Hall-ChR • 5
Mainstream Terminal Index
Ultrasound Measurements for Body Composition
NALF’s mainstream terminal index ($MTI) is a multiple- trait selection index, expressed in dollars per head, designed to assist beef
producers by adding simplicity to genetic selection decisions. It measures differences in expected profit per carcass produced on a
mainstream grid (yield grade 1 or 2, Select to low-Choice quality grade, and no over- or underweights or dark cutters).
The $MTI is based on the assumption that Angus- Hereford cows in a two-breed rotation are mated to Limousin-influenced terminal
sires with the resulting calves sold into the commodity beef market with premiums and discounts based on both quality and yield grade and
quality grade and gives an estimate of how future progeny of each sire are expected to perform, on average, compared to progeny of other
sires in the Limousin herdbook if the sires were randomly mated to similar herds of black baldy cows and if the calves were exposed to the
same environment.
When using $MTI, it is wise to simultaneously select for eonomically important traits that are not included in the index. The index was
developed for use in a terminal sire situation in which Limousin and Lim-Flex bulls are bred to mature British cross cows and all calves are
placed in the feedlot and sold on a mainstream grid. Maternal traits, such as calving ease, are not included in the index. If you intend to use
high $MTI bulls on first-calf heifers, you will also want to use EPDs for calving ease direct to minimize calving difficulty. Likewise, in order
to keep pace with the Limousin breed’s tremendous improvement in temperaments, selection for superior docility EPDs is also advised.
It is also important to understand the assumptions behind $MTI and consider genetic differences among animals in the component traits
that contribute to $MTI.
Because $MTI is composed of EPDs for several traits, it does not have an associated value for accuracy. However, animals with high
accuracy values for each of the component traits (weaning weight, yearling weight, yield grade and marbling score) have more reliable $MTI
that are less subject to change than animals with low accuracies for each of the component traits.
$MTI index and ranking is the last trait listed on the EPD portion of each pedigree in the catalog.
Procedure:
Body composition measurements are taken with an Aloka 500 real-time ultrasound machine connected to a transducer
that has been designed specifically for animal use. Scan measurements for fat thickness and ribeye area are taken at the 12-13th ribs on each
bull during the off-test weigh date. Scan measurements for intramuscular fat (marbling) are also taken at the 12-13th ribs. The scans are
stored and returned to the ISU Animal Science Department for processing.
Adjustments:
Ribeye area is adjusted to 365 days using a linear area per day of age adjustment. Fat measurement of external fat is
not adjusted while marbling as percent of intramuscular fat is adjusted to 365 days.
Ribeye aRF:
Ribeye area is measured in square inches. Ribeye area is positively and highly correlated with retail product. The
heritability of this trait is moderately high meaning that the trait will be passed on to progeny. According to NCBA National Beef Quality
Audit—1991, ribeyes of 11 to 14 square inches are most appropriate for cutting 1-inch thick steaks weighing 8 to 10 ounces.
Fat thickness:
Fat thickness is measured in inches. Fat thickness is a good indicator of percent retail product. The higher the fat
thickness, the lower the percent retail product. Fat thickness is lightly lower in heritability than ribeye area.
Marbling:
Marbling is measured as percent fat. Heritability for marbling is in the moderate range. Beef carcasses are traded on the
amount of intramuscular fat (marbling) they contain at the 12-13th ribs. However, marbling is a very subjective score. Real-time ultrasound
has the capability to predict the actual percent fat in the ribeye muscle which is what the USDA grader is primarily trying to visually
evaluate. Several research studies have indicated that the genetic correlation between marbling and fat thickness are very low, meaning that
producers can select for increased marbling and not have to be concerned about having animals put on additional external fat.
Ultrasound body composition measurements for fall yearling bulls were taken October 28, 2000, yearling bulls will be taken before sale
day. We will furnish this information to you in a manner that will be both helpful and informative.
A carcass information box accompanies each bull’s pedigree in the catalog (or will be provided at the sale on the supplement sheet). The
bulls are in two contemporary groups—fall yearlings and spring calves. The most important thing to consider about the animal’s percent
of intramuscular fat is—does the animal rank in the top 1/3 or middle 1/3 of its counterparts. The individual measurements may not be
totally accurate, but eventually through sorting sire groups and the use of the EPD system, they can assist us greatly in making selections for
intramuscular fat and other carcass traits.
Example:
ADJ REA —Actual ribeye area adjusted to 365 days.
ADJ IMF — Percent intramuscular fat adjusted to 365 days.
17.0
ADJ REA:
1.83
ADJ IMF:
1,2,3,4,5,6 8,9,10,11,12,13,14,15,16,17,...40