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  • The Cornell Nutrition Conference 2020

    “…an imbalanced rumen environment is the basis of milk fat depression.”

    This annual event was held as a virtual conference this year, and there were several papers devoted to current knowledge relating to milk fat. Although most of the work is based on research in North America, the conclusions are relevant everywhere. Dr. Kevin Harvatine from Penn State gave a talk ‘Maximising Milk Fat Yield’, noting that where farmers are paid for milk solids, it is usually more economical to raise low milk fat than low milk protein. Milk yield is affected by genetics, season (see below), days post-calving and parity. Milk fat percentage is the most heritable production trait, and Dr. Harvatine showed that average milk fat percentage in the US had been about 3.7% consistently from 2000 to 2010, since when it had risen steadily to 3.9% in 2018. There are probably many reasons for this, including selecting for milk fat. However, nutritional factors are very important, as well. Diet-induced milk fat depression (MFD) is a well-recognised condition. A cow’s diet usually contains about 3% fat (without any fat supplements); the majority of these fatty acids are unsaturated, which are toxic to rumen bacteria. Therefore, the rumen microbes saturate (or biohydrogenate) these fatty acids – the normal chemical route is the ‘trans-11 biohydrogenation pathway’. However, in certain circumstances – low rumen pH and/or a high level of dietary unsaturated fatty acids – the ‘trans-10 biohydrogenation pathway’ is used to de-toxify these fatty acids. These trans-10 fatty acids have been shown to cause milk fat depression by reducing fat synthesis in the udder.

    To avoid MFD, we need to maintain a healthy rumen environment, by considering: quantity and type of dietary fatty acids (of the unsaturated fatty acids, linoleic acid (C18:2) is the most important), rumen fermentability of the diet (carbohydrate quality and quantity), adequate RDP, feeding management (avoid sorting and slug feeding), forage characteristics, rumen pH, as well individual cow factors – higher yielders are more prone to MFD. One of the intermediates in the ‘trans-10 pathway’, trans-10 18:1, seems the best monitor of MFD; normal level of this fatty acid in milk is 0.3-0.5% (of total fat) and levels > 1.0% are highly correlated with total milk fat < 3.2%. Levels of ‘De novo’ fatty acids, which are those synthesised in the udder, aren’t as highly correlated with total milk fat.

    Dr. Harvatine also discussed dietary supplements that can increase milk fat. ‘De novo’ fatty acids contribute about 45% of the fatty acids in milk; the remaining 55% are (preformed) long chain fatty acids from dietary sources (and fat mobilisation in early lactation). Determination of how much and of which type of fatty acids to feed is complex. Palmitic acid (C16:0) is used at 15-20% efficiency, ie for every 100g fed, 15-20g appears as milk fat. He showed studies with different rumen-protected methionine sources, which increased milk fat levels, although the mechanism is unknown. Recent work has shown that dietary sodium acetate increases milk fat, by supplementing the acetic acid produced as an end product of rumen fermentation and subsequently used as fatty acid building blocks in the udder.

    Dr. Tom Jenkins from Clemson began his talk ‘Managing the Diet to Control Ruminal Fatty Acid-Microbial Interactions that Reduce Milk Fat Synthesis’ by drawing attention to the quote at the top of this article, from an Invited Review in the Journal of Dairy Science (Dewanckele et al., 2020); this highlights the integral role of a healthy rumen in maintaining normal milk fat levels. Dr. Jenkins developed the concept of RUFAL (Rumen Unsaturated Fatty Acid Load), which is measure of those fatty acids referred to above, which, when present in large amounts, divert rumen biohydrogenation down the trans-10 pathway.  The transition from trans-11 to trans-10 pathway, when RUFAL is high, occurs because the activity of the main bacteria of the trans-11 pathway, B. fibrisolvens, is reduced by more than that of the trans-10 pathway’s main bacteria, C. acnes (52% vs 14%). This reduction in activity is due to the disruption of the bacterial outer membrane by unsaturated fatty acids. The trans-11 pathway bacteria are more sensitive to the effects of RUFAL, when pH is low (or lactic acid concentration high). Additionally, ionophores (eg monensin) can contribute to increased sensitivity to RUFAL, whereas it appears that methionine reduces sensitivity, although this mechanism in unknown. (This parallels the methionine result described above.) Therefore, a low rumen pH coupled with high levels of unsaturated fatty acids, especially linoleic acid (C18:2), will cause a drop in milk fat via the rumen biohydrogenation pathway shift.

    Dr. Jenkins showed a graph of several studies correlating increased dietary starch with decreased milk fat (R2 = 0.67), which we would expect from the rumen pH effect described above. However, perhaps less expected, was recent work (Razzaghi et al., 2020) showing that feeding molasses resulted in increased milk fat, decreased time of rumen pH < 5.8 and decreased levels of trans-10 fatty acids. This does support earlier work where increased dietary sugar levels resulted in higher rumen pH values. The mechanism for this effect is unclear. While focusing on RUFAL, Dr. Jenkins also drew attention to increased feeding frequency, increased feed space, decreased over-crowding and decreased sorting of feed all being associated with increased milk fats.

    A RUFAL risk factor is applied which reflects the proportion of the fatty acids available in the rumen; for instance, hay has a low risk and bakery waste a high risk. The use of RUFAL to calibrate rations is particularly relevant in North America, where dried distillers grains (DDG) are often readily and cheaply available as a by-product of bioethanol manufacture. DDG has a high RUFAL risk factor, so care must be taken over its inclusion rate.

    Dr. Harvatine gave a second presentation, ‘Impact of Daily and Seasonal Rhythms in Maximising Milk Production’. Seasonal rhythms have evolved to coordinate metabolism with weather and food supply. The cue is light/dark, both day length and rate of change of day length; although in real life these change with outside temperature, and the effects of each are sometimes hard to separate. Data from the US showed that milk fat (and protein) % peaks in December and is at its lowest in June. In contrast, milk yield has a peak in March and trough in September. This milk fat % rhythm was seen across different systems, parities and breeds, with high milk fat breeds, eg Channel Islands, displaying a greater difference between peak and trough. Is this variation due to temperature and heat stress rather than light/dark? Heat stress results in a reduction in protein % and yield, but an increase in fat %, so this seasonal rhythm appears to be a day-length effect. We know that 16-18 hours of daylight (vs 8-10 hours or constant daylight) leads to a 5-10% increase in yield with no effect on milk solids. It seems that 16-18 hours light ‘fixes’ the cow’s biological clock at the spring equinox for maximum production.

    There are also daily rhythms, with cows ‘naturally’ feeding in the morning and late in the day (dawn and dusk), but more in the afternoon/evening. This is thought to have evolved to (a) avoid predators and (b) synchronise maximum intakes with the highest concentration of nutrients in grass late in the day. If feeding once a day, doing so in the morning results in more level intakes throughout the day. Feeding once a day in the afternoon leads to a higher peak intake, with increased risk of lower rumen pH. When housed and fed during the day (7am to 11pm), milk yield is highest at 2am and lowest at 2pm, and milk fat and protein % both highest 12-4pm and lowest 12-4am. Feeding at night (7pm to 11am) reversed these patterns. Expression of some core clock genes in the mammary gland was also altered by changing feeding times. However, delivery of fresh feed is also a stimulus to eat, and hence the need to keep food pushed up and available. Dr. Harvatine predicted more work being done on feeding different rations at different times of day to different groups! Perhaps topping up highest yielders in the afternoon with a different ration?

    The proceedings for this conference can be found at: https://ansci.cals.cornell.edu/news-events/cornell-nutrition-conference/proceedings/