A series of studies carried out at Lethbridge Research Centre indicates withdrawing feed from beef calves for one or more 48-hour intervals during the backgrounding period is a simple and low-cost way to promote marbling in the finishing stage.
In the latest trial, heifers denied feed for one 48-hour cycle just before stepping-up to the high-grain finishing diet were 1.84 times more likely to grade Canada AAA than those fed continuously throughout the backgrounding period. Any weight loss the heifers experienced during the feed withdrawal was made up quite handily and there were no differences in carcass weight, salable meat yield, rib-eye area or average backfat cover between the two groups.
The trial was designed to test whether a natural survival mechanism could be triggered to gain a production advantage for beef producers. It has been hypothesized that in times of feed deprivation, the body senses a need to ready itself for survival by preparing to store more energy as fat, explains Priya Mir, senior ruminant physiologist at Lethbridge. This appears to be true for all mammals including wildlife and humans.
The advantage to producers is an increase in the value of the finished animal as well as savings on labour costs from not having to put out the feed. There really isn’t a reduction in total feed costs because the animals make up the difference once feeding resumes.
FEED WITHDRAWAL TRIALS
Previous trials at Lethbridge in the past 15 years looked at the role of glucose and insulin in influencing marbling fat deposition. Insulin enables muscle tissue to absorb glucose and convert it to fat. Glucose is the building material for marbling fat, whereas, subcutaneous (under the hide) fat is formed from acetate.
A study published in 1998 showed a tight correlation between marbling and glucose availability following a 48-hour fast. A followup study in 2002 produced conflicting results.
This led Mir to question whether feed withdrawal itself rather than high levels of glucose in the bloodstream affects marbling fat deposition. To test the hypothesis, 120 British-Exoticcross heifers weighing an average 1,290 pounds were equally divided and placed on a ration of steam-rolled barley and barley silage. One group had free-choice access to feed components, while the other was fed a total mixed ration. Prior to the two-week step-up to the finishing diet, 30 heifers in each management group were denied feed for 48 hours.
Blood samples were taken. As expected, glucose and insulin levels decreased in the heifers denied feed, but the ratio wasn’t affected.
At slaughter, 81 per cent of the heifers that experienced the feed withdrawal graded Canada AAA for marbling, whereas only 68 per cent of the heifers that weren’t denied feed graded Canada AAA. Both groups were above the national average, but feed withdrawal increased the odds for a Canada AAA grade. Canada’s national beef audit in 2001 indicated 41 per cent of heifers processed here make the Canada AAA grade.
Mir says the answer to how the body prepares to store energy during times of feed deprivation may lie in identifying an insulin-like growth factor secreted during the feed withdrawal period. A related study published in 2005, suggests a growth hormone may be responsible for triggering the proliferation of new fat cells in muscle tissues. Alternatively, mature fat cells may change from dedicated fat storage cells to supply additional fat cells.
Though the exact mechanism has yet to be identified, all of the work to date indicates that the increase in marbling fat after feed withdrawal as cattle enter the finishing period is consistent enough to be economically significant, Mir says.
A subsequent study, now being reviewed for publication, shows that the marbling and financial benefit realized from one 48-hour fast is amplified with each additional fast. Four 48-hour fasts eight weeks apart prior to finishing were better than one.
Another related study, also in review, looks at the potential for using RNA (the translators for DNA) markers for fat cells to confirm the effect of feed withdrawal on increasing the number of fat cells in the muscle and subcutaneous fat.
Mir cautions that withdrawing feed to promote marbling is a management practice that can only be implemented when grain makes up less than 40 per cent of the ration on an as-fed basis. Once the cattle go on to finishing diets this practice would be hazardous to their health and your pocketbook.
During the backgrounding period there are no associated disadvantages. The 48-hours is well within the 72-hour limit allowed by Canada’s Health of Animals regulations for beef cattle to go without feed, which can often happen while animals are in transit.
Rather than four feed withdrawals eight weeks apart as implemented in the study, you may want to trim that to three withdrawals about six weeks apart to facilitate a shorter backgrounding period.
The work with beef cattle at Lethbridge is now being tied to research in the human field. Some individuals who are deficient in fat cells experience symptoms related to Type 2 diabetes because the glucose stays in the bloodstream rather than being absorbed and stored as fat, Mir explains. The question to be answered is whether fasting could initiate the development of new fat cells to store the glucose, thereby reducing glucose concentration in the bloodstream.
A recent trial carried out in co-operation with the University of Alberta and Washington State University, indicates a 24-hour feed withdrawal increased the number of fat cells in the abdominal area of rats. The amount of circulating glucose in the bloodstream of the rats was lowest when they had medium-size fat cells in the abdominal area. Further research may some day provide the scientific data necessary to support the concept of periodic fasting as a non-pharmaceutical, low-cost treatment option to delay the onset of, or control the expansion of Type 2 diabetes in humans.