The concepts of least cost diet formulation, feeding of alternative feed ingredients and production throughput have become common practice in the swine industry. The inclusion of alternative ingredients containing greater levels of unsaturated fat in the diet can have negative effects on fat quality that reverberate throughout the entire pork chain. The literature indicates that carcasses with increased unsaturated fatty acids, particularly polyunsaturated fatty acids (PUFA), are subject to fat oxidation, color change, shortened shelf life and reduced processing efficiencies as a result of soft pork fat. Hence, the industry needs rapid predictive methods to quantify the impact of production factors (i.e. diet, heat stress, growth) on pork fat quality.

Iodine value (IV) is used as a predictive measure in determining pork fat quality by measuring the ratio of unsaturated to saturated fatty acids present in carcass adipose tissues. The greater the ratio of unsaturated to saturate fatty acids present, the greater the corresponding IV value. The objectives of this study were threefold. The first objective was to analyze the inherent biological variation that exists in fatty acid composition when all genetic, environmental, and managerial practices are held constant. The next objective was to quantify the impact of diet, sex, season and market pull on fatty acid composition and iodine value. The final objective was to develop accurate and precise predictive equations to determine iodine value of two relevant adipose depots in grow-finish pigs.

The experiment consisted of 480 crossbred pigs (240 barrows and 240 gilts) replicated over two seasons (winter and summer) and assigned to one of four dietary treatments. The dietary treatments consisted of: 6% supplemental beef tallow, 6% supplemental corn oil, 6% supplemental beef tallow and Ractopamine HCL and 6% supplemental corn oil and Ractopamine HCL. When pigs approached target market weights the heaviest hogs were marketed first removing 80 pigs (16 pens) in each of the first, second and third marketing cuts. Backfat and belly fat samples were collected at harvest from a commercial abattoir. Fat IV was calculated from fatty acid profile using the following equation (AOCS, 1998): IV = C16:1 × 0.9976 + C18:1 × 0.8986 + C18:2 × 1.810 + C18:3 × 2.735 + C20:1 × 0.8175 + C22:1 × 0.7497.

Results showed fatty acid composition from both sampling sites was greatly affected by the degree of unsaturation of the supplemental dietary fat source. Thus, diets containing corn oil as a dietary fat source had a higher concentration of monounsaturates and polyunsaturates when compared to diets containing tallow as the dietary fat source. Iodine values were also greater for corn oil diets when compared to tallow diets.  Moreover, iodine value of both sampling locations were also affected by ractopamine, sex, season and market pull.

In the development of equations to predict iodine value of backfat and belly sample sites, dietary IVP was the first variable selected (backfat: R2 = 0.74; belly: R2 = 0.66). The inclusion of ractopamine, sex, season and market pull increased backfat and belly R2 values to 0.78 and 0.69, respectively. In other words, 66% of the variation in belly IV was explained by dietary IVP and approximately 3% of the variation in belly IV was explained by ractopamine, sex, season and market pull. Equations produced by this project will enable swine production companies to better manage fat quality and optimize feed costs.

Key findings
– Dietary IVP explained the majority of the variation in IVP for both backfat and belly fat (R2 = 74 and 66%, respectively)
– Accounting for ractopamine, sex, season and market pull improved IVP R2 for backfat and belly fat by 4 and 3%, respectively