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 differing primarily in degree of saturation (tallow, animal-vegetable fat, vegetable oil, and hydrogenated fats). Data were standardized as proportions of the respective controls to reduce variation among individual studies and were subjected to stepwise multiple regression against the iodine value of fats, the percentage increases of total fatty acid's in diets above the respective controls, or the ratio of total C-16 to C18 fatty acids in fats (only for hydrogenated fats). Increased iodine value (increasing unsaturation) of fats increased apparent fatty acid digestibility, especially as iodine value increased from 11 to 27. 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 (negative interaction). Dry matter intake and FCM production decreased as iodine value increased, perhaps because of inhibition of fiber digestion or metabolic regulation of DMI. Milk protein percentage depression averaged 2 percentage units for most fats. However, as partially hydrogenated fat sources became more saturated, milk protein depression appeared to be less evident; increased ratio Of C-16:C18 of fatty acids appeared to increase milk protein percentage. Despite the lower apparent digestibility of fatty acids of hydrogenated fats, increased milk production and percentages of fat and protein appeared to make them more economical than more unsaturated fats.