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• Conjugated linoleic acid (CLA), a component of milk fat, has potential human health benefits.
• The proportion of CLA in milk fat can be increased by feeding and management practices.
• Feeding dairy cows a ration with 3 percent added grease doubled the proportion of CLA in the milk fat.
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The goal of our present research is to determine if the health benefits of CLA can be obtained from consuming CLA-enriched milk fat.

INTRODUCTION

An article we wrote for the 1998 Illinois Dairy Report (Can Milk Fat be Beneficial to Your Health?) provided a brief description of conjugated linoleic acid (CLA), a minor component of milk fat. Briefly, CLA is a fatty acid that is synthesized in the rumen during biohydrogenation of linoleic acid (C18:2), the predominant fatty acid in vegetable oils. The pathway of biohydrogenation is C18:2 CLA trans 18:1 C18:0. Research conducted over the past ten years suggests there may be many health benefits from the consumption of CLA. Conjugated linoleic acid is an anti-carcinogen in many cell culture and animal models. It has positive effects on growth, the immune system, is anti-atherogenic and may improve glucose tolerance (help control diabetes).

We and others are interested in modifying the diet of cows to increase the synthesis of CLA in the rumen, and thus increase the proportion of CLA in milk fat. The proportion of CLA in milk fat is highly variable. Milk fat is enriched with CLA when the cows diet is supplemented with oils rich in linoleic acid such as corn or soybean oil. The highest proportions of CLA in milk fat have been observed when cows graze pasture; higher than when the cows consume conserved forage. However, in one study, cows were fed ad lib or restricted rations with forage to concentrate ratios of either 35:65 or 50:50. The highest concentration of CLA in milk fat was found when the cows were fed restricted amounts of the high concentrate ration. Consequently, the synthesis of CLA in the rumen appears to be affected by factors other than just the amount of C18:2 in the diet.

There is little information available about the effects of source of dietary energy on CLA in milk fat. The source of energy in the diet affects rumen fermentation, which is known to affect C18:2 biohydrogenation and thus CLA synthesis. Therefore, we analyzed milk samples from an earlier trial designed to examine production responses of dairy cows to supplemental fat or an isocaloric high grain ration.

MATERIALS AND METHODS

Nine cows and nine heifers were fed one of three diets 1) control (35 percent alfalfa haylage, 25 percent corn silage, 40 percent concentrate, DM basis), 2) control + 3 percent choice white grease, and 3) high grain (15 percent alfalfa haylage, 25 percent corn silage, 60 percent concentrate). Diets 2 and 3 were isocaloric. All cows received each treatment for 28 days.

RESULTS AND DISCUSSION

The major milk fatty acids are presented in Table 1. The proportion of CLA in milk fat also is shown graphically (Figure 1). The proportion of CLA in the milk fat was increased more than 100 percent when fat was added to the ration. There was no effect of feeding the high concentrate ration. Although our methodology does not permit a definitive identification of the specific isomer of CLA in the milk fat, we have tentatively identified it as the cis-9, trans-11 isomer, which is believed to be responsible for the beneficial health effects reported.

Choice white grease and tallow often contain relatively high amounts of trans isomers of fatty acids. The trans 18:1 fatty acid was increased when grease was included in the ration, but was also increased when the high concentrate ration was fed. Conjugated linoleic acid is a direct precursor of trans 18:1 and there was a high correlation between the amount of trans 18:1 in the milk fat and CLA. Additionally, there was a negative correlation between the proportions of both trans 18:1 and CLA with the proportion of linoleic acid in milk fat. Linoleic acid was decreased, apparently being converted to trans 18:1 and CLA. Trans fatty acids in the rumen have been implicated as causing reduced milk fat percentage. In this study, there was a negative correlation between the trans 18:1 and milk fat percent, that is, milk fat was lower when trans 18:1 in the milk fat was increased. This relationship also was observed with CLA, however it was not as strong. Generally, there was a decrease in the short and medium chain fatty acids (C4 to C14; not all are shown) when diets 2 and 3 were fed. This has been shown in many other studies when a diet high in concentrate or supplemented with fat is fed.

IMPLICATIONS

Although by supplementing the ration with choice white grease we were able to substantially increase the proportion of CLA in milk fat, it must be increased even further if the health benefits of CLA are to be derived from milk consumption. By combining the dietary and management factors that have been shown to increase CLA, we believe higher concentrations of CLA in milk fat can be achieved. If profitable systems to do so can be developed, both consumers and the dairy industry will benefit.

Table 1. The effect of feeding high fat or high concentrate diets on milk fatty acid composition.

WORK IN PROGRESS

We have completed studies with beef cattle examining the synthesis of CLA in the rumen when steers were fed high amounts of soybean oil. Preliminary results indicate that the rumen micro-organisms do synthesize increasing amounts of CLA when the diet is supplemented with high amounts of C18:2 from soybean oil. We are presently analyzing tissue samples from heifers finished on similar rations containing 5 percent soybean oil to determine if the meat is enriched with CLA. We have two studies about to begin. Modified tall oil, a byproduct of the pulp industry that contains about 60 percent CLA, will be infused into the abomasum of dairy cows to produce high CLA milk fat. This milk fat will be included in diets for rats to determine the effect on growth and body composition. In a second study, milk samples will be collected throughout the year from several locations in Illinois. This milk will be analyzed for total fatty acids, including CLA. The information will be used to formulate rations for rats that contain CLA and other fatty acids in ratios representative of consumption patterns by Illinois residents. The overall goal of this study is to determine if the amount of CLA in typical human diets impacts mammary cancers in laboratory rats. Additionally, when the milk samples are collected we will gather information describing the feeding and management practices of the dairy farms. This will allow us to examine on-farm practices that control milk fat CLA concentrations. Dave Fisher, Wes Winter, Jim Endress and Dave Seibert will assist with the milk sample collection for this project.