You wouldn't think that Jerry Craveiro has to worry about cold stress affecting the 14,000 heifers he raises. That's because his Cameiro Heifer Ranch, located 25 miles north of Mexico in Brawley, Calif., sees mostly sunny weather and little rain.
However, the veteran heifer raiser watches the weather closely, and adjusts the ration when temperatures drop. "It's hard to believe that we'd have to worry about the cold," he says. "But, for 30 to 60 days, beginning around Dec. 1 when night-time temperatures drop to around 40 F, we have to give the heifers a more energy-dense ration to get the growth rates we want," he says. Craveiro consistently returns heifers to dairy producers ready to calve at 23 months of age.
Whether for 30 days in California, 90 days in Missouri, or for 190 days in Minnesota, cold stress can slow growth rates in heifers. "Failure to adjust rations for these conditions by providing additional nutrients typically decreases daily gains by 0.2 to 0.4 pounds per day," says Bob James, dairy nutritionist at Virginia Tech, with severe environmental conditions lowering gains even more.
If left unchecked, that could delay the breeding of heifers and could cause pregnant heifers to weigh less than desired at calving, which could result in milk production losses and possible dystocia problems.
More than calves
Producers typically think of cold stress as a problem affecting newborn calves. Indeed, outdoor temperatures which fall below the lower limit of a newborn calf's thermal neutral zone - between 50 F to 78 F - can increase the animal's energy needs.
But, young stock and adult cows have a thermal neutral zone, too. For mature animals, outdoor temperatures below 41 F and above 77 F cause animals to use additional energy to regulate their body temperature. Although not specifically calculated for heifers, the lower critical temperature is between 41 F and 50 F, depending on the animal's size. The animal can tolerate more cold as its body weight increases, says Pat Hoffman, dairy scientist at the University of Wisconsin.
Unlike calves, low temperatures do not have an immediate, life-threatening impact on large growing heifers. But, cooler temperatures do add to their maintenance requirements, which can reduce their rate of gain. A 1994 field study of 1,281 heifers on 18 Wisconsin dairies, conducted by Hoffman, shows this effect. During the summer, the heifers had average daily gains of 1.88 pounds. But from December through February, the gains declined by 0.17 pounds per day to 1.71.
More than temps
Many producers feel immune to this problem if they house their heifers indoors. That may be true for animals housed in five-star accommodations or total-confinement barns with amply-bedded free-stalls and excellent ventilation. But, if your heifer facilities are more like a cheap motel - old, three-sided barns with bedded packs - you may need to re-evaluate the animals' nutritional needs.
And, certain environmental factors can make the problem worse. Just as humidity adds to an animal's heat stress, the presence of mud and manure on the animal's haircoat plays a role in creating cold stress. In his study, Hoffman found that heifers with no mud or manure on their bodies had an average growth rate of 1.98 pounds per day, while animals that had mud or manure covering their legs and abdomen grew at 1.74 pounds per day. Heifers with mud covering their sides and flanks grew at 1.7 pounds per day.
"A cold and wet environment destroys the thermal capacity of the hair coat," says Hoffman. This causes the animal to experience cold stress as though the outdoor temperature were about 20 degrees lower. Indeed, the spring and fall growth rates of 1.73 pounds per day (found in the Wisconsin study) were not much better than those in the winter.
Small losses add up
With this wide seasonal window, growth rates can quickly fall behind.
For example, say you have a pen of seven-month old heifers that weigh around 480 pounds on Nov. 1. Ideally, in the next six months, the heifers would gain 1.77 pounds per day and reach around 800 pounds at 13 months of age on May 1. That would be an ideal size to breed them and allow them to calve at 23 months of age.
However, if between Nov. 1 and May 1 the heifers only gained 1.37 pounds per day - a drop of 0.4 pounds per day due to cold stress - they would weigh about 725 pounds on April 1, about 75 pounds short of the recommended breeding weight. Even if the heifers resumed the ideal growth rate of 1.77 pounds per day, it would take them six weeks to reach the ideal breeding size, thus delaying calving by one to two months.
Heifer budgeting suggests that older heifers cost around $50 to $60 per month to raise. Thus, delays in breeding past 22 to 24 months quickly become expensive. In comparison, the cost of nutritional and management remedies to maintain growth rates during cold stress are minor.
For example, when temperatures turn cold in Minnesota City, Minn., heifer growers Ron and Marianne Scherbring, watch closely the growth rates of the 2,000 heifers they manage. "Selected groups of animals are measured for weight and height every two weeks," says Ron Scherbring. When growth rates decline and outdoor temperatures dip, he provides more energy to the heifers. Animals weighing between 200 to 500 pounds receive about 3 percent to 3.5 percent more energy while heavier animals receive less of an increase - usually around 2 percent.
For the Scherbrings, this change can result in an increase in feed cost per day of 4 to 5 cents per heifer. However, depending on the feeds available and the quality of forage in the diet, the ration can be readjusted to provide the needed energy with a smaller increase in feed cost or no change in the ration expense.
Even at the maximum rate of 4 to 5 cents per day - or $1.20 to $1.50 per month - the extra feed is more economical than growing heifers beyond 22 to 24 months.
With cost-effective ration solutions available, you must adjust for cold stress - no matter where you live. Talk to your nutritionist about what you can do.