increased more rapidly and linearly to peak oestrus, followed by an exponential decay with no refractory period post-oestrus.
The optimal time of AI based on pedometer readings was predicted using mathemati- Ž
. cal models and rectal palpation of 171 cows Maatje et al., 1997 . The pedometers
Ž
w
. Boumatic Heat-seeker-TX ; Dairy Equipment, Madison, WI were set to produce an
Ž .
alarm signal flashing light when the mean activity of the last six 2-h periods was more than double the mean activity of the last six corresponding 2-h periods for the previous 2
days. Retrospectively, the time of the onset of oestrus and the number of hours from the beginning of increased activity to the time of AI was determined. The onset of oestrus
was determined as the first 2-h period in which the mean pedometer reading for the current 12-h window was twice the running mean of a comparable period in the previous
2 days. Chance of pregnancy was highest between 6 and 17 h after increased pedometer activity, and the calculated optimum time of AI was 11.8 1.7 h. The aim of most
Ž .
published applications of pedometers until Maatje et al. 1997 has been to improve rates Ž
. of oestrus detection. A review by Lehrer et al. 1992 stated that 70 to 80 of cows in
oestrus are detected by pedometer measurements. Pedometry systems, which allow identification of the onset of oestrus in addition to efficient and accurate oestrus
identification, will increase the usefulness of such technology in animal breeding. However, all current pedometry systems do not use real-time data transfer, thus
requiring the activity information to be retrieved by an interrogation device. Therefore, system requirements dictate that retrieval of activity measurements can occur two or
three times daily, usually at milking, reducing the effectiveness of determining the timing of insemination.
3. Pressure sensing radiotelemetric system
Radio frequency data communications is the base technology employed by the
w
Ž commercially available pressure sensing radiotelemetric HeatWatch
system DDx, .
Denver, CO . A radiotelemetric device attached to each cow consists of a miniaturized radiowave transmitter, powered by a lithium 3-V battery and linked to a pressure sensor
enclosed in a hard plastic case 5.3 = 8.1 cm and 1.8 cm in height. Each device is secured in a water-resistant pouch, attached to a 35 = 20 cm saddle-shaped nylon mesh
patch that is glued with contact-type adhesive to the hair caudal to the sacral region. Activation of the pressure sensor by weight of a mounting herdmate for a minimum of 2
Ž .
s produces a radiowave transmission 0.4-km range . Transmitted data consists of sensor Ž
. Ž
. identification, date month, day, and year , time h and min , and duration of sensor
Ž . activation s . Transmitted signals are sent to a microcomputer via a fixed radio antenna.
The remote signal receiver should be centrally located on each farm to maximize transmission area and situated to minimize transmission interference. Transmitted data
from a remote receiver are chronologically stored in a buffer external to the microcom- puter and transferred to a microcomputer at request of the software. The software
generates both fixed management reports and individual cow files that can be viewed or printed.
Ž .
Duration of oestrus, determined by video recordings Hurnik et al., 1975 , varied with the number of dairy cows in oestrus simultaneously, increasing from 7.5 to 10.1 h, with
one or three cows in oestrus, respectively. Duration of oestrus ranged from 2.6 to 26.2 h and averaged 14 h for 50 oestrus-synchronised beef heifers monitored with the pressure
Ž .
Ž .
sensing radiotelemetric system Stevenson et al., 1996 . Many heifers 20.5 had periods of oestrus activity - 10 h in duration. Monitoring two seasonal dairy herds on
Ž .
pasture with a similar radiotelemetric system, Xu et al. 1998 reported the average duration of oestrus for 89 cows was 8.6 h. There were 11.2 recorded mounts during each
oestrus for a total mounting duration of 29 s with an average duration of 2.5 s for each standing event.
Using the radiotelemetric system to monitor mounting activity and ultrasonography to determine the time of ovulation, a significant, positive relationship between duration of
Ž .
oestrus and time of ovulation was reported Walker et al., 1996 . A prolonged duration of mounting activity was associated with an extended interval from first mount to
Ž ovulation. However, this relationship existed over a relatively brief time interval 25 to
. 34 h ; therefore, differences in duration of oestrus would have limited importance in the
timing of AI. The average oestrus duration was 9.6 h with a large standard deviation of 6.9 h. In a subsequent study involving 2661 inseminations, the duration of oestrus did
Ž .
not have a significant effect on conception rate Dransfield et al., 1998 . Duration of oestrus, defined as the time interval from first to last standing event recorded by the
radiotelemetric system, averaged 7.1 5.4 h for 2055 oestrus periods. The duration of oestrus varies greatly not only among cows in the same herd but also among different
studies. Differences in age, herd size, management conditions, frequency of observation, and definition of onset of oestrus may account for most of the variation in duration of
oestrus among studies.
Determination of the onset of oestrus is only possible with continuous monitoring for behavioral activity to accurately determine the first standing event when a herdmate is
allowed to mount the animal in oestrus. Using time-lapsed video recording, Hurnik et al. Ž
. 1975 disclosed the highest frequency of onset of standing oestrus was observed during
1800 to 2400 h. After 12 years, when the study was repeated with more advanced video equipment, the hourly distribution of oestrus onset was tested against a normal distribu-
Ž .
tion and no significant pattern of onset was revealed Amyot and Hurnik, 1987 . Pooled 6-h intervals demonstrated the highest frequency of the onset of oestrus occurred
between 1200 to 1800 and 0600 to 1200 h in the primiparous and multiparous groups, respectively. Conflicting data concerning diurnal and nocturnal oestrual activities are
found in the literature. While the factors underlying these discrepancies are probably of a complex nature, a photoperiod effect directly or indirectly influencing oestrual
Ž .
expression may be a predominant one. In Amyot and Hurnik’s 1987 study, the sensitivity of the cameras permitted a nocturnal illumination of less than 3 lx, which is
Ž .
90 less than the illumination used in the earlier study Hurnik et al., 1975 . If oestrual activities are light-mediated, then suppression of oestrus expression should be expected
during the nocturnal period. Using the radiotelemetric system to monitor mounting activity in pasture-fed cows,
the onset of oestrus and distribution of total mounting activity occurred equally Ž
. throughout the day when grouped into 6-h periods
Xu et al., 1998 . However,
individual hourly variation did occur with the greatest number of first mounts or oestrus onsets between 1200 and 1500 h and between 2100 and 2300 h. Total mounting activity
did not parallel the hourly distribution of oestrus onsets and was more evenly distributed across the day despite a trend toward more mounting activity in the afternoon.
Since 1994, the HeatWatch
w
system has been the exclusive method used for the identification of oestrus at the Virginia Tech University Dairy Center. The distribution in
Ž .
Ž . Ž
onset of oestrus is shown graphically for heifers n s 393 and cows n s 1075 Fig. 1
. and Fig. 2, respectively . There were no differences for onset of oestrus among hourly
periods for either heifers or cows although it appeared that management practices influenced the onset of oestrus. Heifers were gathered between 0700 and 0800 h daily
Ž .
for concentrate feeding, which corresponded to the hour of peak initial activity Fig. 1 . During the remainder of the day, heifers were on pasture with very limited human
interaction and only a trend toward a diurnal pattern with more activity during the afternoon. A greater variation in the onset of oestrus was detected for cows. It appeared
that the three periods of peak onset activity occurred when cows were either being
Ž .
moved in groups during milking 2300 to 0000 and 1100 to 1300 h or loafing on dirt Ž
. lots during barn cleaning 0800 to 0900 h .
When the precise onset of oestrus has been determined for a significant number of oestrus periods, all studies have revealed that initial standing behavior occurs evenly
distributed throughout the day. The rise in estradiol-17b concentrations which occurs Ž
. almost simultaneously with the onset of oestrus activity Stevenson et al., 1998 is
responsible for the initiation of behavioral oestrus. Estradiol-17b is also indirectly responsible for the release of LH by the modification in the amplitude and frequency of
Fig. 1. Distribution in onset of oestrus at different hours of the day for 393 heifers monitored by the
w
Ž .
radiotelemetric HeatWatch system DDx .
Fig. 2. Distribution in onset of oestrus at different hours of the day for 1075 cows monitored by the
w
Ž .
radiotelemetric HeatWatch system DDx .
release of gonadotropin releasing hormone in the absence of progesterone, thus, estra- diol-17b is ultimately responsible for ovulation. The rise in Estradiol-17b, which
initiates this cascade of events, is probably independent of most environmental influ- ences, thus the onset of oestrus is equally distributed during the day and should dictate,
within management constraints, the timing of insemination.
Biological events that affect the successful timing of AI are the length of the Ž
. functional viable life of gametes sperm and ova , transport time of viable sperm from
the site of insemination to fertilization, and timing of ovulation in association with insemination as shown graphically in Fig. 3. The transport of viable spermatozoa to the
oviducts requires a minimum of 6 h to obtain a population capable of fertilization, and
Fig. 3. Average time relationships among reproductive events associated with fertilization in the bovine.
Ž sperm numbers progressively increase over 8 to 18 h Thibault, 1973; Wilmut and
. Hunter, 1984; Hawk, 1987 . The functional viable life of bovine spermatozoa in the
Ž .
reproductive tract has been estimated at 24 to 30 h Trimberger and Davis, 1943 . Ž
Ovulation following the onset of oestrus is approximately 27 h Chenault et al., 1975; .
Walker et al., 1996 . 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
Ž .
remarkably transitory, estimated at 6 to 10 h Brackett et al., 1980 . Thus, with the availability of a 24-h surveillance system to monitor behavioral events associated with
oestrus, it seems appropriate to re-examine timing of AI in dairy cattle. Therefore, the Ž
. primary objective of the study conducted by Dransfield et al. 1998 was to evaluate
timing of AI in dairy cows where the onset of oestrus was precisely determined using the radiotelemetric HeatWatch
w
system. Each farm selected a 3-h interval to inseminate cows identified in oestrus during the
previous 24 h. Pregnancy status was determined by data for return to oestrus and palpation of the uterus 35 to 75 days following insemination. Logistic regression
analysis for the probability of pregnancy was performed with a model including herd, interval from onset of oestrus to AI, standing events per oestrus, season, and days in
milk at insemination. The time interval from onset of oestrus to insemination signifi-
Ž .
cantly influenced P - 0.01 the percentage diagnosed pregnant 35 to 70 days following insemination. Odds of pregnancy resulting from AI increased approximately 34 for
cows inseminated between 4 and 12 h after onset of oestrus when compared with a baseline interval of 0 to 4 h after onset. Intervals from onset of oestrus to AI 16 h
were related negatively to the probability of conception. The bar graph shown in Fig. 4 represents the cows that were diagnosed pregnant relative to 4-h intervals from the first
standing event to insemination. A curvilinear relationship between interval and preg- nancy is unmistakable; conception rates were highest for cows inseminated from 5 to 16
h following the first standing event of oestrus. Inseminations performed between 4 and
Fig. 4. Percentage pregnant by 4-h intervals relative to timing of AI from first standing event detected by the
w
Ž .
radiotelemetric HeatWatch system DDx across 17 herds and 2661 inseminations. Number of inseminations
for each 4-h interval is within parentheses.
12 h following onset of oestrus achieved a conception rate of approximately 50 vs. Ž
. 30 for inseminations performed after 16 h from onset Fig. 4 . From previous reports
Ž .
Trimberger, 1948; Trimberger and Davis, 1943; Nebel et al., 1994; Maatje et al., 1997 , near-optimal conception rates would be expected for cows submitted for insemination 12
to 18 h after detection of oestrus. Mathematical modeling to predict the optimal time for AI using activity pedometers and visual signs of oestrus, estimated 11.8 h from onset
Ž .
Maatje et al., 1997 , which coincides with the approximate midpoint of the 5 to 16 h optimum using the HeatWatch
w
system.
4. Conclusions
