The relationship between reproductive success and body condition at calving is based to a great extent on energy. The energy demands of a lactating cow can be very high.

While Jack Berger, Saratoga, WY, doesn't formally body condition score (BCS) his cows, he knows intuitively it's important that the cow is in adequate body condition at calving so she has the necessary energy stores to meet her needs.

“If she's a middle-aged cow that's a little thin, I don't worry too much,” says Berger, who operates a large family cattle operation on a mix of public and private land in southern Wyoming.

The Bergers run their herd of about 1,250 cows on range in the winter, and feed a protein cake supplement that's adjusted depending on available forage and weather conditions. This usually provides a good mix of energy and protein to keep his cowherd in adequate body condition.

“My feeling is that middle-aged cows (5-8 years old) ought to be able to keep up under normal situations,” he explains. “If she can't, then maybe she shouldn't be in the herd.”

But, before his cows achieve that middle-age status, Berger sorts his herd into age groups, paying special attention to the first-calf heifers and two-year-olds. Often, he sorts off the thinner three- and four-year-olds and feeds them a little higher energy ration along with the young cows.

Table 1. Priority of energy use by the cow
1. Basal metabolism
2. Physical activities including grazing
3. Growth
4. Supporting basic energy reserves
5. Maintaining an existing pregnancy
6. Milk production
7. Adding to energy reserves
8. Estrous cycling and initiating pregnancy
9. Storing excess energy
Short et al., 1990 — USDA-ARS
Table 2. Influence of high and low energy diets fed for 90 days precalving on length of the postpartum anestrous period in beef cows.
Postpartum Diet Precalving Anestrous Duration
Higha 51 days
Lowb 67 days
aHigh = 14.1 lbs. of total digestible nutrients/day
bLow = 7.1 lbs. of total digestible nutrients/day
Adapted from Bellows and Short, 1978 — USDA-ARS

He prefers to sort after weaning, but sometimes will sort again a few weeks before calving if he thinks some cows need a little extra attention. Such attention is all-critical in keeping cows in shape for the rigors of lactation and rebreeding.

“If those cows don't have enough stored energy at calving, they must gain weight during lactation in order to have enough energy left over to begin cycling again,” says Clay Mathis, Extension beef specialist at New Mexico State University's Clayton Research Center.

“That's a tough thing to ask of a cow,” he says, adding that some functions have a higher priority for energy use than others (Table 1).

Sorting Prior To Calving

Creating groups according to BCS has long been a way to manage cows through the winter. Most often, the best time to sort spring calving cows according to body condition is in the fall after weaning. It's when cows' energy needs are the lowest, so it's the most economical time to add body condition to the thinner cows, Mathis says.

“Another viable time to sort is 60-90 days before the calving season is expected to begin,” he says. “There's still time to feed the thinner cows to meet a target BCS by their calving date.”

He warns though, that this may require more intensive inputs since forage quality is often at its lowest during the late winter and the cow's nutritional requirements are increasing to support the developing calf.

“The efficiency of gain is usually lower during this period than the period just after weaning,” adds Jason Sawyer, beef cattle nutritionist, Texas A&M University.

He notes that it takes approximately 40-55 days to increase BCS by one unit when cows are gaining 1.5-2.0 lbs. of non-fetal weight/day. Thus, large BCS gains may not be feasible at this time. This period does allow producers to maintain some flexibility and take advantage of favorable weather conditions if they exist, he explains.

“This strategy may carry slightly more risk than sorting at weaning, though,” Sawyer says.

The Best Chance Cow

Mathis says research shows that producers who provide cows with a higher level of precalving energy can impact the length of the postpartum anestrous period, as well as the potential age and weight of the following calf crop at weaning (Table 2).

However, it's important to note that both levels of precalving nutrition yielded a postpartum anestrous period of less then 82 days. Cows fed the higher level of energy precalving had two chances to become pregnant and maintain a 365-day calving interval. The cows fed the low-energy diet had only one chance to become pregnant.

Sawyer and Mathis agree the cost and returns of feeding either diet needs to be closely evaluated.

“If a high level of energy is provided for an extended period of time precalving,” Mathis says, “the effort should be limited to cows considered at high risk of calving too late or being open at the end of the breeding season.”

In general, Sawyer says, it's better for a thin cow to gain weight after calving than a well-conditioned cow to lose large amounts of body condition between calving and breeding.

“Nevertheless,” he says, “it's still more desirable for all cows to be in the targeted BCS range at calving than to sort off and feed thin cows extra feed while they are nursing a calf.”

Berger, who shoots for at least a 95% pregnancy rate, puts it another way.

“We'll sort some of those thinner cows off before calving and feed them separately if we need to,” he says. “But, if by the time calving time roles around and they aren't in the kind of condition we want, they might not be an economical animal anyway. It won't be long before they'll be culled.”

Contact Clay Mathis, New Mexico State University, Las Cruces, at 505/646-8022 or cpmathis@nmsu.edu.

Jason Sawyer is now at Texas A&M University, College Station. Contact him at 979/845-5083 or J-sawyer@tamu.edu.

Filling Up With Reproductive Fuel

Dietary fat, and possibly more specifically unsaturated fatty acids, may be important components of the “reproductive fuel” required for optimal reproductive activity.

Bob Bellows, retired reproductive physiologist, USDA-ARS, Livestock and Range Research Laboratory, Miles City, MT, pioneered research into the effects of feeding fat supplements to heifers.

His diets were formulated with safflower and sunflower seeds to contain equal amounts of energy and protein with additional fat added to the treatment diets. The diets were fed for an average of 65 days to examine the effects of supplemental fat on subsequent reproductive performance of heifers.

The research demonstrated the benefits of feeding supplemental vegetable fat to pregnant heifers during gestation. They included increased re-breeding pregnancy rates and better calf weaning weights. Plus, a 13% increase in fall pregnancy rate and 30-lb. increase in calf weaning weight was observed.

Feeding fat resulted in a non-significant increase in calf birth weight and in the incidence and severity of calving difficulty.