ANOTHER LOOK AT TIMING OF A I

Transcription

1 ANOTHER LOOK AT TIMING OF A I Ray L. Nebe l Department of Dairy Scienc e Virginia Polytechnic Institute and State Universit y Blacksburg, Virgini a Adoption by dairy producers of Al has made it one of the most important technologies of this century; Al has been important in reducing disease transmission, allowing fo r genetic selection, and ultimately increasing the health, longevity, and yield of dairy cattle. However, an estimated annual loss of >$300 million to the US dairy industry because o f the failure to detect estrus or the misdiagnosis of estrus has reduced the positive economic impact of Al. Thus, the efficient and accurate detection of estrus and the timin g of resulting Al remain major challenges to improving reproductive and economi c efficiencies of many dairy farms. Chronology of Events Between Onset of Heat and Fertilizatio n Onset of heat is physiologically linked to the ensuing ovulation through the surge o f luteinizing hormone (LH). The rise in blood estradiol-171 from the preovulatory follicl e illicit the surge of LH that then sets in operation a series of events in the follicle that lead s to ovulation. In the intervening period, the cow shows heat, is mated or inseminated an d sperm are transported for fertilization of the ova. Biological events that affect the aspect of timing of Al and fertilization are functiona l viable life of gametes (sperm and ova), transport time of viable sperm from the site of Al to fertilization, and timing of ovulation in association with Al. Using intrarecta l ultrasonography to detect ovulation and the estrus detection system HeatWatch to determine onset of standing activity associated with estrus, the interval from first standin g event of estrus to ovulation was determined to be 27.6 ± 5.4 h (Walker, et al., 1996). The transport of viable spermatozoa to the oviducts requires a minimum of 6 h to obtain a population capable of fertilization, and sperm numbers progressively increase over 8 to 18 hours. The functional viable life of bovine spermatozoa in the reproductive tract has been estimated at 24 to 30 hours. Although the maximum length of time the ovum may retain its capacity for fertilization is 20 to 24 h, the optimum period of retention of this capacity is remarkably transitory an estimated at 6 to 10 hours. The timing of these events is important for normal fertility and their chronology is illustrated in Table 1. There is very little evidence to suggest that delayed ovulation is a significant problem in cattle. Basically, once the LH surge occurs, the biochemical events set in motion within th e follicle are on a fairly predictable time course that leads to ovulation. Since the LH surg e sets these events in motion, it is possible that a delayed surge would lead to a delayed ovulation, but there is little evidence that delayed surges are a problem, although this has not been studied extensively. 60

2 It is now recognized that nearly all sperm are lost from the tract in one way or anothe r within 30 hours after insemination. The few that remain accumulate in a reservoir in lowe r isthmus region of the oviduct. They apparently stream slowly from this reservoir to th e ampullae of the oviduct near the time of ovulation. Sperm can move from one horn to another through the uterine body and apparently to some degree through the abdomina l cavity. Fertility is influenced by site of semen deposition during Al, with the most seriou s mistake being placement in the mid or posterior cervix or in the vagina. Horn breeding has resulted in improvement of fertility in some studies, probably because it leads to fewe r cervical or vaginal deposits. Table 1. Chronology of Events Around Onset of Heat Hours relative to Onset of heat Event or process to -30 Estradiol begins to ris e to -24 Estradiol surpasses threshold to induce heat to -20 Estradiol surpasses threshold to induce LH surg e 0 Onset of Standing Heat to - 6 Estradiol levels in blood peak to + 6 LH surge occurs, duration about 8 hours + 4 to +14 Probably best time to inseminate + 8 to +16 Sperm accumulate in lower isthmus region of oviduct +24 to +32 Ovulation occurs, fertilization occurs soon thereafter Timing of Insemination Optimal timing of insemination relative to stage or onset of heat has been unde r investigation for nearly 70 years. As early as 1918, research recommended that idea l timing of insemination occur 10 to 24 hours after onset of heat. Hammond in reported that the length of estrus for dairy cattle varied from 6 to 30 hours with a mean o f 17 hours. He found that cows averaged 19 hours as compared with 16 hours for heifers. Researchers from University of Wisconsin (Werner, et al., 1938) examined 35 heifers an d found that ovulation did not occur during the standing period of estrus. Most earl y researchers used the end of standing activity to time when ovulation occurred because o f the difficult of determining the onset of standing heat. In general, it was reported tha t ovulation transpired 6 to 19 hours after the end of standing activity with an accepte d average of 10.5 hours. 6 1

3 Early investigations into the use of Al recommended that timing of Al occur eithe r during standing activity, toward the end of standing activity, or soon after the end o f standing activity. However, recommendations were given against the insemination of cows during the first 10 hours of estrus. However, Henderson (1939) admitted that "th e recommendation of timing of Al was a rough classification and that probably a goo d proportion of inseminations would not be accurate because the farmers merely estimate d the stage of estrus". All studies until the late 1950's used fresh unfrozen semen, usually a 1-ml dose generally deposited in the mid-cervix region of the reproductive tract. Two important factors is that we now generally accept that an insemination in the mid-cervi x will have a profound effect on lowering fertilization and fresh semen used prior to the mid 's did not contain antibiotics which also is now known to greatly influence fertilizatio n rates. A.M. - P.M. guideline In the late 1930's and 1940's, George Trimberger and co-workerss evaluate d conception rates in dairy cattle at various periods during estrus and even today this body o f research has been the most quoted for the timing of Al. Cows and heifers were observe d three times per day for signs of heat and once heat was identified that individual wa s observed every 2 hours. A few important points of the study should be noted : 1) a total of 295 cows and heifers and 489 breedings were evaluated ; 2) Only 25 cows were bre d before the middle of standing heat (in heat for at least 6 hours before insemination) ; 3) routine heat checks were performed 7 a.m., noon, and 6 p.m., while individuals due in heat were observed every two hours, so intense heat detection was performed; and th e production and management style was vastly different then currently practiced on today' s dairy farms. From this research developed the industry standard "AM - PM" guideline, where cows first observed in heat in the AM should be inseminated the afternoon of th e same day, and cows first observed in the PM should be inseminated before noon th e following day. Once-a-day Al Two large field trials have evaluated the use of once-a-day versus AM - PM Al. In a tria l conducted in New York State using professional Al technicians and fresh semen (no t frozen), 44,707 cows were bred either before noon the same morning of observation, between noon and 6 p.m., or after 6 p.m. the same day (Foote, 1979). There was no difference in non-return rates (a measure of pregnancy rates) for cows bred the sam e morning or during the PM following a morning detection. Foote stated that with unfroze n semen, the best time for Al of dairy cattle is midmorning suggesting that most cows ha d been in heat between 12 to 18 hours. The second study was conducted in Pennsylvania again using professional A I technicians, but with frozen semen (Nebel, et al., 1994). A total of 7,240 first service inseminations were evaluated. Half the herds bred cows using the AM - PM guideline an d 62

4 the other half of the herds bred only once daily during a predetermined 3-hour period, afte r three months the herds switched so each herd used both methods and at anyone time half the herds were using each method. Non-return rates (90-day) for the once-a-day was 58.4 versus 57.8% for cows bred following the AM - PM guideline. A slight advantage was revealed for herds that used once-a-day Al during the mid-morning. Timing when onset of heat is known The optimal time of Al was predicted using mathematical models based on pedomete r readings and rectal palpation of 171 cows (Maatje, et al., 1997). Chance of pregnancy was highest between 6 and 17 h after increased pedometer activity, and the calculated optimum time of AI was 11.8 h. Optimal time of artificial insemination (Al) was determined from services (n = 2661) performed in herds (n = 17) utilizing the HeatWatch system (Dransfield et al. 1998). Each farm selected a 3-hour interval to Al cows identified in estrus during the previous 24 hours. This allowed for inseminations to occur at all intervals from onset of heat (the first standing event detected by HeatWatch ) to breeding. Pregnancy status was determine d by data for return to estrus and palpation of the uterus 35 to 75 days following Al. Time from first standing event to Al affected the probability of pregnancy ; highest conceptio n rates for Al occurred between 4 and 15 hours after the onset of standing activity (Figure 1). A higher probability of pregnancy was revealed for cows >100 days in milk, exhibiting > 2 standing events during estrus, and inseminated during March, April, or May. Conclusion s Unfortunately, studies designed to evaluate the optimal time of Al generally contained two technical deficiencies: inadequate numbers of cows for valid statistical comparison s and inaccurate knowledge of the onset of estrus because of low frequency and efficienc y of methods used for estrus detection. When observations for heat are frequent (every 2 to 4 hours) it has been recommende d that cows be inseminated approximately 12 hours following detection, thus the "AM - PM guideline". However, the common management practice is that only one or two daily hea t observation periods are utilized ; therefore, results following the AM - PM guideline or usin g once-a-day Al usually produce similar pregnancy rates because accurate timing of onset is not known. Movement of cows immediately prior to estrus observation and an observatio n period greater than 15 minutes in length are two management practices that have bee n shown to improve conception rates. With HeatWatch the electronic heat detection system the first mount of "standing" heat can be identified, thus allowing accurate timing of insemination 4 to 15 hours after the first standing even t References 1. Dransfield, M. B. G., R. L. Nebel, R. E. Pearson and L. D. Warnick Timing of insemination for dairy cows identified in estrus by a radiotelemetric estrus detection 63

5 system. J. Dairy Sci. 81:(Accepted). 2. Foote, R. H., Time of artificial insemination and fertility in dairy cattle. J. Dairy Sci. 69 : Hammond, J The Physiology of Reproduction in the Cow. University Press. Cambridge, England. 4. Henderson, J. A New Jersey artificial breeding results. Jersey Bull. 58: Maatje, K., S. H. Loeffler, and B. Engel Optimal time of insemination in cows that show visual signs of estrus by estimating onset of estrus with pedometers. J. Dairy Sci. 80: Nebel, R. L., W. L. Walker, M. L. McGilliard, C. H. Allen and G. S. Heckman Timing of artificial insemination of dairy cows : Fixed time once daily versus mornin g and afternoon. J. Dairy Sci. 77: Trimberger, G. W Breeding efficiency in dairy cattle from artificial inseminatio n at various intervals before and after ovulation. Nebraska Agric. Exp. Stn. Bull. No. 153, Lincoln. 8. Trimberger, G. W Ovarian functions, intervals between estrus, and conceptio n rates in dairy cattle. J. Dairy Sci. 39: Trimberger, G. W. and H. P. Davis Conception rate in dairy cattle by artificia l insemination at various stages of estrus. Nebraska Agric. Exp. Stn. Bull. No. 129, Lincoln. 10.Trimberger, G. W. and M. G. Fincher Regularity of estrus, ovarian function, and conception rates in dairy cattle. Cornell Agric. Exp. Stn. Bull. 911, Ithaca, NY. 11. Walker, W. L., R. L. Nebel, M. L. McGillard Time of ovulation relative t o mounting activity in dairy cattle. J. Dairy Sci. 79: Werner, G. W., L. E. Casida and I. W. Rupel Estrus, ovulation and artificia l insemination in cattle. Amer. Soc. Anim. Prod. Proc. 31:54. 64