Data . . .
Nutrition - and Bull IMF
By Allen R. Williams, Ph. D
For over a decade there has been confusion
among cattlemen regarding the results of live animal carcass ultrasound
and how to interpret data collected between seedstock producers.
In no trait is this confusion more notable than with % IMF (intramuscular
fat or marbling). This particular trait measurement is heavily impacted
by several factors, including bull masculinity, degree of nutrition
and bull activity (related to masculinity). These factors can greatly
impact the degree to which IMF is expressed — although they
do not, in any way, change the genetic potential.
|Bull Masculinity and Activity
and degree of testicular development clearly impact the expression
and ultrasonic measurement of IMF. It is important to remember
that the male sex hormone testosterone is a potent IMF inhibitor.
In over a decade of collecting ultrasound IMF measurements on yearling
to long yearling bulls, it has become very apparent that the level
of masculinity and testicular development plays a significant role
in the magnitude of the ultrasound IMF measurements. In other words,
bulls that are thick-muscled, exhibiting signs of heavy development
of secondary sex characteristics (crest to the neck, heavy shoulders,
etc.) and have good to excellent testicular development will have
IMF measurements that are biased downwards due to the inhibitory
effect of testosterone. Statistical analysis of IMF data, coupled
with scrotal circumference data, clearly indicates a significant
negative correlation between bull masculinity and actual IMF. What
this means is that in many instances (not all), bulls that have
a very high ultrasound IMF are often not very masculine bulls.
The lower testosterone activity in these bulls allows for greater
phenotypic expression of IMF, but does not give a true picture
of the IMF genetic potential as compared to the very masculine
Level of nutrition
leading up to the yearling stage greatly impacts expression of
IMF. It has long been known that bulls developed on high concentrate
rations will exhibit a higher magnitude of IMF deposition, at the
same age, than bulls developed on lower concentrate or forage rations.
That is one of the reasons most seedstock operations develop bulls
on 50% or greater concentrate rations. The bulls gain quite a bit
of condition (backfat) and lay down a higher level of intramuscular
fat. Fat, slick bulls appeal to a lot of bull buyers. The problem
is, you often don’t know what you are really buying because
fat covers over a lot of faults and can give a very false impression
of actual fleshing ability. Bull buyers also like to see high ultrasound
IMF scores. Again, this can lead to misinterpretation by mistaking
IMF due to high concentrate feeding with actual genetic potential
for IMF. Bulls can only lay down IMF to the degree to which their
nutritional environment will allow. Therefore, just as with bull
masculinity and activity, it is important to compare bulls within
contemporary groups only and not from ranch to ranch.
|The PCC Bulls
case of the PCC bulls, it is evident that many of these bulls are
thick, heavily-developed bulls that exhibit a high degree of masculinity
at 12-18 months of age. This masculinity, in and of itself, will
inhibit phenotypic expression of IMF. Couple masculinity with high
levels of activity — and then consider the fact that these
bulls have not been fed a high-concentrate ration. Therefore, it
is important that these bulls be compared for their genetic potential
for IMF — not for the magnitude of their actual IMF measurements.
analyzed actual carcass data from progeny produced from many of
the prominent bloodlines utilized within PCC seedstock, and the
results clearly show that there is true genetic potential for marbling
or IMF. The majority of the progeny sired by these genetics will
produce grass-fed and grain-fed cattle that grade USDA Choice-
or better at harvest.