyte, or other indices) between control and treatment groups containing PO and SBO. Thus, the addition of PO and SBO supplement to the diet did not lead to disease-related chemical changes in dairy cows (Supplemental Table S2).
Some researchers have found a relationship between fat supplement and blood metabolites 39,40. According to Henderson et al. 39, fat addition in ruminant diet affects the synthesis of microbial protein in rumen. The added fat is used as an energy source for protein synthesis, resulting in increasing amino acid-N and decreasing ammonia-N in the rumen, eventually reducing nitrogen content in the blood. It is known that fat degraded by lipolysis in rumen is fermented to increase propionate content in VFA compositions, thereby increasing glucose content in the blood by gluconeogenesis 41. Similar to results of previous studies, the present study also showed that the PSO group containing oil supplement in feed had lower blood BUN but higher glucose concentration than the control group (Supplemental Table S3). Blood magnesium was also higher in the oil supplement group compared to the control. In general, most of the digestive and absorption lipid is transferred itself or binding to protein. However, some proportions of the lipid binding to magnesium are presented in the blood. Thus, the higher blood magnesium in our result might be affected by the oil supplement in the diet 42. The reason why blood Ca showed a tendency to be significant was unknown. This parameter was inconsistent with other plasma outcomes.
Fecal microbes are microorganisms involved in the digestive physiology of livestock. Their digestibilities have been evaluated according to their populations 43. Currently, antimicrobial effects of essential oils have been extensively studied in rumen. However, their effects on post-ruminal digestive have not been studied in detail yet. Furthermore, although the essential oil PO used in the experiment is also known to have antimicrobial effects, it is not yet known whether such effect could affect the whole ruminant digestive system 44. However, as shown in the present study, numbers of pathogens in fecal microbes decreased due to the addition of PO and SBO. This demonstrates that the antimicrobial effect of PO also affects post-ruminal digestive system. Thus, the result of decrease in pathogenic microbes by PSO could support that PO might also be able to improve the energy availability of feed in ruminant (Figure 1).
Generally, some studies have reported that milk production is positively correlated with the energy content in feed 45. As expected, this study also showed that the addition of 2% fat source, a high energy source, resulted in an increase in milk yield. Bauman 45 has reported that the increase in glucose in the blood is positively correlated with milk yield. In particular, the fat supplement used in this study increased glucose in the blood by increasing propionate in the VFA composition due to the effect of SBO and PO on rumen fermentation in in vitro experiment. Furthermore, the antimicrobial effect of PO might have affected fecal microbes. Thus, PO might be able to increase milk yield by improving the energy availability of SBO and basal diet (Table 4).
From milk composition data (Table 5), milk fat content (kg/d) and 4% FCM were similar between control and treatment groups. This is different from the result of Rico and Harvatine 15. This might be because SBO has an effect on milk fat depression by increasing trans-10, cis-12 CLA known to inhibit the synthesis of milk fat. In our result, the unchanged milk fat in the treatment group may be explained by the effect of PO on isomerization to cis-9, trans-11 CLA rather than to trans-10, cis-12 CLA. Milk nitrogen is generally influenced by blood nitrogen and ammonia-N in rumen. Thus, decreased MUN in the PSO group could be explained by the flow of decreased BUN in blood (Supplemental Table S3).
In summary, a mixture of 0.1% phytoncide oil (PO) and 2% soybean oil (SBO) in diet can lead to an increase in CLA and a decrease in methane production in rumen by inducing hydrogen ion from methane production to biohydrogenation, especially isomerization of linoleic acid with no change in milk fat. Therefore, a combination of PO and SBO could result in increased milk CLA and depressed methane production with no influence on milk fat.