Recently, we completed an experiment examining whether supplementation of choline would impact liver metabolism of fatty acids and subsequent metabolic health of transition dairy cows. When cows mobilize body condition during early lactation, the fat appears in the blood in the form of nonesterified fatty acids (NEFA). The NEFA are used for fuel by tissues such as muscle and are used by the mammary gland to synthesize milk fat during early lactation; however, the liver extracts NEFA from the blood in proportion to their supply. The NEFA that are taken up by liver are either oxidized in the mitochondria and peroxisomes with resulting production of energy and ketone bodies or esterified back into the triglyceride form. These triglycerides are either exported from the liver in very-low density lipoproteins (VLDL) or stored as triglycerides within the liver tissue itself (Figure 1).

Figure 1. Liver metabolism of NEFA. Exceeding capacity of liver to either oxidize or export fat results in buildup of fat in liver.

Recent evidence from workers at the University of Wisconsin has demonstrated that as triglyceride concentrations in liver increase, the capacity of the liver to detoxify ammonia to urea and to synthesize glucose become impaired. These metabolic impairments may be the underlying mechanism responsible for generally decreased performance and increased morbidity for cows accumulating moderate amounts of fat in their livers during early lactation.

If we consider the timeframe in which NEFA mobilization is of major concern, we see that the "window of concern" lasts from approximately one week prepartum through the first three weeks postpartum (Figure 2) and is associated with changes in dry matter intake that occur during transition. There are two major strategies by which we can "manage" NEFA metabolism in transition cows. The obvious first step is to employ ration and feeding management strategies to decrease mobilization of body condition and hence NEFA supply during early lactation. Well-managed dry and fresh cow programs, including closeup dry and fresh cow rations, go a long way toward achieving this goal. A second practice that will be successful in decreasing NEFA supply, but labor-consuming and requiring investment upwards of $ .75 per treated cow per day, is drenching propylene glycol.

Figure 2. Typical relationships between dry matter intake (DMI) and nonesterified fatty acid (NEFA) concentrations
in plasma during the transition period (Burhans et al., unpublished data).

The second major strategy to manage NEFA metabolism during the transition period, and the one that was the focus of our experiment, is to enhance the capacity of liver to metabolize NEFA. Limited evidence suggests that both the oxidation and export pathways of NEFA disposal from liver (Figure 1) may be sensitive to nutritional modulation by amino acids such as methionine and lysine, or other methyl donors such as choline.

In our experiment, we used 48 Holsteins entering second or later lactation and fed them either 0, 45, 60, or 75 grams per day of a rumen-protected choline product (supplied by Balchem Corporation, Slate Hill, NY) from 21 days before expected calving until 63 days of lactation. Biopsies of liver tissue were conducted before assignment to treatment and on 1 and 21 days after calving. Samples of blood were taken three times per week during the last 21 days before expected calving and the first 21 days after calving. Milk yield was measured at all milkings, and a composite milk sample from one day each week for each cow was analyzed for milk components.

Dry matter intake was not affected by treatment during either the prepartum phase or the postpartum phase of the experiment (Figure 3). Yields of milk and percentages of fat, protein, and solids in milk were not affected by treatment; however, choline supplementation tended to increase yields of fat, 3.5% fat-corrected milk, and total solids (Table 1).

Figure 3. Dry matter intakes of cows fed increasing amounts of rumen-protected choline from 21 days before expected calving
through 63 days of lactation.There were no significant effects of treatment or interactions of treatment with day.

We also made measurements of the capacity of liver tissue from cows fed the different amounts of rumen-protected choline to either oxidize NEFA or convert them to esterified products (essentially triglycerides) within the liver slices themselves. The capacity of the liver tissue at 1 and 21 days of lactation to convert radiolabeled palmitate to CO₂ was not affected by treatment; however, the rate of conversion of radiolabeled palmitate to esterified products within the liver slices was decreased linearly by supplementation with rumen-protected choline (Table 2). These data suggest that the flux of fatty acids through the export pathway (assembly and secretion of VLDL back into the blood) was increased by choline supply; however, more research is required to elucidate whether this is the mechanism of action of choline in the liver of transition cows.

What do these data mean? They suggest that provision of a rumen-protected choline source to the transition cow may decrease triglyceride accumulation in the liver postcalving. Analyses of liver composition in our laboratory are pending, but should help to answer this question. This should translate into improved metabolic health and performance postcalving. We are in the middle of an experiment involving over 800 cows on commercial dairies in NY State to determine whether there are performance advantages to feeding 60 g/d of rumen-protected choline to cows during the closeup dry period and first 30 to 40 days of lactation.

In which situations should we consider supplementing rumen-protected choline? Our results suggest that rumen-protected choline is beneficial in terms of facilitating metabolism of NEFA by liver; therefore, herds in which cows likely are have higher concentrations of NEFA in their blood (overconditioned cows that do not eat as well as more moderately conditioned cows) would be potential candidates for supplementation in addition to greater attention paid to the nutritional factors that caused them to become overconditioned in the first place. Cows in our experiment were not overconditioned at calving (most cows were in the 3.25 range), but we need further data before recommending supplementation in all situations. A second factor that should be evaluated is the current methionine status of the diets. Our basal diet was formulated to be adequate in methionine supply using a factorial estimation, but interrelationships between choline and methionine would suggest that this factor should be evaluated as well. Finally, choline that is not protected is rapidly degraded in the rumen; therefore, it must be fed in a protected form. I am aware of two rumen-protected choline products on the market, but only one product (the one that we fed) has published data in the scientific literature (Deuchler et al., 1998. J. Dairy Sci. 81:238-242) demonstrating efficacy at delivering choline to the postabsorptive tissues of the cow.

A more detailed report can be found in a paper in the Proceedings of the 1999 Feed Dealers Seminars on this website and authored by Piepenbrink and Overton.

T. R. Overton

It’s time again for the annual Feed Dealer Seminars. Larry Chase and Tom Overton will be discussing what’s new in forage analyses, amino acid nutrition of dairy cows, ionophores in relation to transition dairy cows, and choline for transition cows. Locations, times, and contacts follow:

Please email Tom Overton at tro2@cornell.edu to be added to our list of subscribers for notification of new issues.