Whenever an unsaturated ingredient is added to a swine diet, the potential economic gain must be weighed against the risk of harvesting pigs that have a high iodine value (IV). Understanding how and where iodine value is measured on the carcass can have a significant impact on the IV of your market hogs.

The inclusion of unsaturated fats in pig diets has raised issues related to pork carcass lipid quality. The objective of this experiment was to develop a more comprehensive understanding of how dietary fat affects the composition of body fat during the finishing period and at slaughter.

A total of 42 gilts and 21 barrows (PIC 337 x C22/29) with an average initial weight of 171.2 lb. were allotted to seven treatments based on sex and weight (Table 1): 3% and 6% each of tallow (TAL, IV = 41.9); choice white grease (CWG, IV = 66.5); corn oil (CO, IV = 123.1), and a control (CNTR) corn-soy based diet with no added fat.

Pigs were individually housed to allow accurate measurement of feed intake and, thus, daily dietary fat and energy intake. Pigs were weighed and adipose samples were collected from the jowl, belly and backfat depots on Days 0, 18, 35 and at harvest (Table 2). IV was determined on diet and carcass lipid samples.

Belly weights were recorded at harvest along with a subjective belly firmness score (1, 2 or 3, with 1 firmest and 3 least firm) recorded 24 hours postmortem.

Carcass lipid IV was affected by source (TAL = 66.8, CWG = 70.3, CO = 76.3, CNTR = 65.4). Carcass lipid IV for TAL and CWG was not affected by inclusion level, although carcass lipid was affected by CO level (3% = 72.6, 6% = 80.0). Carcass lipid IV was also affected by sex (barrows = 69.1, gilts = 71.5). The correlation between carcass lipid IV and dietary lipid IV was R2 = 0.592.

Belly weight was increased by inclusion level (CNRT = 18.3 lb.; 3% = 19.4 lb.; 6% = 20.7 lb.). Belly firmness score was also affected by source (CNTR = 1.8; TAL = 1.7; CWG = 2.0; CO=2.2) and sex (barrows = 1.6; gilts = 2.3).

Average daily gain was increased by inclusion level (CNTR=2.05 lb.; 3% = 2.29 lb.; 6% = 2.42 lb.). Gain/feed was also improved by inclusion level (CNTR = 0.301; 3% = 0.337; 6% = 0.358).

This study shows that an increase of dietary fat can improve feed efficiency and performance; however, including a dietary fat that is highly unsaturated will result in an increase in carcass fat IV. The inclusion of a fat source with an IV less than 66 can be used without repercussions, but an IV greater than 122 (dietary ingredients high in unsaturated fats, such as distillers’ dried grains with solubles) should be limited whenever possible. A diet including 6% corn oil (IV = 123) will raise iodine value beyond the industry standard of 74.

It is also important to know the sampling site on the carcass and the procedure used to calculate/measure IV. Carcass fat sampled from the jowl will have higher IV than when sampled from the belly or backfat when no additional fat is included up to 3% levels. However, with inclusion levels at 6%, regardless of source, the difference between sample sites is no longer significantly different.

The use of dietary fats can improve the bottom line with performance and efficiency gains. If you use a fat source with an IV less than 90, or limit the use of an unsaturated source, you will gain these advantages without running the risk of harvesting hogs that possess a high iodine value.

Researchers: John F. Patience, Ken Prusa and Trey Kellner, Iowa State University, Ames, IA. Contact Patience by phone (515) 294-5132 or e-mail jfp@iastate.edu.