What is the loin eye?

The loin eye size is a measurement of the large muscle in the pigs back that makes up the meat in a pork chop. Packers and retailers are looking for a larger loin eye, so are paying a premium for it. The loin eye is measured with the same probe that measures backfat , but because it play a large role in determining the index, a loin eye bonus is a premium above and beyond the index.

Figure 1 cross section of a pig’s loin at the point where it would be measured. The loin eye is the large round muscle in the middle of the picture.

The loin eye is measured in millimetres as a straight line through the back of the pig. Below is a cross section of a pig’s back showing both the backfat and loin-eye at the location grading probe would pass through them.

The loin-eye is associated with the lean growth of the pigs. This lean growth is when the pig primarily grows the muscles in its body; which is primarily during the starter and grower phase of the pig. However as we select for faster growing and leaner pigs we extend the lean deposition of the pig into the finisher phase. Muscles are primarily made of protein, and thus adequate feed protein must be supplied to the pig for the loin eye to grow. The key factors that will affect the loin eye growth are the protein content (amino acid balance) of the ration, the genetics of the pig, and the health of the pig during the starter and grower periods of the pig’s life, and the carcass weight of the pig. The following are some examples:

Protein Content of feed

The graph below is from a producer, who changed his diet and was not feeding adequate lysine levels. His loin eye size dropped from an average of 55 mm to less than 50. The levels improved once lysine levels were increased. For an optimal loin eye size its critical the protein and amino acid levels are adequate for each stage of the pig’s growth.

Animal Health

Any factor that will slow a pig’s growth, particularly when the pig is young will reduce the eventual loin eye size. Even if the pig recovers and grows well in the finishing phase, it has lost an opportunity to grow muscle, and the eventual loin eye size will not be as large as it could be. Below is a chart showing a producer who had a nursery that didn’t function well, Pigs from that nursery has smaller loin eye sizes. When the nursery was changed and reduce the incidence of diarrhea, the loin eye size again improved.

Genetics

Loin eye size is dependant on the genetics of the pig. Using a sire line that is selected for lean growth will increase loin eye size. Below are some research results from a sire trial we did in 2000. Both PEI Yorkshire and Duroc sired pigs had a higher loin eye size than Landrace sired animals. Landrace are generally selected for maternal traits. Boars have breeding values for loin eye area. Producers have the ability to choose boars with higher loin size should they want to increase that aspect of their market hogs.

Index	110.68	111.06	111.09
Lean Yield %	60.66	60.79	60.80
Back fat mm	18.60	17.81	18.17
Loin eye mm	60.75	57.43 p (0.02)	60.29

Carcass weight

For the most part, our pigs are selected for lean growth and will continue to increase the loin eye size as we increase the carcass weight. This may not be the case for farms where the pigs grow slower or have unimproved genetics, but ASRP data from the last fill suggest that loin eye size will increase as we increase the carcass weight. The chart below describes loin eye size and carcass weight. Producers can test this relationship with their own carcass data. Gilts appeared to have no difference in lean deposition rates compared to barrows, but as you can see, the pigs raised in the bedded section of the barn had a smaller loin eye. If pigs eat the bedding, they may be reducing the amount of protein they ingest.

Source: Atlantic Swine Research Partnership - June 2004
©PEI Pork 2004