CAROTENOIDS 



reflect quantitatively but not qualitatively ^ ^ '• ^ ^ ' the dietary intake 

 of carotenoids. Sharp increases in plasma values are obtained just after 

 going out to grass during the spring flush ^^'-^^^ although maximum 

 levels are generally reached later in the summer. ^ ^ ® Even in the case 

 of cows which are grazing all the year round, the seasonal variation 

 corresponding to periods of active growth and dormancy of the forages, 

 is still marked. i»o {see Fig. 31) 



Fig. 31. — Illustrating the seasonal variations in the carotene content of butter 

 fat and blood plasma of Ayrshire Cows {from Lord, J. W. 1945, Biochem. 

 J., 39, 372.) 



The carotenoid level of cow plasma can be up to five times greater 

 than bull plasma, 1 1 ». 1 2 o ^pi^js may be due to the greater metabolic 

 turnover in females, ^ ^ 1 to the greater efficiency of bulls in converting 

 carotene into vitamin A {see p. 275), or to different management of 

 bulls which are often out at grass only for very short periods. Which 

 is the true reason has not yet been finally decided. 



As well as seasonal variations, there are marked variations in plasma 

 levels ^ 2 2 in diff'erent breeds of cattle (Table 43) and these are reflected 

 in the colostrum, ^ 2 3 miij^ 1 2 2. 1 2 4 ^^d butter. 1 1 ?. n s. 1 2 4- 1 2 9 



Tarassuk and Regan ^ ^ * claimed to have established a direct relation 

 between carotene levels in plasma in the corresponding milk fats ; 

 the numerical values expressed as mg. per 100 ml. plasma and mg. per 



238 



