Reproduction is the foundation upon which all other factors concerning living things are built. The cattle industry is no different, as evidenced by the number of people in attendance at the Applied Reproductive Strategies in Beef Cattle conference held December 3-4, at the Ramkota Hotel and Convention Center in Sioux Falls, S.D. Three hundred and fifty commercial cattlemen, seedstock producers, veterinarians and others were present at the convention, along with an additional 30-40 listening to each presentation live via the Web.
The event has been held yearly between rotating states as far east as Kentucky and as far west as Idaho. With these numbers, this one proved to be the second largest so far.
Topics covered during the event included heifer maintenance, control of estrus, timing of A.I., managing bull development, DNA testing, handling of frozen semen, nutrition, cattle temperament, vaccination, embryo transfer, and sexed semen.
One of the first speakers was Dr. Eric Mousel of the Dept. of Agricultural Sciences at Northwest Missouri State University. His presentation was “Effect of Heifer Calving Date on Longevity and Lifetime Productivity.”
Dr. Mousel began by stating that fertility is a key component of longevity in the cow herd. Early calving increases both longevity and productivity. Females with longer reproductive lives wean more calves and thus have the potential for a higher lifetime weaning weight average.
He cited a survey which indicated that 33 percent of cows are culled because they don’t become pregnant; it also reported that 15.6 percent of all culled cows leave the herd before age five, and still another 31.8 percent leave between ages five and nine. It takes five calves to pay for the development costs and annual maintenance of a replacement heifer. It makes sense to manage the herd to reduce the number of cows culled at a young age.
One such management practice is to ensure that heifers conceive in their first breeding season. If they breed early, that is even more desirable as they will calve early. This helps ensure that they will have time to recover for rebreeding. “Get early calvers in the herd and keep them in the herd,” emphasized Dr. Mousel. “Increased profitability will result.”
“Physiological Factors that Affect Pregnancy Rate to Artificial Insemination” was covered by Dr. Michael Smith, Professor of the Division of Animal Sciences at the University of Missouri. He explained that heifers should meet several criteria before beginning a successful estrus synchronization and A.I. breeding program. First, the determination should be made as to what the pregnancy rate has been in heifers over the past few years. Also, have there been any growth promoting implants? Previous studies show that implanting heifer calves within 30 days of birth impairs uterine function. Third, has the target weight (65 percent of their mature body weight) been attained, and fourth, is their reproductive tract score (RTS) greater than or equal to 4?
The RTS is a subjective measurement of the sexual maturity of a heifer, as performed by a veterinarian four to six weeks prior to the breeding season. Each heifer is assigned a score from 1 to 5, with 1 being prepubertal and 4 or 5 referring to a pubertal or cycling heifer.
Criteria for postpartum mature cows include an unassisted calving, be in good body condition (BCS at calving of greater or equal to 5), disease free, and allowed an adequate period of recovery between calving and rebreeding.
Once females have met the appropriate criteria, A.I. can be performed using either an estrus-based program where animals are visually monitored for estrus and subsequently inseminated, or by fixed-time insemination, in which precise synchronization culminates in a fertile ovulation at a predetermined time.
Dr. Joseph Dalton, Associate Professor of the Dept. of Animal and Veterinary Science at the University of Idaho followed up the previous discussion with his own presentation of “Insemination Related Factors Affecting Fertilization in Cattle.” Dr. Dalton listed a number of elements that are important for fertilization. Semen quality and number of sperm topped the list, followed closely by accessory sperm, bull effect plus timing of A.I., semen handling and fertility associated antigen.
Accessory sperm identification is a research technique which involves quantifying the number of sperm trapped in an embryo which has been recovered by uterine flushing six days after A.I. Accessory sperm are thought to be a measure of sperm available and competing for fertilization.
Bull effect refers to a bull’s ability (or inability) to gain access to the egg. For example, a large number of sperm from a particular bull that reach the egg may indicate that this bull may be less vulnerable to semen handling and insemination errors.
As for handling semen, how many straws should be thawed at one time? Dr. Dalton says, “Know your comfort zone.” Handle no more than can be used in 10-15 minutes. Do not allow straws to touch each other when thawing, and use multiple thaw baths if possible. “Time, temperature, hygiene and skill are the important factors here,” added Dr. Dalton.
Fertility associated antigen (FAA) is a protein on the sperm cells of some bulls which is absent on those of others. It was initially felt that FAA-positive bulls are more fertile; however, further testing did not support this theory.
Following a dinner of beef brisket on Monday evening, attendees were able to rotate through several hands-on workstations. These included ultrasound of a pregnant uterus, semen handling, an A.I. simulation box, an embryo development display, synchronization injections, a carcass quality display and taste testing of different quality grades of beef. There was also an exhibit of six Angus heifers which had been DNA tested for marbling and rate of gain, with printouts of their individual scores available. A trade show was in operation both days of the conference.
On Tuesday the 4th, 13 more speakers were on the schedule. One of these was Dr. Reinaldo Cooke, Assistant Professor and Beef Cattle Specialist at Oregon State University. He discussed “Effects of Temperament and Handling on Fertility.”
“What is temperament?” Dr. Cooke began by asking the audience. It is a behavioral response of cattle when exposed to human handling. An inability to cope with this type of situation affects their well-being, causing a stress response in the animal. This manifests itself as excitable and/or aggressive behavior.
One of the main hormones produced during a stress response is cortisol. Elevated cortisol levels negatively influence reproduction, either by decreasing nutritional status or by altering the physiological mechanism required for ovulation and conception.
Cattle temperament can be evaluated by many methods. One example is by chute exit velocity, in which the speed of an animal leaving a chute is actually measured in feet per second, using a predetermined distance. Another method is the chute score, where cattle are individually restrained in a chute and given a score from 1-5 based on their behavior. Using this procedure, 1 equals calm with no movement, and 5 equals violent and continuous struggling. Producers can utilize both techniques and average both scores to obtain an overall temperament score.
“One means to improving temperament is to acclimate cattle to human handling,” concluded Dr. Cooke. This is best accomplished early in their lives in order to not only improve temperament but also to hasten reproductive development.
“Sex-Sorted Semen for Beef Cattle” was presented by Dr. George Seidel, Professor of the Dept. of Biomedical Sciences in the Animal Reproduction and Biotechnology Laboratory at Colorado State University.
“Sex is the most important genetic trait,” acknowledged Dr. Seidel. In a normal situation, averaged over thousands of animals, 49 percent of calves born will be heifers. This percentage can be changed through the use of sexed semen.
Semen can be sexed with 90 percent accuracy routinely, using an instrument called a flow cytometer/cell sorter. The process, however, is slow relative to the number of sperm in a typical A.I. dose. As a result, sexed semen for A.I. has lower sperm numbers per dose than are used conventionally. Pregnancy rates reflect this; rates are about 10 percent lower than with unsexed semen. Also, sexed semen is packaged in 0.25 ml straws, which require a different insemination gun as well as more careful handling. Sperm insults, too, abound, such as time spent waiting to be sorted, the addition of a dye for DNA, plus the sorting process itself (for example, sperm exit the flow cytometer at 50 mph.)
Despite these drawbacks, sexed semen has several potential applications in beef cattle. One is to increase the percentage of heifer calves born, either to expand the herd or to sell as replacements. Also, heifers can be bred to have heifer calves, thus decreasing calving difficulty. Conversely, the percentage of bull calves can be increased, either for meat production or seedstock producers.
Sexed semen programs can result in more efficient meat and milk production, but producers still need to understand that there are limitations to its use.
The conference was hosted by the Beef Reproduction Task Force, the Beef Reproduction Leadership Team and South Dakota State University in cooperation with the University of Missouri Conference Office. ❖