ISRAEL JOURNAL OF
VETERINARY MEDICINE
VOLUME 54 (1), 1999
TITLE: FACTORS AFFECTING THE RATE OF TRUE POSITIVE
PREGNANCIES BASED ON ROUTINE MILK PROGESTERONE
EXAMINATION
AUTHOR: G. Francos
35 Shoham str 34679 Haifa Israel
Summary
In a study comprising 18233 milk progesterone P4 examinations performed
on 26 kibbutz herds three weeks after insemination, the association between
the progesterone concentration, the conception rate and the rate of true
positive cows (having
positive progesterone concentration values and
pregnant on rectal examination) was investigated. Cows with progesterone
concentrations of less than 5 ng/ml were classed as negative; those having
progesterone concentrations of 5.5 ng/ml or more were classed as positive.
The rates of negatives during the moderate and hot seasons were 27.4 %
and 40.9 % respectively (0.0001) For the rate of positives, the corresponding
values
were 72.6% and 59.1% (0.0001) respectively.The rates of true
positives in the moderate and the hot seasons were 51.3%
and 36.1%
respectively (0.0001).
The distribution of positive concentrations of P4 during the moderate and
hot seasons was similar. The cows with a high positive concentration (21or
more ng/ml) comprised during the moderate and the hot seasons 53% and
54.2 % of all positives respectively. Corresponding values for cows
with
medium positive (11-20 ng/ml) and low positive (6-10 ng/ml) concentrations
were 34.7% and 32.8%; and 12.3% and 13% respectively.
During the moderate seasons herds with a high average percentage of true
positives had higher overall conception rates than cows with medium or low
average percentages of true positives (0.00001). Correlation coefficients
between overall conception rate and the rate of true positives for the
moderate and hot seasons were 0.649
and 0.852 respectively. The
correlation coefficient between the overall conception rate and the rate of
cows with negative P4 concentrations for these periods was -0.307 and 0.353 respectively.
During the moderate seasons in the group of herds with the highest average
rate of true positives (59.7 %) and highest conception rate (43.5 %) in the
subgroups of cows with low, medium and high positive P4 concentrations,
the rate of true positives was
43.6%; 52.6%
and
65.7% (0.00001)
respectively. In contrast, the group of herds with the lowest average rate of
true
positives (42.4%) and the lowest conception rates (31.2%) in
subgroups of cows with low, medium
and high
positive progesterone
concentrations, the rates of true positives were 29.7%; 41.8% and
%(0.0001) respectively. For the
two groups
the
46.2
of herds with intermediate
average rates of true positives (52.8 % and 48.4 %) and conception rates of
37.7 % and 36.1 % the association between P4 values and the rate of true
positives was evident as well.
The average progesterone values of true positives were significantly higher
compared
to
false
positives
(0.0001).
The
average
progesterone
concentration of positive cows was the same during the moderate and hot
seasons.
The rate of true positives arriving pregnant to term of rectal pregnancy
examination was associated with progesterone concentration three weeks
after insemination and with the herd’s conception rate. Further studies on the
factors
influencing
progesterone
concentrations
three
weeks
after
insemination are indicated.
Introduction
Examination of progesterone in milk about three weeks after insemination
is highly indicative for the early diagnosis of no pregnancy in cows which do
not return in visible heat. The studies confirm that 94%-97% of cows with
negative progesterone values come in heat or are not pregnant according to
routine rectal pregnancy diagnosis (1,2,3,4,5,6).
The percentage of true positives, positive by progesterone examination and
positive by routine rectal pregnancy examination, according to different
studies is within the range of 52%-86% (1,2,3,4,7). Different workers found
that the rates of true positives vary strongly between the herds under
investigation and emphasized the role of the herd factor (2,5,9,10,11).
The discrepancy between the rate of cows apparently pregnant based on
progesterone values 19-23 days after the insemination and the rate of
pregnancy according to routine rectal examination reflects to a large extent
embryonal loss 16 to 35 days post insemination (12), but other factors, such
as breeding the cows in luteal phase (13), variation in cycle length (6,13), and
incorrect time of artificial insemination (14) are also involved.
A significant association was found between progesterone concentrations
during the last days of the luteal phase preceeding the first insemination and
the conception rate from the first insemination (15,16). An association
between progesterone concentrations on days19 to 23 and the conception
rate was demonstrated (1,3,9). Francos1998(17) showed that the
progesterone concentration three weeks after the first insemination is
connected positively with the conception rate during the entire lactation and
associated negatively with the incidence of repeat breeders. (cows
inseminated four times or more).
The aims of this work was to establish the factors influencing the rate of true
positives in the herd and to investigate the connection between the
progesterone concentration three weeks after insemination and the
conception rate.
Materials and Methods
The study was carried out on 26 Israeli Holstein kibbutz dairy herds
comprising about 300 cows and located throughout the country. The annual
milk yield was in the range of 8800-10500 kg.
As a basis for this work served the reports of examinations of progesterone
(P4) in whole milk sent routinely to three district laboratories during 19901992, 20-23 days after insemination, from cows which had not returned to
heat by this time. The milk samples were preserved with potassium
bichromate. P4 determination was carried out by quantitative enzyme
immunoassay using Ovucheck kits (Cambridge Veterinary Sciences. u.k)
The reports included following data: the cow’s indentity number, date of
sampling and the results of progesterone examinations. P4 concentrations of
5 ng/ml or less were considered negative, while a concentration of 5.5 ng/ml
or more was considered positive.
For some calculations the positive cows were divided into subgroups of 6-10
ng/ml;(low positive),11-20 ng/ml (medium positive), and 21or more ng/ml (high
positive) of progesterone. For one calculation the last subgroup was divided
again into 21-25 ng/ml and 26 or more ng/ml subgroups. Altogether data on
18233 examinations were analysed. Additional data related to the cows were
obtained from the Israeli Herd Book, which together with the progesterone
examination report, were combined in one computerised report which
included the following data:
Indentity number; date of calving; number of lactations; dates of
inseminations and their number; dates and results of progesterone
examinations; results of rectal pregnancy examination performed routinely by
the veterinary surgeon 40-50 days after insemination.
All herds were under intensive veterinary supervision; inseminations were
performed by qualified technicians working at the On and Sherut Insemination
Centers.
Definitions
True positive: progesterone-positive cows confirmed pregnant on routine
rectal pregnacy examination
False positve: progesterone-positive cows returning to heat or found not
pregnant
True negative: progesterone-negative cows returning to heat or found not
pregnant
False negative: progesterone-negative cows not returning to heat and found
pregnant
Average percentage of true positives in the herd= ratio of true total positives
to positives.
Overall conception rate: (OCR)-all pregnant according to rectal examination
X 100‫ ק‬all inseminations
All herds in the study were divided into subgroups according to the herd
percentage of true positives in the moderate climatic season as following:
Group A: above 57.5 %; group B: 50-57%; group C: 45-49.9%; group D:
below 45%.
This division was maintained during the hot season. It means that a herd
which was classified during the moderate period as belonging to group A was
classified as group A during the hot period as well. The calculations of the
association between the progesterone concentration and the true positive
rate and between average herd true positive rate and the conception rate
were performed separately for the moderate and the hot climatic periods.
Progesterone examinations which were carried out from July 16 to November
15 were considered as the hot climatic period, while examinations made from
November 16 to July 15 were considered as related to the moderate climatic
period.
Statistics
P4 values and the differences in the distribution of P4 values were
evaluated by Chi-square test; those of P4 concentration in the cows pregnant
and not-pregnant were analyzed by T-test.
Coefficient of correlation between conception rate of the herds surveyed
the rate of true positives and the rate of negatives were calculated separately
for the moderate and hot climatic periods.
Results
During the moderate and hot periods, negative values were found in 27.4 %
and 40.9 % of examinations, respectively; while the corresponding data for
positive values were 72.6% and 59.1% respectively (0.0001) (Table1). The
percentages of true positives during the moderate and the hot period was
51.3% and 36.1% respectively (p= 0.0001). (Table 1).
Table 1: Distribution of negative and positive p4 Vs. concentration..NARY
MEDICI
Moderate
season (% of
P4 (ng/ml)
% Pregnant
all cows)
Hot season
(% of all cows)
% Pregnant
< 5.0
27.4
4.0
40.9
1.9
p<0.0001
> 5.5
72.6
51.3
59.1
36.1
p<0.0001
No. of
examinations
12524
5709
During the moderate climatic period the distribution of negative and positive
values in subgroups A,B,C,D were numerically similar. (Table 2).
Table 2: Distribution of negative and positive p4 concentration according to herd
group and season:
Herd
Group
Moderate
Season
Hot
Season
P4
P4
(No. of
examinations
<5.5
>5.5
(No. of
examinations
<5.5
>5.5
A
2495
27.1
72.9
1143
37.7
62.3
B
5671
28.6
71.4
3107
44.2
55.8
C
1640
25.4
74.6
588
44.6
55.4
D
2718
26.4
73.6
871
30.3
69.7
Total
12524
27.4
72.6
5709
40.9
59.1
The rate of medium positive and high positive values from all examinations
during the moderate period was higher than in the hot period. (Table 3).
Table 3: Distribution of negative and positive p4 concentration according to
season:
P4
conecentration
<5.5
6-10
11-20
21+
total
No. of
examinations
Moderate
Season
27.4
8.9
25.2
38.5
100
12524
Hot Season
40.9
7.7
19.4
32
100
5709
p<0.001
p>0.001
Among the cows with positive P4 values, the distribution of low , medium
and high positives during the moderate period was not different from those in
the hot period. (Table 4).
Table 4: Distribution of P4 positive concentration according to season:
P4
conecentration
6-10
11-20
20+
total
No. of
examinations
Moderate
Season
12.3
34.7
53
100
9092
Hot Season
13
32.8
54.2
100
3374
The percentage of cows in the first lactation with negative P4 values during
the moderate period was significantly lower than cows in later lactations.
(Table 5).
During the moderate and the hot period the percentage of cows with 21or
more P4 value was significantly higher in the first lactation than during later
lactations. (Table 5).
No effect of the insemination number on the distribution of P4 values was
found.
Table 5: Distribution of P4 positive concentration in the first and following
lactation according to season:
Moderate period -12524 examinations
P4 (ng/ml)
First lactation
Second
lactation
< 5.0
25.3
28.6
6-10
7.2
9.7
11-20
22.2
26.6
21+
45.3
35.1
p<0.00001
Hot period -5709 examinations
P4 (ng/ml)
First lactation
Second
lactation
< 5.0
40.7
40.9
6-10
6
8.4
11-20
15.1
21.6
21+
38.2
29.1
p<0.00001
During the moderate period the average percentage of true positives in
subgroups A,B,C,D, was 59.7%, 52.8%, 48% and 42.4% respectively. During
the hot season the percentage of true positive in A,B,C,D subgroups was
41.9%; 34.%; 33.8% and 35.8% respectively. (Table 6) The differences in the
percentage of true positives between subgroups during the moderate season
were significant (Table 6), and the differences between subgroup A and
subgroups B,C,D during the hot period were also significant (Table 6). Thus
an overcarrying effect on the rate of true positive values from the moderate to
the hot season was evident for subgroup A only.
The percentage of true positives in a given herd subgroup was higher during
the moderate than in the hot climatic season. (Table 6).
The decrease in the rate of true pregnants between subgroups was
connected with a parallel decrease in the conception rate of those subgroups.
(Table 6).
Table 6: Precentage of true positive and overall conception rate of herds during
the seasons:
Moderate Period
Hot Period
Herd
Group
% True positive
OCR
% True positive
OCR
A
59.7
43.5
41.9
26.1a
B
52.8
37.7
34.1
19b
C
48.8
36.1
37.1
20.6b
D
42.4
31.2
33.8
23.6b
Total
51.3
38.3
36.1
22
p<0.00001
ab p<0.0001
The coefficient of correlation between the average rate of true pregnants
and the conception rate in the moderate and hot season was 0.649 and
0.852, respectively. (Table 7).
Table 7: Correlation coefficients between overall conception rate, percentage of
true positives and percentage of negatives:
% true positive
% negative
Moderate period
0.649
-0.307
Hot period
0.852
-0.353
The coefficient of correlation between the average rate of negatives and
conception rate in the moderate and hot season was -0.307 and -0.353,
respectively.
In progesterone-positive cows the P4 concentration determined three
weeks after insemination had a highly significant effect on the rate of true
positives which was lowest in the 6-10 ng/ml subgroup, intermediate in the
11-20 ng/ml subgroup and highest in the 21-25ng/ml and 26 or more ng/ml
subgroups. (Table 8) During the moderate climatic season with given P4
positive values belonging to subgroup A had a higher percentage of true
positives than cows with the same P4 values belongin to the subgroups B, C
or D. The overcarrying effect of these features from the moderate to the hot
season was evident for subgroup A only. (Table 8) Thus the percentage of true
positives was affected by P4 values determined three weeks after
insemination and by the herd status related to the rate of true positives and
the conception rate. (Table 8).
Table 8: Percentage of true positives in groups of cows with different positive P4
concentrations.
Herd Group
6-10
P4 (ng/mg)
11-20
21-25
26+
Moderate season
A
43.6
52.8
64.3
67.1
p<0.00001
B
40.9
50.1
54.7
58
p<0.00001
C
33.6
46
50.9
53.3
p<0.05
D
29.7
41.8
41.9
50.5
p<0.0001
Hot Season
25
37.4
45.1
50.6
p<0.00008
17.3
32.4
38.7
40.8
p<0.00001
14.3
28.8
42.6
50.9
p<0.00009
14
28.9
36.4
39.9
p<0.0014
True positive cows found pregnant according to rectal examination had
significantly higher P4 values three week after insemination, compared to
false positive cows returning to heat or not pregnant on rectal examination.
(Table 9) There was no difference in the average progesterone concentration
of positive cows between the moderate and the hot seasons. (Table 10).
Table 9: Average progesterone concentrations of true and false positive T-test.
Season
Moderate season
Hot Season
X
S.D
X
S.D
True positive
22.4361
7.670
23.2897
7.568
False Positive
20.2852
8.145
20.5713
8.312
Signif.
0.0001
0.0001
Table 10: Average progesterone concentrations of positive cow T-test.
No. of Examinations
X
S.D
Moderate Season
9092
21.3883
7.978
Hot Season
3374
21.5537
8.155
Signif
0.307
Discussion
The distribution of progesterone concentration.
The rates of the negative and the positive P4 concentrations examined
three weeks after insemination during the moderate climatic season were
27.4% and 72% respectively. (Table 1) Other studies report similar rate of
negative concentrations (27.2-29.4%) (3,9,18) with very similar rates of
positive P4 concentrations (% negative +%positive =100%) to those found in
our study.
The rate of true positives from all positives during the moderate season
being 51.3% (Table 1) is considerably lower compared to the reports of 81.2%
(18) or 71%-73% rate for true positives (1,2,3) but similar to the results of
(53.7%) (9) and not far from the findings of 60% (4) and (64.3%) (5).
In this study 96 % of the cows with negative P4 concentration were not
pregnant on the rectal examination (Table 1) close to other reports. (1,3,4)
The rate of false positives reflects to a considerable extent the incidence of
late embryonal death, (12) occurring from the time of progesterone
examination and until the time of the rectal or ultrasound negative pregnancy
examination.
The rate of negative concentrations reflects a fertilisation failure or early
embryonal deaths occurring before the term of P4 examination (19)
During the hot season the rates of negative concentrations were
considerably higher and the rates of positive concentrations considerably
lower compared to the moderate climatic season. (Table 1) It appears
therefore that heat stress contributes to the increased rate of early (until 20
days post-insemination) and late embryonal death.
The distribution of low, medium and high positives was similar in the
moderate and hot seasons indicating a trend of relatively constant distribution
of P4 concentrations in the positive group (Table 4)
Similarly the average P4 value in the positive group during the hot and
moderate season were similar (Table 10) and is consistent with ref.12.
The rate of 51.3% true positives from all positives is an average of 9092
examinations performed in 26 herds during the moderate climatic season.
(Table 6) In the subgroups the rate of true positives was in the range of 42.4%
to 59.7%. (Table 6)
The diversity in the rate of true positives reported in this and in other studies
- from 42.4% to 81.2% (1,5,9,18) indicates that the rate of false positives
ranging from 19.8 % to 57.6 % is extremely wide.
It is generally thought that most embryo mortality occurs until 20 days post
insemination. (10,20,21)This claim is supported by the low estimates of late
embryonal death (4.8%-6%) in a series of studies (18,22,23)
In some later studies the revised estimates of late embryonal death are
considerably higher 22%-29 %. (10,11) There is evidence for an increase in
the rate of late embryonal deaths as a result of possible stress associated
with breeding or management for high level milk production. (10,11,24) The
high rates of false positives in a recent study performed in high producing
herds are in accordance with these assumptions.
During the moderate and the hot climatic seasons a correlation of 0.64-0.85
was found between the percent of true positives and between the overall
conception rate of cows in surveyed herds. The correlation between the
overall conception rate and the rate of negative values is much lower and is
negative -0.30 and -0.35 (Table 7) what leads to the conclusion that one of
the main factors connected with low herd fertility is the high incidence of late
embryonal death as reflected by the low rate of true positives. The term of
routine P4 coincides with the period of implantation. (10) It seems therefore
that in low fertility, high producing dairy herds late embryonal death is more
important as a pregnancy reducing factor than early embryonal death
occurring before 20 days after insemination. It is likely that uterine rather than
embryonical factors are responsible for the late embryonal death. (10) In an
extensive epidemiological study, nutritional factors explained 67% of the
differences in kibbutz herds conception rates. (25) It is postulated that
nutritional imbalances occuring in high producing herds contribute to the
creation of unfavorable uterine environment detrimental for embryos in the
phase of implantation and a high rate of late embryonal death in
consequence.
Average progesterone concentrations in true-and false - positive cows.
In the progesterone-positive cows, the average P4 values were significant
higher in the true positive compared to the false positive cows. (Table 9) The
differences in the milk progesterone concentrations between inseminated
cows subsequently shown not to be pregnant and those subsequently
diagnosed as pregnant were reported by others. (26,27,28,29). These
differences, although highly significant, do not have any predictive use
because in absolute values they are small.
The association between the progesterone concentration and the rate of
true positives
Progesterone concentrations estimated three weeks after insemination
appeared to be important factor in determining the rate of true positives cows
arriving pregnant to the term of routine rectal examination. During the
moderate climatic season the rate of true positives was lowest in the low
positive group (6-10 ng/ml), intermediate in the medium positive group (11-20
ng/ml), and highest in the high positive group (21or more ng/ml). The
differences being prevalently highly significant. (Table 8) The same trend was
evident during the hot season, but the rate of true positives was lower than in
the moderate period. (Table 8) The connection between progesterone values
on days 19 through 23 and the chances of pregnancy were demonstrated in
other studies. (1,9) Cows with P4 values of 21+ ng/ml had the biggest
chances of pregnancy. In our study 53-54.2% of all cows with positive
values. (Table 4)
The rank of the herd according to the average rate of true positives was
another significant factor influencing the rate of true positives in low-medium
and high positive subgroups. During the moderate season, low positive cows
in subgroup A had a rate of true positives of 43.6 %. In subgroup D the rate of
true positives of low positive cows was 29.7 % only (Table 8).
Medium positive cows of subgroup A had a rate of true positives of 52.8 %
compared to 41.8% of true positives in subgroup D. The same trend was
evident for high positive subgroups. (Table
8). During the hot season herds in subgroup A had higher true positive rates
than the other three subgroups. (Table 8) It should be remembered that for
the hot season the division of the herds into groups A to D was made
according to the herd status during the moderate season and not according
to the actual herd status.
High correlation coefficient between overall conception rate and percentage
of true positives during the moderate and the hot seasons (Table 7) as well
the association between the average percentage of true positives in the
subgroups and the overall conception rate of these subgroups during the
moderate season (Table 6) demonstrated the association between the
conception rate and the rate of true positives of the herd. It is probable,
therefore, that the big differences in the rates of true positives reported in the
various investigations discussed above were connected with differences in the
conception rate of the herds in which the investigations were conducted.
The average progesterone concentration of positive cows examined three
weeks after insemination was not influenced by the climate (Table 10). This
was consistent with ref.12. On the other hand the rate of true positives that
were pregnant on rectal examination during the hot season was highly
significantly lower compared to the moderate season (36.1% v. 51.3 % (Table 1). It appears therefore that the increased rates of embryonal loss and
not the lowered progesterone concentration as proposed (7) are responsible
for low conception rates during the hot season.
Conclusions
Progesterone concentrations measured by milk examinations performed
three weeks after insemination and the interaction between the rate of true
positives and the conception rate are important factors subsequently
influencing the rate of cows pregnant at routine rectal examination. Further
studies for the determination of the factors influencing progesterone
concentrations three weeks after the insemination, particularly after the first
insemination, would provide information useful for planing measures to
improve the fertility of high producing dairy herds.
References
1. Cavestany D., and Foote R.H. The use of milk progesterone and electronic
vaginal probes as aids in large dairy herd reproductive management.
Cornell Vet. 75:441-453, 1985
2. Pennigton J.A., Schulz L.H. and Hoffman W.F. Comparison of pregnancy
diagnosis by milk progesterone on day 21 and day 24 postbreeding:Field
study in dairy cattle J. Dairy Sci 68: 2740-2745
3. Rajamahandrem R., Burton B., and Shelford J. A field study on usefulness
of milk progesterone determination to confirm estrus and pregnancy of
dairy cows in Fraser Valley area of British Columbia. Can. Vet. J. 34: 349352, 1993
4. Kovacevic R., Krsmanovic L., Maric D., Perkucin R.,and Veselinovic S.
Investigetion of hormone profiles in dairy cattle with a view to controlling
fertility and increasing reproductive efficiency. In proceeding of the
coordination meeting on isotope aided studies on livestock productivity in
Mediterranean and North African countries 255-274 1988 Rabat
5. Markusfeld (Nir) O., Nahari N., and Adler H. A routine 21 to 23/24
postservice milk progesterone monitoring for reducing the interinsemination
interval in dairy cows. Isr. J. Vet. Med. 44, 111-119 1989
6. Gowan E.N., Etches R.J., Bryden C., and King C.J. Factors affecting
accuracy of pregnancy diagnosis in cattle. J. Dairy Sci. 65:1294-1302 1982
7. Markusfeld (Nir) O., Adler H., Nahari N., and Kessner D. Impaired summer
fertility associated with lower milk progesterone levels in dairy cows. Isr. J.
Vet. Med. 46:54-56 1991
8. Noseir M.B., Gyawu P., and Pope G.S. Progesterone concentration in
defatted milk of dairy cows in early pregnancy. Br. Vet. J. 148: 45-53 1992
9. Klawonn W.,Leschhorn E.,and Jonas D. Milchprogesterontest: Ausagewert
und-sicherheit im Routineinsatz durch den Landwirt. Mh. Vet.-Med. 47:187192 1992
10. Ball P.J.H., Later embryo and early fetal mortality in the cow Animal
Breeding. Abstracts 65: 167-175 1997
11. Pursley J., R.Silcox R.W., and Wiltbank M.C. Effect of time of artificial
insemination on pregnancy rates, calving rates,pregnancy loss and gender
ratio after synchronization of ovulation in lactating cows. J.Dairy Sci
81:2139-2144 1988
12. Howell J.L., Fuquay J.W. and Smith A.E. Corpus luteum growth and
function in lactating Holstein cows during spring and summer.J.Dairy Sci
77:735-739
13. Wimpy T.H., Chang C.H.,Estergreen V.L. and Hillers J.K. Milk
progesterone enzyme immunoassay: Modification and a field trial for
progesterone detection in dairy cows. J. Dairy Sci. 69:1115-1121 1986
14. Noakes D. Pregnancy diagnosis in cattle. In Practice March, 46-51, 1985
15. Folman Y.,Rosenberg M., Herz Z. and Davidson M.The relationship
between plasma progesterone concentration and conception in post-partum
dairy cows maintained on two levels of nutrition. J. Reprod. Fert. 34 267278 1973.
16. Fonseca. F.A., Britt J.H., McDaniel B.T., Wilk J.C. and Rakes A.H.
Reproductive traits of Holstein and Jerseys, Effects of age, milk yield and
clinical abnormalities on involution of cervix and uterus, ovulation, estrus
cycles, detection of estrus, conception rate and days open. J.Dairy Sci
66:1128-1148,1983
17. Francos G. Association between milk progesterone concentration after
first insemination and conception in dairy cattle in Israel. Vet. Record.
142,63-64 1998.
18. Humblot P., Dalla Porta M.A. Effect of conceptus removal and intrauterine
administration of conceptus tissue on luteal function in the cow. Reprod.
Nutr. Develop. 24: 529-541 1984.
19. Kastelic J.P. Noninfectious ebryonic loss in cattle. Veterinary Medicine
584-589 June 1994.
20. Sreenan J.M., and Diskin M.G. Early embryonic mortality
in the cow: its relationship with progesterone concentration. Vet.Record.
112:517-521 1983.
21. Sheldon M. Bovine fertility-practical implications of the maternal
recognition of pregnancy.In Practice 546-556 November/December 1997.
22. Moller K., Lapwood K.R. and Marchant R.M. Prolonged service intervals in
cattle. New Zealand Vet. J. 34:128-132 1986.
23. Baxter S.J., and Ward W.R. Incidence of fetal loss in dairy cattle after
pregnancy diagnosis using an ultrasound scanner. Vet. Record. 140:287288 1997.
24. Peters A.R. Embryo mortality in the cow Animal Breeding. Abstracts
64:587-598,1996.
25. Francos G., Distl O., Kraeusslich H., and Mayer E. Epidemiological study
on factors influencing fertility indices in Israeli dairy herds. Theriogenology
38:385-405, 1992.
26. Bulman D.C., and Lamming G.E. Milk progesterone levels in relation to
conception, repeat breeding and factors influencing acyclity in dairy cows.
J. Reprod. Fert. 54,447-458, 1978.
27. Kastelic J.P.,Bergfelt D.R., and O.J. Ginther Relationship between
ultrasonic assesment of the corpus luteum and plasma progesterone
concentration in heifers. Theriogenology 33: 1269-1278, 1990.
28 .Larson S.F., Butler W.R., and W.B. Currie Reduced fertility associated
with low progesterone postbreeding and increased milk urea nitrogen in
lactating cows. J. Dairy Sci 80:1288-1295, 1997.
29.Hansel W. Plasma hormone concentrations associated with early
embryonal mortality in heifers. J. Reprod. Fert. 30: 231-239, 1981.