Reproductive tract scoring can help determine heifer breeding readiness
—Progesterone inserts studied in improved pregnancy rates research.
The goal of cow/calf operators is to produce as many calves per year as possible. Yearling heifers should be bred to calve as 2-year-olds to maximize the number of calves produced in their lifetimes. They must be sufficiently mature to be bred at 14 to 15 months of age. Determining sexual maturity as a predictor of future reproductive efficiency is important so producers can retain the heifers most likely to become pregnant at a young age.
Reproductive tract scoring (RTS) is performed by transrectal palpation and can be combined with ultrasonography. Heifers are categorized into five different groups based on uterine size and tone, and ovarian findings. The method considered most accurate measures progesterone levels in two separate blood samples taken 10 days apart in order to determine whether the heifer is prepubertal or pubertal.
As females reach puberty, concentrations of sex hormone become influenced by the estrous cycle. After the first ovulation (pubertal estrus), progesterone levels are higher than any values previously observed. After the onset of pubertal estrus, progesterone follows a cyclic pattern. It is lowest on days 0 to 2, increases to a maximum between days 12 and 15, and decreases rapidly 3 to 4 days before the next estrus.
The use of progesterone to influence the onset of puberty and heat synchronization has been widely studied. Findings have supported the possibility progesterone can successfully induce cyclicity in prepubertal heifers, and this cyclic activity can be maintained.
Another progesterone based product is an intravaginal insert known as a controlled internal drugreleasing (CIDR) device.
Unlike the standard protocol with oral progesterone, CIDR inserts are in place for seven to 10 days, reducing the treatment time and avoiding delays in the onset of the breeding season.
In a series of studies conducted at the University of California (UC) Sierra Foothill Extension Center, after the CIDR implant was removed on the seventh day of treatment, a fixed-time artificial insemination was performed at the resulting estrus. Prepubertal heifers that responded favorably to the treatment were artificially inseminated at the pubertal estrus.
Unfortunately, the results found lower pregnancy rates may result due to lower fertility in pubertal estrus in beef heifers and other female mammals. Previous studies have found that pregnancy rates were higher in heifers bred on the third estrus compared to those bred on the pubertal estrus.
The goals of the study were to determine the accuracy of RTS as a measure of pubertal status and whether the treatment of heifers with CIDR inserts, followed by breeding on the second estrus following removal of the insert, could result in increased conception rates compared to untreated heifers under the same management conditions.
The two-year study was performed using 201 Angus/Hereford cross yearling heifers belonging to the UC Sierra Foothill Research and Extension Center.
Heifers’ RTS were determined via transrectally palpated and ultrasound. Ultrasonography was used to measure ovarian structures and uterine horn diameter to more accurately determine the RTS (see table 1). A blood sample was collected to measure serum progesterone concentration, and a second blood sample was collected 10 days later.
Heifers with RTSs of three or less were classified as prepubertal. Heifers with higher RTSs were classed as pubertal. Both groups were randomly divided into control groups and groups to be treated with CIDRs.
CIDR implants were placed in the treatment group when the second blood sample was taken.
This was considered day zero of the clinical trial. The CIDR implants were removed on day 9, and both groups were observed for estrus behavior.
In both years of the study, all heifers were artificially inseminated by two experienced inseminators using frozen semen from a single bull. Three weeks after artificial insemination, the heifers were gathered in a single group and bulls were turned in for two months. All bulls had previously passed a breeding soundness examination. The bullto-heifer ratio was approximately 1:20. At 30 days and 140 days after artificial insemination, all study animals were examined using ultrasound to detect pregnancy.
In this study, a greater number of CIDR-implanted heifers showed estrous behavior a week after treatment than nontreated heifers. Despite this, the estrous response to treatment was lower than expected based on prior research. There were no significant differences between the CIDR and control groups in overall pregnancy, pregnancy at first breeding, or time to conception.
The researchers believe this result detail is attributable to nutrition deficiency and the test animals’ young age. The age of puberty onset is influenced by a heifer’s nutritional status and genetic background, so nutrition is an important variable in fertility. The average breeding age of the heifers in the study was lower than normally accepted in beef operations. The youth of animals in this study undoubtedly affected the onset of puberty.
Based on the results of this trial, the researchers concluded RTS can provide important information related to the pubertal status of heifers, but should not be used as the sole criterion for selecting replacement females. Also, CIDR inserts only marginally improve the breeding efficiency of prepubertal beef heifers when nutrition is limiting.
Nutrition is clearly an important factor for the success of any reproductive management tool. Adequate nutrition and optimal breeding efficiency enable producers to reduce the economic costs associated with increased feeding to maintain nonpregnant females, the loss of nonpregnant breeding animals due to culling, and the need to provide replacement breeding females. — UC Sierra Foothill Research and Extension Center