ASSESSMENT OF THE EFFECTS OF IODINE VALUE ON FATTY-ACID DIGESTIBILITY, FEED-INTAKE, AND MILK-PRODUCTION

Data were pooled from 11 studies evaluating supplemental fat sources d iffering primarily in degree of saturation (tallow, animal-vegetable f at, vegetable oil, and hydrogenated fats). Data were standardized as p roportions of the respective controls to reduce variation among indivi dual studies and were subjected to stepwise multiple regression agains t the iodine value of fats, the percentage increases of total fatty ac id's in diets above the respective controls, or the ratio of total C-1 6 to C18 fatty acids in fats (only for hydrogenated fats). Increased i odine value (increasing unsaturation) of fats increased apparent fatty acid digestibility, especially as iodine value increased from 11 to 2 7. For partially hydrogenated fat sources, as the ratio of C-16 to C18 fatty acids increased, digestibility also increased, especially with increasing iodine value (positive interaction). Beneficial effects of higher C-16:C18 ratio were reduced as amount of added fat increased (n egative interaction). Dry matter intake and FCM production decreased a s iodine value increased, perhaps because of inhibition of fiber diges tion or metabolic regulation of DMI. Milk protein percentage depressio n averaged 2 percentage units for most fats. However, as partially hyd rogenated fat sources became more saturated, milk protein depression a ppeared to be less evident; increased ratio Of C-16:C18 of fatty acids appeared to increase milk protein percentage. Despite the lower appar ent digestibility of fatty acids of hydrogenated fats, increased milk production and percentages of fat and protein appeared to make them mo re economical than more unsaturated fats.