RED DEER – Understanding how cattle grow could produce better marketing decisions.
Because cattle are generally sold by the pound rather than by carcass quality, putting on weight is foremost in producers’ minds.
“The most valuable animal in the pen is the one that weighs one lb. below the heavy weight discount,” researcher Ty Lawrence, who heads the beef carcass research centre at West Texas A and M University, told a recent beef education session in Red Deer sponsored by the pharmaceutical company Schering-Plough Canada.
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“It does not matter what it grades because the key driver of carcass value is weight. It trumps quality and yield.”
The most valuable component of a carcass is red meat yield with some fat for flavour.
“Muscle grows at a decreasing rate as the animal reaches maturity,” he said.
“Fat growth increases at an increasing rate as the animal reaches its physiological end point. They will die with the same number of muscle fibres they were born with. You do not get more muscle fibres after you are born.”
Naturally occurring anabolic hormones such as insulin, testosterone and estrogen cause growth of muscle cells, which is called anabolism.
A process known as catabolism, driven by hormones such as adrenalin, destroys body components when the animal is living off itself to survive.
“If anabolism exceeds catabolism, you have growth,” Lawrence said.
As animals mature, their muscles become longer and rounder by adding more protein into the system.
Muscle fibres are all about the same size. For example, the proteins in an elephant’s muscles are no bigger than those in a mouse.
The muscle fibre adds protein for length and width growth. Bulls have more protein in their muscles than steers because of testosterone. As a result, animals receiving growth hormones tend to produce less fat, yield more red meat and have less bone.
There are two main types of muscle: red and white.
Red muscles are built on aerobic metabolism and are used for work such as walking.
Therefore, the rounds and chucks at the front and back ends of the animal have more muscling than the loin and do not appear to have much fat. However, the body stores more fat in those areas for extra energy.
The rib and loin muscles stay contracted to support the middle of the body such as the rumen, other internal organs and ribs. They use anerobic metabolism and do not carry much fat.
“The body wants to put the fat where it can use it,” Lawrence said.
Fat is stored as an energy reserve, and the body uses insulin to take up fatty acids and glucose. The liver stores it and then releases glucose into the blood for energy.
“This is a survival mechanism our bodies have adapted to.”
Cattle store fat in four main areas of the body:
- Intermuscle fat or seam fat between the muscles is 42 percent of the average fat in a carcass.
- Subcutaneous fat or back fat under the skin is 30 percent of all fat.
- Intramuscle fat within the muscles is about 15 percent of the carcass and is referred to as marbling. It is the last fat to develop as the animal matures.
- The remainder of fat is found around the organs.
When animals become sick they go off feed and first draw energy from intramuscular fat.
“A sick animal that goes off feed for a day or two is probably two to three weeks behind his pen mates to the finished end point,” Lawrence said.
It is rare for those animals to achieve a higher marbling score at slaughter because they drew on that reserve first.
Another quality determinant is bone. Bone growth starts during gestation with the development of the skull and jaw, followed by the fore and hind limbs and finally the vertebral column.
Growth plates start as cartilage at the end of the bone and appear pink. These convert to bone as the animal matures.
Estrogen causes closure of the growth plate, an indicator of carcass maturity. Heifers are at greater risk of grading mature because estrogen stops bone growth sooner than a steer regardless of breed.
However, a heifer is fatter than a steer at the same weight at the same number of days on feed.
Growth patterns can be changed using hormone implants. They cause protein to be built first and marbling last, which means animals have to be fed to a heavier weight to achieve desired marbling. Individuals vary.
“Unless you are feeding clones of the same weight and age, there is no one-size-fits-all implant,” Lawrence said.
Studies show animals with no implants can achieve the highest grade, AAA, at about 1,145 lb.
To reach AAA, implanted animals need to weigh an extra 38 lb. It takes more days on feed to reach a heavier weight, but the producer is rewarded for the added pounds, Lawrence said.
If an animal is slaughtered with four millimetres of back fat, it has a 20 percent chance of grading AAA or better. Between four and 14 mm back fat, there is a 60 percent probability of making the top grade.
As an animal gets close to its finished weight, the heavier animals are likely to grade higher, but they also carry more subcutaneous and seam fat, which is considered waste.
