Optimal reproductive performance of two- and three-year-old cows is a challenge for most U.S. cow-calf producers. The goal of ongoing research at New Mexico State University (NMSU) is to increase pregnancy rates of such females, while decreasing days to first estrus and maintaining or decreasing costs.

“Our objective is to evaluate postpartum supplements for their potential to improve reproductive performance of young postpartum range cows,” says Rachel Endecott, Miles City, MT. Now a Montana State University Extension beef specialist, she recently received her doctorate in ruminant nutrition while working with Mark Petersen at NMSU's Corona Range and Livestock Research Center.

The supplements were fed to varying numbers of cows each year from 1995-2004. To compare supplementation strategies, they fed either “traditional,” “bypass” or “propionate” supplements. Supplements used in the research were fed after calving and contained 30-36% crude protein (CP):

• Ration 1 — traditional cottonseed meal-based cubes.

• Ration 2 — bypass protein cubes with 50% of CP supplied as feather meal, plus animal protein product.

• Ration 3 — the bypass protein cubes, plus propionate salt.

The idea is to provide one group of research cows a propionate salt — a volatile fatty acid used as an additive in many feeds and foods — that can be readily converted to glucose by the liver.

“We're thinking if we can increase the cow's glucose supply, we can shift nutrient partitioning towards weight gain and reproduction,” Endecott says.

The supplements were fed at a rate of 2-2½ lbs./head/day twice weekly (7-8¾ lbs./head/feeding). In normal years, cows were fed until the start of the breeding season. In drought years, cows were fed through the first 21 to 30 days of the breeding season.

Cows on all supplements had similar pregnancy rates, although those on Rations 2 and 3 returned to estrus sooner than cows fed Ration 1 (Table 1).

Because calcium propionate is an expensive supplement ingredient, the researchers wanted to refine the level of propionate to see if less can be fed with the same beneficial results. They began using 100 gms/day, which Endecott reduced to 80 gms/day with the same results. “We're looking at levels as low as 40 gms/day,” she says.

Financial analysis

Table 2 summarizes NMSU researchers' financial analysis. Traditional calves were assumed to be 205 days old at weaning, and calf average weight per day of age was assumed to be 2.2 lbs. All calves were valued at $1/lb. at weaning.

Even though annual feed costs were higher for cows in the bypass and propionate groups, their calves had potential to be heavier at weaning because cows fed bypass and propionate bred back sooner than cows fed traditionally.

This resulted in an increase in net income of $22.95 and $27.95, respectively, when bypass and propionate were compared to traditional supplementation.

“If you just analyzed pregnancy rate, you'd think this work is no big deal,” Endecott explains. “But by getting cows to cycle 10 days sooner, we're gaining half an estrus cycle per year — which provides an opportunity to have calves earlier the next year. That can have important management implications.”

That's the big challenge throughout the West, if not many other areas of the country — to get young cows bred back, with the second and third conceptions the critical times.

“Getting yearlings bred isn't necessarily the challenge it once was,” Endecott adds. “In a lot of cases, while we're able to get the two-year-olds bred again, it's with the three-year-olds that we're seeing the fallout.”

Clint Peck is a former BEEF Senior Editor based in Billings, MT.

What is bypass protein?

Bypass protein is a common term referring to dietary protein that isn't degraded in the cow's rumen, says Charles Stallings, Virginia Tech dairy science professor in Blacksburg.

He says dietary protein is either undegradable (bypass) or degradable. Rumen microbes break down degradable protein to small peptides, amino acids and ammonia. These products can, in turn, be used by rumen microbes to produce microbial protein digestible by the cow in the small intestine.

Microbial protein is an excellent quality protein. Unfortunately, not enough may be produced to supply the requirement for the high-producing cow. Therefore, undegradable protein must be available to make up the difference between what the cow requires and what the microbial protein supplies.

Feeds vary in their ability to supply undegradable protein; its quality varies by source and depends on digestibility and amino acid composition. If the undegradable protein is indigestible, or has a poor amino acid profile, it will be of little value.

Table 1. Pregnancy rates and days to first estrus for young cows fed three different postpartum supplements from 1995-2004 at the NMSU Corona Range and Livestock Research Center.

	Supplement
Item	TRADITIONAL	BYPASS	PROPIONATE
Pregnancy rate, %	86	87	90
Days to first estrus	96	86	87

Table 2. Financial comparison of three different postpartum supplements fed to young cows at the NMSU Corona Range and Livestock Research Center

	Supplement
Item	TRADITIONAL	BYPASS	PROPIONATE
Supplement cost/ton, $	230	245	345
Feed cost/cow, $	16.10	17.15	24.15
Calf age at weaning, days	205	215	214
Calf weaning weight, lbs.	451	473	471
Lbs. calf weaned/cow exposed	388	412	424
Calf value at weaning, $	388	412	424
Calf income difference, $	---	24	25
Feed cost difference, $	---	1.05	8.05
Net income difference, $	---	22.95	27.95