PhD defence: Dietary Medium-Chain Fatty Acids for Sustainable Ruminant Nutrition

Summary of dissertation

Methane produced by ruminants is considered a relevant contribution to the emission of greenhouse gasses and therefore impacts the climate on earth. In the current thesis, the potential of dietary fat sources rich in medium chain fatty acids, such as lauric acid and myristic acid, to reduce methane production is investigated. The inhibitory effect of coconut oil on methane production is well known and it’s depressant effect on rumen methane synthesis is attributed to the high lauric acid content of coconut oil. The use of coconut oil is, however, not feasible under economically less viable conditions. Thus, investigating an alternative source of lauric acid was considered opportune. Krabok oil and coconut oil have a similar lauric acid content but the myristic acid content of krabok oil is almost 2.5 times greater compared to coconut oil.
In contrast to lauric acid however, the potency of myristic acid to reduce on methane production was not clear. Therefore, the effects of krabok oil on methane production were compared with that of coconut oil. In the first in vitro experiment described in the current thesis it was shown that both of krabok- and coconut oil were equally effective in reducing methane production. The observed decrease in methane production was associated with a shift from acetate and butyrate production to propionate thereby explaining the lower methane production caused by coconut- and krabok oil. In a subsequent study with rumen-cannulated beef cows, the effect of krabok oil on rumen protozoa were investigated. In the rumen, methane is synthesized by Archaea which are one-cellular microorganisms who live to a great extent in symbiosis with rumen protozoa. Thus, in case dietary krabok oil reduces the number of rumen protozoa, the Archaea population may also decrease which can potentially lead to a lower methane production. In order to test this notion, beef cows were fed rations supplemented with either 25.5 g/kg of tallow (control) or the same quantity of coconut oil or krabok oil. Unfortunately, a clear depressant effect on the rumen protozoa population could not be demonstrated in the latter study. The lack of effect was most likely related to the low dose of supplemental fat. Therefore, a second in vivo experiment was conducted using a higher dosage of supplemental fat, i.e., 35 g/kg dry matter. In addition, an in-depth assessment of the methanogenic community in the rumen was performed so as to potentially gain more insight in the mode of action of krabok oil. In this study, rumen-cannulated bulls were fed a total mixed ration supplemented with either coconut- or krabok oil.  Both krabok oil and coconut oil caused greater concentrations of rumen volatile fatty acids and shifted the proportions of individual volatile fatty acids from acetate to propionate. Protozoal numbers were reduced by either source of medium chain fatty acids and the strongest reduction was observed when supplemental krabok oil was fed. The abundance of Archaea was likewise affected by the experimental diets. These results are in line with the idea that krabok- and coconut oil, at least potentially, are equally effective in reducing methane production. 
In the aforementioned experiments extracted krabok oil but not the whole krabok seed was used. For practical reasons, it was considered opportune to evaluate the inhibitory effect of whole krabok seed (contains ~ 56% krabok oil) on methane production. Therefore, an in vitro experiment was designed to compare the effects of whole krabok seed versus krabok oil on methane production. 

The results from this study indicated that supplementation of krabok oil  in the form of whole krabok seed is not warranted because the fat extracted residue of whole krabok seed is poorly degraded during fermentation. Overall, it can be concluded that krabok oil is instrumental in mitigating methane emission. However, in view of the potential issues related to fiber digestibility and feed intake, future studies should be conducted to evaluate the practical use of krabok oil in formulating ruminant diets.