In the previous 12 interesting facts on milk fat depression, we focused on the effect of rumen pH on milk fat depression. In these 12 interesting facts, we look at the effect that unsaturated fatty acids may have on milk fat depression. Combination of low rumen pH and high UFA may have a large negative impact on milk fat yield.

1. Different factors influence milk fat depression (MFD). This includes rumen pH, unsaturated fatty acids (UFA) and digesta passage rate (Jenkins et al., 2020).
2. Grass, especially young pastures, can contain between 3 – 10% lipids (DM). Much of the lipids is contributed by the chloroplasts (Bauchart et al., 1984). Approximately 95% of the lipids consist of Linolenic acid (C18:3), Linoleic acid (C18:2) and Palmitic acid (C16:0).
3. UFA has antibacterial characteristics and can inhibit or kill rumen bacteria. Maia et al. (2010) reported a > 96% reduction in metabolic activity in the biohydrogenating ruminal bacterium, Butyvibrio fibrisolvens, within 20 minutes following the addition of 0.2 mg/mL of linoleic acid to cultures.
4. Low pH and lactic acid accumulation were both shown to accentuate antibacterial effects of unsaturated fatty acids (Maia et al., 2010).
5. To protect the rumen against the negative effects of certain UFA’s, fibre-digesting bacteria are primarily responsible for ruminal biohydrogenation of fatty acids (Harfoot and Hazlewood, 1997). Biohydrogenation is a process in the rumen where unsaturated fatty acids are enzymatically converted to saturated fatty acids (Jenkins et al., 2008).
6. Normal biohydrogenation pathway produces milk fat friendly intermediates, like trans-11C18:1. When the pH in the rumen drops and/or high levels of UFA’s, the biohydrogenation pathway changes and milk fat depressing intermediates, like trans-10 C18:1 and trans-10 C18:2, are produced (Jenkins, 2020).
7. Not all UFA’s are biohydrogenated in the rumen. According to Loor et al. (2004b) between 75% – 97% of C18:3 and C18:2 are biohydrogenated in the rumen. Therefore, between 3% – 25% of UFA’s can escape the rumen and could have a possible negative effect on milk fat yield.
8. There is a strong correlation between trans-10CLA flowing out the abomasum and milk fat yield. 5g of trans-10 C18:2 can reduce the milk fat yield by almost 30% (Bauman & Griinari, 2003).
9. Decreasing the rumen fluid pH from 6.4 to 5.6, while using the same diet, increased trans-10C18:1 (milk fat threat) from 0.35% to 18.3% of total C18 fatty acids, indicating the large impact the rumen fluid pH has on the amount of trans-10 C18:1 production (Fuentes et al., 2009).
10. The biohydrogenation process takes time. Increasing the digesta passage rate due to low eNDF levels will decrease the % of UFA’s that are biohydrogenated, increasing the risk of trans-10C18:1 and trans-10C18:2 to escape the rumen and reduce the milk fat yield (O’Dell et al., 1963).
11. It can take up to 21 days for the milk fat % of a dairy cow to recover from a MFD incident (Rico et al., 2013).
12. Dairy cow on young pasture or high concentrate diet: High UFA content in diet + low rumen pH + high digesta flow rate = MFD. Higher rumen pH will allow microbes to handle high UFA levels, reducing the risk of MFD.