Masculinity, 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 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 bulls.

     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.

     In the 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.

     For your benefit in selecting bulls for IMF genetic potential, the decision was made to convert IMF measurements to an IMF ratio, calculated within each contemporary group. This allows for an accurate and true comparison between bulls and alleviates the temptation to compare solely based on the actual IMF measurement. I have 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.