Somewhat controversial in the early days of cattle genetic coding, leptin — a hormone that regulates appetite in animals and humans — is making a comeback among some cattle breeders.
The difference this time is its common inclusion in a broad panel of beneficial genes tested in cattle, said Stephen Moore, a leading researcher in the field of genetic coding in cattle.
“When we do genetic association studies for a number of different traits — marbling or fattening, for example — leptin is one of these genes that keeps coming up time and time again, along with genes such as the Growth Hormone Receptor and Insulin Like Growth Factor,” said Moore, a former University of Alberta professor now at the University of Queensland in Australia.
“They’re essential in a complex interaction of genes that affect a lot of different traits but they don’t do it on their own.”
Leptin is a hormone secreted by fat cells that essentially tells the brain when to stop eating. There are more than 20 mutations of the gene in cattle, and all can create subtly different effects, said Moore. The fully mutated version in the single gene test of the leptin gene (genotype TT) has been associated with better back fat growth, higher dairy production, and improved feed efficiency.
The value of these mutations in leptin for breeding programs has been debated in the cattle genetics research community for some time, said Moore. Some argue that the difference in animal performance is significant while some say otherwise. But with most testing for leptin now done as part of a broad panel test for a wide range of genomics — as opposed to the older single gene test — it’s become clear that leptin mutations work best when considered in the context of other genetic variations.
“With most single gene tests we only see a small amount of variation in a trait such as fatness or growth rate,” said Moore. “When many mutations in many genes are tested, much more of the variation can be explained.”
Panel tests offer breeders more accurate predictions when selecting for various traits, he said. “Leptin may form part of a much larger panel of up to 700,000 mutations in some cases. Depending on the breed, they can predict a whole lot of traits with pretty good accuracy. Depending on your bull or your cow herd’s key performance target, you might want a bull that is good for one particular thing and you can make that choice based on these genomic predictions.”
Panel testing has also changed the game when it comes to the turnover of high-demand bulls. “It’s transformed the bull breeding in dairy cattle like you wouldn’t believe,” said Moore. “Five or six years ago, you had these elite Holstein bulls — Goldwin was the big one in Canada. He was still the drop bull four years after he died with producers still using his semen because it took five years to get accurate genomic predictions for him.
“With this newer technology you get an accurate estimate of the bull when it’s one year old. What that’s done to bull breeding in dairy cattle is you have a one-year-old bull that’s the top bull and you collect semen from that bull, but next year you find a better bull. You don’t have to wait five or seven years to get an accurate prediction. The bulls are turning over much more quickly.”
Still, the research on leptin is unclear when it comes to efficiency. Moore and his associates from the University of Alberta have tested the relationship between leptin and residual feed intake (RFI), which is the difference between an animal’s actual feed intake and its expected feed requirements for maintenance and growth.
“Efficiency is different things to different people, so we tested the RFI trait which is a measure of efficiency that is not dependent on growth rate,” said Moore. “It’s a trait that’s important in your cow herd — you want cows that are low maintenance and having calves that go to the feedlot. The problem is all the RFI measurements have been measured in a feedlot and not in the cow herd because it’s very difficult to measure feed intake on pasture.”
For that reason, Moore said it’s difficult to determine a direct connection between leptin and RFI. “Leptin came up in some of those studies. It was there, it’s important. But as to how it’s affecting the whole suite of genes in feed-to-gain ratio or residual feed intake we don’t know. We haven’t established those pathways.”
Sold on leptin testing
While the scientific debate over leptin isn’t settled, Lon Carlson and Lorraine Beaudin swear by it.
The owners of Carlson Cattle Company, a purebred Red Angus and Gelbvieh operation located near Magrath, have been testing for leptin in their herd for the past 10 years, using only bulls that have been tested 100 per cent positive for the TT mutation in their program.
Although Carlson Cattle Company uses the single gene test for leptin, Beaudin said they test for a number of other genes linked to specific traits as well.
“We test our herd sires for other traits that (genetics information supplier) Quantum Genetix has identified, such as rib-eye area (IGF2), tenderness (PMCH), stress (CrH4), and more recently POMC which is linked to feed efficiency and rate of gain.”
“Our interest in the leptin gene started when Lon worked with several feedlot owners and operators who were interested in leptin,” said Beaudin. “The leptin program was being introduced to the feedlots through Quantum Genetix as a means of selecting cattle for slaughter based on their DNA and condition score.
“Lon found this of interest and felt that testing and breeding for leptin would be beneficial in the seedstock industry since the positive bulls could then only pass on the leptin mutation to their offspring.”
“The more we researched leptin, the greater our interest,” added Carlson. “What impressed Lorraine the most was the empirical research linking leptin to improved milk production, primarily in dairy cattle.
“To have a DNA marker that is proven to increase ADG (average daily gain) and milk production in cattle could only be a positive addition to our program.”
The Carlsons have been seeing improvements in average daily gain in their herd for some time. “We first noticed this when our four kids’ 4-H steers were consistently in the top end of the 4-H club,” said Beaudin.
They are just beginning to explore the relationship between the TT leptin mutation and RFI, said Carlson.
“In our most recent RFI test, six of our bulls were tested against 24 other bulls,” he said. “Four of our bulls were ranked in the top five. In that test, two of the bulls were 100 per cent leptin and two were 50 per cent. The top-ranked bull was 100 per cent leptin and the next two bulls 50 per cent leptin.”
The same test also measured average daily gain, with 30 bulls averaging 3.26 pounds per day.
But once again, his bulls stood out, said Carlson.
“Three bulls out of the 30 on test had an ADG of over four pounds per day — and all three were ours, two of which were 100 per cent leptin.”