IMPORTANCE OF HIGH-PERCENT CALF CROP

Percent calf crop weaned is influenced more by management decisions than by any other single factor in a cow herd, and therefore can be a very important factor in annual returns for a cow-calf operation. Percent calf crop weaned is calculated by dividing the number of calves weaned by the total number of females exposed during the breeding season. As percent calf crop weaned increases, pounds of calf weaned per cow exposed increases and production cost per hundred pounds of calf produced decreases. Increasing weaning weights approximately 50 pounds is equal to an increase of 10% in calf crop weaned.

CAUSES OF A REDUCED CALF CROP

The greatest single loss in potential calf crop is in the failure of cows to become pregnant during the breeding season (Wiltbank et al. 1961). There are four main factors during the postpartum to rebreeding period that contribute to infertility in beef cows: 1) a general infertility, 2) short estrous cycles, 3) uterine involution, and 4) anestrous (Short et al. 1990). A general infertility related to the breeding of a cow at any time exists (Short et al. 1990). Short estrous cycles occur in approximately 80% of all cows following the first ovulation after calving (Yavas and Walton 2000). Short estrous cycles occur when the cow cycles

Proper nutrition for a cow plays a major role in her ability to reproduce. Body condition scores are a subjective but effective way for ranchers to evaluate the nutritional status of their herd. To maintain a 12 month calving interval, cows must be bred within 80 days of calving. Body condition scores allow a producer to determine if cows are losing weight during the postpartum period. A body condition score of ¡Ý5 has been determined to be the minimum score at calving that allows for adequate postpartum reproductive performance. Cows having a body condition score ¡Ý5 at calving return to estrus sooner and rebreed more quickly than cows with body condition scores ¡Ü4 at calving (Richards et al. 1986; see Table 2). In addition, cows that have inadequate energy intake during the late precalving and postcalving periods have lower pregnancy rates when compared to cows receiving adequate energy intake during the late precalving and postcalving periods (Randel 1990).

B. Herd health

Herd health can be divided into two categories:

1) Diseases that affect the reproductive performance of the cows.

2) Diseases that cause calf loss from birth to weaning.

CONTROL OF DISEASES THAT AFFECT REPRODUCTIVE PERFORMANCE

The goal of a good herd vaccination program is not necessarily to render each individual animal immune to a disease, but rather to stimulate a sufficient immunity in a sufficient number of animals in the herd so that an epidemic, or widespread outbreak, does not occur. Therefore, it is important to vaccinate all animals in the herd. For maximum protection during the breeding season, cattle should be vaccinated 30 to 45 days before the breeding season. This gives sufficient time for animals to build immunity and for antibody levels to remain elevated during the breeding season. All animals in the breeding herd¡ªcows, heifers, and bulls¡ªshould be vaccinated annually for reproductive diseases such as Bovine Viral Diarrhea Virus (BVDV), Leptospirosis, Vibriosis, and Infectious Bovine Rhinotracheitis (IBR).

Bovine Viral Diarrhea Virus is a virus that is widespread throughout the world. Reproductive signs of BVDV in a cow herd depend on the stage of gestation in which cows or heifers were infected. BVDV infections during early gestation result in low conception rates, due to early embryonic death. If cattle are infected during mid-gestation, BVDV may result in persistently infected calves; this occurs when animals are infected during the specific period of fetal development when the fetus is differentiating its own cells from foreign materials (roughly, between 40 and 120 days of gestation); this results in a calf that has incorporated BVDV into its own body and that will shed high levels of virus persistently throughout the calf¡¯s life. Late-gestation BVDV infections may result in congenital defects, late-term abortions, or the birth of congenitally infected calves, which are weaker and more prone to illness than normal calves.

Leptospirosis has long been recognized as a cause of infertility in cattle. Symptoms of

Vibriosis is a bacterial disease that affects the reproductive tract of male and female cattle. Symptoms of vibriosis manifest themselves as infertility (decreased pregnancy

CONTROL OF DISEASES THAT CAUSE CALF LOSS FROM BIRTH TO WEANING

Neonatal diarrhea

Because most diarrhea-causing organisms are common in cattle herds, steps should be taken to reduce the exposure of calves to high levels of these agents. Ideally, calves should not spend their first hours of life in a dirty, contaminated environment. Paying attention to calving area sanitation and calving cows and heifers on clean lots will drastically reduce calf exposure to these pathogens. An adequate amount of high-quality colostrum consumed within the first 12 hours of life will provide the calf with some passive resistance against common pathogens. The quality of this colostrum may be enhanced by vaccinating pregnant cows in late gestation with appropriate vaccines (i.e., "scour shots").

Following the neonatal period, illnesses that may result in death losses include blackleg and pneumonia, among others. Depending on pasture location and disease history, a vaccination program should be implemented¡ªwith close consultation with a veterinarian for calves being turned out to pasture. Typically, vaccination programs include a 7-way clostridial vaccine and possibly vaccines against viral (IBR, BVDV, Bovine Respiratory Syncytial Virus (BRSV) and Parainfluenza (PI-3) and bacterial (Pasteurella and Mannheimia) pneumonia pathogens.

C. Defined breeding and calving seasons

Many advantages can be gained from defined breeding and calving seasons. Defined breeding and calving seasons can both match the time of year most suitable for each and limit the time required to accomplish each. And defined breeding and calving seasons focus the labor required for each to a limited length of time. And defined breeding and calving seasons also allow for easier management of the herd.

BREEDING AND CALVING SEASON

There are tremendous benefits to a short, defined breeding season. A short breeding season results in a longer interval from calving to the start of the breeding season, which allows cows to recover from calving and initiate estrous cycles before the start of the breeding season. A short breeding season also results in a shorter calving season, which focuses the labor of calving into a shorter period of time and produces a more uniform calf crop. Health benefits are also realized when calf ages do not vary greatly. Calves born late are more likely to become exposed to the diarrhea-causing organisms that have been building up from the beginning of the calving season.

To read the entire study and view corresponding tables and charts, link to SDSU Extension Extra, Increasing your calf crop by management, pregnancy testing and breeding soundness examinations.

WAYS TO INCREASE CALF CROP

A. Nutrition