ARTICLE FEED & ADDITIVE MAGAZINE June 2022 65 data it is abundantly apparent that choline is in fact an essential nutrient for transition dairy cows and their calves. CHOLINE IS VITAL FOR TRANSITION Several studies have shown 50 to 60% of transition cows experience moderate to severe fatty liver (Bobe et al., 2004). Because fatty liver is a classic deficiency symptom for choline, it is reasonable to question if transition cows are deficient in choline. At calving there are hormonal changes that trigger an intense period of lipid mobilization from adipose tissue and as a result, blood nonesterified fatty acid (NEFA) concentrations typically increase 5- to 10-fold (Grummer, 1993). NEFA remain elevated, albeit to a lesser extent, during early lactation when cows experience negative energy balance. Due to increased NEFA concentration and increased blood flow during this period, daily fatty acid uptake by the liver increases 13-fold at calving (Reynolds et al., 2003). The most desirable fate of fatty acids entering the liver would be complete oxidation to provide energy to the liver or reesterification and export as triglyceride from the liver as part of a VLDL. Though the liver becomes more metabolically active in terms of oxidation during the transition period, the increases are not sufficient to cope with the increased load of fatty acid being presented. This leads to fatty liver and increased ketones in the blood (ketosis) which can negatively impact feed intake and further exacerbate negative energy balance, leading to even more NEFA mobilization. It is apparent that choline deficiency is the limiting factor for VLDL export from the liver. It has been shown in many species, using a wide variety of experimental approaches, the rate of VLDL export is highly related to the rate of phosphatidylcholine synthesis (Cole et al., 2012). Hence when animals are deficient in choline they are prone to develop fatty liver. In fact, during the first week of lactation when the cow is undergoing negative energy balance, choline metabolites in the plasma are at their lowest (Artegoitia et al., 2014; Imhasly et al., 2015).
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