A number of pork processors are concerned about the quality and cutability of fat in pork for domestic and export markets.

Adding fat in the form of soybean oil to the diet of grow-finish pigs increases pig performance, but it also contributes to the softness of the fat, even if removed from the diet up to 56 days prior to marketing, according to researchers at Kansas State University (KSU).

Soybean oil or choice white grease (CWG) is routinely added to pigs' diets to improve growth performance and help reduce dust levels in hog barns.

But a new KSU swine nutrition study, led by Extension Swine Specialist Mike Tokach and graduate student Justin Benz, has found that added fat could compromise pork fat quality.

And some pork packers, including Smithfield Foods, have already taken steps to firm up the backfat and belly fat in their hogs by reducing levels of unsaturated fat in the last finishing diet fed before slaughter.

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Study Results

In the KSU study, 144 barrows and gilts (PIC genetics) weighing about 97 lb. were evaluated for the impact of fat source and feeding period on growth performance and fat quality.

Nine treatment regimens were tested: a control, corn-soy diet with no added fat; a diet plus 5% CWG fed from Day 0 - 26, Day 0 - 54, Day 0 - 68 or Day 0 - 82; or a diet containing 5% soybean oil fed from Day 0 - 26, Day 0 - 54, Day 0 - 68 or Day 0 - 82. Pigs were fed the control diet after feeding the 5% fat diets.

At the end of the study, jowl samples were collected at the packing plant and tested for iodine value.

“The iodine value is used to predict the softness of the carcass fat and whether there will be problems in processing bellies,” Tokach explains. A value of 73 or below is being used by a growing number of plants as the cutoff level for acceptable carcass fat.

Data is depicted in Figure 1. It clearly shows that when pigs remained on the control corn-soy diet for the entire 82-day experiment, iodine value levels for fat averaged 67.1, well within the range sought by packers.

The iodine levels for pigs fed diets with choice white grease stayed in an acceptable level below 73, rising only slightly as days fat was removed from the diet before marketing decreased.

“Some people argue you can't feed CWG all the way to market without fat problems, but our data from midwestern plants says you can,” Tokach remarks.

The results for soybean oil were not as positive. “The big thing that this study shows is that even if you take out the fat (soybean oil) 28 days before marketing, you don't change the fat composition very much. It drops some, but it is not where it needs to be,” he observes.

The iodine levels for pigs fed soybean oil ranged from 73.6 when withdrawn at 56 days before marketing, to 82.0 when not withdrawn at all from the diet (Figure 1).

Soybean oil is often fed (at levels of 1-2% in the diet) because of ease of handling. “You don't have to heat it to put it into the diet. But producers need to be careful about using it in late-finishing diets to make sure they don't go over the limit their packer has set,” comments Tokach.

Benz also indicates that barrows in the trial had lower iodine values than gilts for both sources of fat. As iodine values increased with feeding duration for fat, the differences between sexes became greater. This could play a major role for producers marketing to plants monitoring iodine values, he notes.

Why Use Iodine Value?

Benz points out that the majority of iodine value and fat quality research has been conducted using backfat and belly samples, with very little testing of jowl samples.

KSU researchers decided to test jowls instead because “that is the fat that plants are using to monitor iodine value,” explains Tokach. “It is an easy sample for the plants to collect, and it doesn't lower the value of any of the cuts. More research needs to be done with the jowls for these reasons.”

What iodine value actually refers to is a laboratory test that determines the relative saturation or unsaturation of the fat, Tokach explains. “Basically, the more iodine that attaches, the higher the iodine value becomes, which means the softer the fat or the more unsaturated the fat is; that represents a bad thing for pork bellies and slicing.”

A Quality Issue

Tokach sums up: “This is a meat quality issue from the bacon side, but it really is a fat quality issue as well. For the Japanese market, only a hard, firm, white fat is acceptable because they judge quality by both the lean and fat appearance.

“But it becomes a practical issue as well in determining whether you can cut it and have it firm enough to market.”

Results of this trial will be presented at the Midwest Section of the American Society of Animal Science meeting in late March in Des Moines, IA.

Related Nutrition Trials

KSU researchers have three other related swine nutrition studies underway of interest to pork producers:

  • Evaluating the fat level in corn vs. milo grain sources and adding CWG to the ration.

  • Analyzing extruded-expelled soybean meal vs. regular soybean meal and adding CWG to the ration.

  • Testing gradient levels up to 20% of dried distiller's grains with solubles in finishing diets to determine the iodine value response.

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