Environmental constrains on post-partum ovarian activity in Tanzanian Zebu cows

Milk progesterone measurements were used to study post-partum ovarian activity and risk factors for ovarian dysfunction in Zebu cows reared under a traditional pastoral management system in three villages in Morogoro region, Tanzania. Milk samples for progesterone analysis were collected twice per week from a total of 98 lactations. The most prevalent ovarian dysfunction detected was delayed onset of post-partum ovarian activity and cessation of cyclical ovarian activity after a period of normal ovarian function. Prolonged luteal phases were not observed. Resumption of post-partum ovarian activity was observed in 61 (62.2%) cows. Pregnancy was diagnosed in 43 (43.8%) out of 98 cows and was terminated by abortion in 7 (16.3%) cows. The median number of days from calving to resumption of ovarian activity was lower in cows that calved during the dry season (167 days) as compared with those that calved in the rainy season (200 days, P=0.03), probably because ovarian function was stimulated in cows entering the rainy season first after the puerperal phase had ended. Cyclical ovarian activity ceased in seven (11.5%) cows, and all but one case of cessation of ovarian activity took place during the dry season. Cows in their first and second parities experienced prolonged intervals (244 days) between calving and onset of ovarian activity compared with cows in their third or later parity (172 days, P=0.03). Cows with marked losses in body condition experienced prolonged intervals between calving and onset of ovarian activity (229 days) as compared with cows that lost <1 body condition unit (172 days, P=0.03). The time interval from calving to resumption of ovarian activity varied between the three villages of the study (142, 192 and 229 days, P=0.05), indicating local differences in management practices.     

Increased incidence of retained placenta associated with heat stress in dairy cows

A dairy herd experienced an increased incidence of retained placenta and postpartum metritis during late spring and summer of 1977 and 1978. The incidence of retained placenta and postpartum metritis was 24.05% during the period of May through September compared to 12.24% the rest of the year. The gestation period of cows having retained placenta and postpartum metritis was an average of 5.25 days shorter than that of cows not exhibiting these symptoms. The gestation period of cows calving during the warm season was an average of 2.82 days shorter than that of the cows calving during the cool season. Heat stress resulting in early parturition was the probable cause of this increased incidence of retained placenta and postpartum metritis. The number of days open was influenced both by abnormal postpartum period and season of calving. Cows having retained placenta and metritis were open an average of 31.69 days longer than cows with normal postpartum periods. Cows that calved during the warm season were open an average of 24.42 days longer than those calving during the cool season.     

Factors affecting postpartum ovarian activity in crossbred primiparous tropical heifers

The relationship between postpartum ovarian activity and a total of 9 variables was studied in a dry tropical environment. Primiparous cows (n=61) that had shown no peripartum abnormalities, and were not suckled but milked twice daily, were used in the study. Independent variables included crossbreeding, sex of the calf, season, body condition, weight of cow at calving, age of dam at calving, uterine involution, calf weight and accumulated milk yield. Diet was a controlled variable. Dependent variables were first estrus postpartum and/or first milk progesterone elevation prior to first estrus. A bull fitted with a chin ball marker was used to detect first estrus postpartum, while ovarian structures were palpated per rectum once a week. Progesterone was measured by RIA in milk samples collected twice weekly. First postpartum estrus was detected at 56 +/- 32 days postpartum, a first milk progesterone elevation was observed in 50.8% of cows at 42 +/- 27 days. Cows calving in the dry season had longer intervals and those who calved males had shorter postpartum intervals. Accumulated milk yield affected both intervals significatly (p < 0.01). Weight, age and uterine involution were asociated with first milk progesterone elevation, while crossbreeding, weight at calving and weight postpartum change were associated with the dependent variables.     

Postpartum resumption of ovarian cycling activity in first-calf suckled beef cows exposed to familiar or unfamiliar bulls

The objective of this experiment was to determine if the proportion of first-calf suckled beef cows that resumed ovulatory cycles and the interval to resumption of ovarian cycling activity differ after exposure to either “unfamiliar” bulls or cows on d 35 postpartum, after exposure to either “familiar” bulls or cows for the first 30–32 d after calving. Fifty Angus × Hereford cows were stratified by calving date, calf BW, and calf sex by d 3 postpartum, and assigned to be exposed to familiar epididectomized bulls (BEF; n = 25) or familiar mature ovariectomized (OVX) cows (CEF; n = 25). On d 35 after calving, 12 BEF cows were assigned to be exposed to unfamiliar bulls (BEU); likewise, 12 CEF cows were assigned to be exposed to unfamiliar OVX cows (CEU). Cows were in their treatments for either 95 d (BEF and CEF) or 60 d (BEU and CEU) during the experiment. Blood samples were collected every third d from the beginning to the end of the experiment. A rise in progesterone concentration of >0.5 ng/mL in consecutive samples was used as the criterion for resumption of ovulatory cycles. Exposing cows to bulls on d 5 after calving and then switching a subset of these cows to be exposed to unfamiliar bulls 30–32 d later did not (P > 0.10) alter: (1) the proportion of cows that resumed cycling activity; and (2) postpartum interval to resumption of ovarian cycling activity compared to cows exposed to familiar bulls. However, 32% more (P < 0.05) cows exposed to bulls (BEF and BEU) resumed cycling activity by approximately 14.8 d before cows that were exposed to OVX cows (CEF and CEU). We conclude that the familiarity of first-calf cows to either bulls or ovariectomized cows, did not affect the postpartum occurrence of cycling activity or the interval from calving to resumption of ovulatory cycles. However, bull exposure, whether familiar or unfamiliar, stimulates first-calf cows to resume ovulatory cycles sooner after calving than if they are not exposed to bulls.     

Resumption of postpartum ovarian cyclicity in high-producing Holstein cows

The objective of this study was to investigate the resumption of ovarian cyclicity postpartum in high-producing dairy cows in commercial dairy farms under subtropical conditions. The cows were kept in a free-stall or tie-stall barn. Milk samples were collected from cows twice weekly, and progesterone in the skim milk was assayed by double-antibody ELISA. Cows were examined rectally and vaginoscopically at 2-week intervals after calving. Body condition score (BCS) and body weights were taken before and after calving. A cow was considered to have resumed ovarian cyclicity on the day of ovulation if followed by regular ovarian cycles. Thirty seven percent (n=20/54) of the cows had normal resumption of ovarian cyclicity (resumption within 45 days after calving), and 63% (n=34/54) had delayed resumption (resumption did not occur until >45 days after calving). Delayed resumption Type I (one or more ovarian cycles with luteal phase >20 days, i.e. prolonged luteal phase; 31.5%) and delayed resumption Type II (first ovulation did not occur until > or =45 days after calving, i.e. delayed first ovulation; 24.1%) were the most common types of delayed resumptions. Almost half (46.3%) of the cows did not resume their ovarian cyclicity until >65 days postpartum. Cows with delayed resumption Type I had a higher incidence of abnormal cervico-vaginal discharge (64.7%) and incomplete uterine involution (94.1%) compared to cows with normal resumption (P<0.01). The BCS of cows with delayed resumption Type II were lower than those of normal resumption cows at 5 weeks and later in the postpartum period (P<0.05). Approximately two-thirds of high-producing cows had delayed resumption of ovarian cyclicity postpartum. Prolonged luteal phase and delayed first ovulation were two important ovarian dysfunctions that delayed postpartum resumption of cyclicity in high-producing dairy cows.     

Postpartum resumption of ovarian cycling activity in first-calf suckled beef cows exposed to familiar or unfamiliar bulls

The objective of this experiment was to determine if the proportion of first-calf suckled beef cows that resumed ovulatory cycles and the interval to resumption of ovarian cycling activity differ after exposure to either “unfamiliar” bulls or cows on d 35 postpartum, after exposure to either “familiar” bulls or cows for the first 30–32 d after calving. Fifty Angus × Hereford cows were stratified by calving date, calf BW, and calf sex by d 3 postpartum, and assigned to be exposed to familiar epididectomized bulls (BEF; n = 25) or familiar mature ovariectomized (OVX) cows (CEF; n = 25). On d 35 after calving, 12 BEF cows were assigned to be exposed to unfamiliar bulls (BEU); likewise, 12 CEF cows were assigned to be exposed to unfamiliar OVX cows (CEU). Cows were in their treatments for either 95 d (BEF and CEF) or 60 d (BEU and CEU) during the experiment. Blood samples were collected every third d from the beginning to the end of the experiment. A rise in progesterone concentration of >0.5 ng/mL in consecutive samples was used as the criterion for resumption of ovulatory cycles. Exposing cows to bulls on d 5 after calving and then switching a subset of these cows to be exposed to unfamiliar bulls 30–32 d later did not (P > 0.10) alter: (1) the proportion of cows that resumed cycling activity; and (2) postpartum interval to resumption of ovarian cycling activity compared to cows exposed to familiar bulls. However, 32% more (P < 0.05) cows exposed to bulls (BEF and BEU) resumed cycling activity by approximately 14.8 d before cows that were exposed to OVX cows (CEF and CEU). We conclude that the familiarity of first-calf cows to either bulls or ovariectomized cows, did not affect the postpartum occurrence of cycling activity or the interval from calving to resumption of ovulatory cycles. However, bull exposure, whether familiar or unfamiliar, stimulates first-calf cows to resume ovulatory cycles sooner after calving than if they are not exposed to bulls.     

Risk factors for resumption of postpartum estrous cycles and embryonic survival in lactating dairy cows

The objectives of this study were to evaluate factors associated with resumption of postpartum estrous cycles and embryonic survival in lactating dairy cows. Holstein cows, 6396 from four dairy farms were evaluated to determine the relationships among parity, body condition score (BCS) at calving and at AI, season of year when cows calved, and milk yield on resumption of postpartum estrous cycles by 65 days postpartum, and all the previous variables, estrual or anestrus and AI protocol on conception rates and embryonic survival at the first postpartum insemination. Cows had their estrous cycle pre-synchronized with two PGF_2α injections given 14 days apart and were inseminated between 69 and 82 days postpartum following either an estrous or ovulation synchronization protocol initiated 12–14 days after the presynchronization. Blood was sampled and analyzed for progesterone twice, 12–14 days apart, to determine whether cows had initiated onset of estrous cycles after calving. Cows were scored for body condition in the week after calving, and again at AI, between 69 and 82 days postpartum. Pregnancy was diagnosed at 30 ± 3 and 58 ± 3 days after AI. Farm influenced all reproductive outcomes evaluated. More (P < 0.0001) multiparous than primiparous cows had initiated estrous cycles. Onset of estrous cycles was also influenced (P < 0.01) by BCS at calving and at AI, BCS change, season, and milk yield. More (P < 0.001) cows that had initiated estrous cycles than anestrous cows were pregnant at 30 and 58 days after AI, but anestrus did not affect pregnancy loss. Conception rates were also influenced (P < 0.01) by parity, BCS at calving and AI, BCS change, and season; however, milk yield and insemination protocol were not associated with conception rates at 30 and 58 days after AI. Factors that reduced conception rate on day 30 after AI also increased pregnancy loss between 30 and 58 days of gestation. Improving BCS at calving and AI, minimizing losses of BCS after calving, and hastening onset of estrous cycles early postpartum are all expected to increase conception because of enhanced embryonic survival.     

Effects of supplementary feeding and suckling intensity on postpartum reproductive performance of small East African Zebu cows

Small East African Zebu cows (n = 120) which calved during the dry season were used to determine the effects of postpartum nutritional supplementation and suckling intensity on body weight, body condition score, reproductive performance and on calf mortality and preweaning growth performance. Cows were allocated based on calving body weight to either receive a supplementary feed of molasses blocks containing 10% urea or to serve as controls and to be grazed only. Within nutritional groups, cows were further assigned either to continuous suckling or to restricted suckling twice daily. Intact bulls were introduced 21 days after the first cow had calved, and data were collected for 8 months. Calving body weight and body condition score of dams ranged from 186 to 192 kg and from 3.8 to 4.2 and did not differ among the treatment groups. Supplementary feeding significantly (P<0.05) reduced calf mortality from 40 to 20%, while restricted suckling reduced mortality by 13% from 36 to 23%. Body weight and body condition score did not vary between groups during the supplementation period. Over the 8-month period, 65% of supplemented and 53% of control cows exhibited estrus (P>0.05). A significantly (P<0.05) karger percentage of restricted suckled cows showed estrus than of continuously suckled cows (74 vs 42%). The postpartum estrus interval was shorter by 54 days in supplemented than in control cows and by 13 days in restricted than in continuously suckled cows. The pregnancy rate was higher in supplemented than in control cows (37 vs 33%; P>0.05) and in restricted than in continuously suckled (46 vs 22%; P>0.05) cows.     

Bull effect on the reproductive performance of mature and first calf-suckled Zebu cows in the tropics

This experiment was designed to determine if continuous bull exposure of heifers 2 wk post partum to a first calf (30 to 36 m old) or mature (42 to 72 m old) Zebú Brahman suckled cows reduces the interval to first estrus and the number of days open. Post partum first calf cows were randomly assigned to be exposed continuously to epidectomized bulls with surgical ablation of the dorsal ligament of the penis (Group 35 FBE, n = 25) or not exposed to bulls (Group FNE, n = 22). Post partum mature cows were likewise assigned to be continuously exposed (Group MBE, n = 24) or not exposed to bulls (Group MNE, n = 25). Milk samples were collected twice a week for progesterone assays to determine resumption of ovarian cyclic activity. All the cows were artificially inseminated for 120 d while in their respective treatment groups. The first postpartum estrus percentages were 60 < 90 d, 24.4%; and > 90 d, 33.7%. The interval to first estrus f-or Groups FBE and FNE was 75.4 and 104.2 d, respectively, and differed significantly (P < 0.01). For Groups MBE and MNE the interval to first estrus was 76.3 and 95.0 d, respectively (P < 0.05). The number of open days for Groups FBE and FNE was 77.3 and 114.9, respectively (P < 0.01); while for Groups MBE and MNE it was 73.0 and 98.6 d, respectively (P < 0.01). The results of the present study indicate that introduction of bulls to first calving cows or mature Zebú Brahman cows after parturition provides a practical and economical option for reducing the extended post partum anestrus interval and days open under tropical conditions.     

Reindeer bull introduction affects the onset of the breeding season

Reindeer are seasonally polyestrus, short day breeders, with estrous cycles of approximately 20 days in length. The objective of this study was to investigate the effects of reindeer bull exposure on the onset of the reindeer cow breeding season and to investigate whether cows with calving experience responded differently than cows with no previous reproductive experience. During year 1, blood samples were collected weekly beginning 14 July and continuing until 15 September (n = 8) or 30 September (n = 8) in order to determine the onset of the breeding season, based on ovarian function, with no bull present. Plasma was stored frozen for later assay of progesterone (P(4)) following the conclusion of sample collection. Eight randomly selected cows were allowed to breed with a bull during year 1. The mean onset of ovarian activity in the first year was 15 September (range: 8-29 September). The bull was removed from cows more than 2 months prior to the start of the experimental period during year 2 and housed at a separate facility approximately 2 km distant. Blood samples were collected during year 2 from 14 cows 3x weekly beginning on 11 August (day 1) and continuing until 29 September (day 50) and plasma was stored frozen for later assay of P(4) following the conclusion of sample collection. On day 6, cows were divided into two groups such that group 1 (early bull exposure; EBE), consisted of four cows that had calved the previous spring and four cows that had no reproductive experience (n = 8). Group 2 (late bull exposure; LBE), consisted of three cows that had calved the previous spring and three cows that had no reproductive experience (n = 6). EBE experienced bull introduction on day 13, 23 days earlier than the average onset of ovarian activity during year 1. LBE experienced bull introduction on day 46, 10 days later than the average onset of ovarian activity during year 1. Progesterone concentrations were analyzed by ANOVA procedures for repeated measures. Bull presence was not requisite for the onset of ovarian activity during either year. Previous reproductive status had no effect on the onset of ovarian activity within EBE (P = 0.61) or within LBE (P = 0.92). Time of bull introduction had a significant effect on the onset of ovarian activity when EBE was compared to LBE (P<0.001). The first sustained increase in mean P(4) concentration above 1 ng/ml occurred on day 25 in EBE reindeer and day 41 in LBE reindeer. EBE reindeer initiated ovarian activity 12 days after bull introduction while LBE reindeer initiated ovarian activity 5 days before bull introduction. All cows penned with the bull conceived during both breeding seasons, demonstrated by production of live calves during the subsequent spring. Cows bred during year 1 all calved within a 9 day-period. During year 2, EBE displayed a more synchronous calving period compared to LBE (P<0.05). Results indicate that bull management affects the onset of the breeding season in reindeer cows, regardless of previous reproductive experience.     

Relationships between body condition score, body weight, and some nutritional parameters in plasma and resumption of ovarian cyclicity postpartum during pre-service period in high-producing dairy cows in a subtropical region in Japan

The objective of this study was to investigate the relationships among body condition score (BCS), body weight and plasma metabolite concentrations, and resumption of ovarian cycles postpartum in high-producing dairy cows in a subtropical region in Japan. The study was conducted between May 2001 and January 2003 in a commercial dairy farm in Hiroshima. Cows were kept in free-stall barns with sawdust and wood shavings for bedding, and fed a total mixed ration consisting of alfalfa, timothy and oat hay, corn, tofu ground-wet, beet pulp, cottonseed and soybean. Milk samples were collected twice weekly from 2 to 11 weeks postpartum, and progesterone concentrations in skim milk were determined by ELISA. At 8 and 2 weeks before calving and 3, 5, 7, 9, and 11 weeks after calving, both BCS and body weights were determined and blood samples were collected to determine plasma concentrations of total cholesterol (T-cholesterol), non-esterified fatty acids (NEFA), total protein, and urea nitrogen. The average 305 days milk yield was 10,422 kg. Out of 110 cows, 44.5% had normal resumption (ovulation occurred < or =45 days after calving, followed by regular ovarian cycles), 33.6% had a prolonged luteal phase (luteal activity >20 days), 12.7% had delayed first ovulation (no ovulation until < or =45 days after calving), 4.6% had a short luteal phase (luteal activity <10 days, except in the first cycle), and the remaining 4.6% had cessation of cyclicity (no luteal activity > or =14 days between cycles). The BCS of cows with delayed first ovulation were lower than the BCS of cows with normal resumption and prolonged luteal phase at 5 weeks (2.9+/-0.1 versus 3.1+/-0.1 and 3.2+/-0.1; P<0.05), 7 weeks (2.8+/-0.1 versus 3.1+/-0.1 and 3.2+/-0.1; P<0.01), 9 weeks (2.7+/-0.1 versus 3.0+/-0.1 and 3.2+/-0.1; P<0.01) and 11 weeks (2.6+/-0.1 versus 3.0+/-0.1 and 3.2+/-0.1; P<0.01) after calving. None of the plasma parameters differed (P>0.05) among different groups of cows. Cows loosing > or =1 unit BCS after calving had a prolonged interval to commencement of luteal activity, and were at greater risk of having delayed first ovulation. Poor postpartum nutritional status was associated with delayed first ovulation postpartum. Regular monitoring of BCS before and after calving would be useful in nutritional management, and help in preventing delayed first ovulation in high-producing dairy cows.     

Effects of abnormal ovarian cycles during pre-service period postpartum on subsequent reproductive performance of high-producing Holstein cows

The primary objective of this study was to investigate the effects of abnormal ovarian cycles during the pre-service postpartum period on subsequent reproductive performance of high-producing Holstein cows. The study was conducted in a commercial dairy farm with approximately 150 lactating cows, in a subtropical region of Japan. Animals were kept in free-stall barn, and fed a total mixed ration. Cows that calved from June 2001 to July 2002 were included in the study. Milk samples were collected twice weekly from 2 to 11 weeks postpartum, and progesterone concentrations in skim milk were determined by ELISA. After a voluntary waiting period of 40 days, cows detected in estrus were bred by artificial insemination (AI). Pregnancy was confirmed by palpation per rectum 40-70 days after AI. Out of a total of 91 cases, 39 (42.9%) had normal ovarian cycles (ovulation within 45 days after calving, followed by normal ovarian cycles), 32 (35.2%) had prolonged luteal phase (i.e. luteal activity for >20 days), and 12 (13.2%) had anovulation or delayed first ovulation (i.e. first ovulation did not occur until >45 days after calving). The remaining (8.8%) had other types of abnormalities. When compared with cows with a normal ovarian cycle, prolonged luteal phase cows had a lower 100 days AI submission rate, conception rate and pregnancy rate (84.2% versus 56.3%; P<0.05, 50% versus 16.7%; P<0.05 and 42.1% versus 9.4%; P<0.01, respectively), and longer intervals to first AI and to conception ( 67+/-6 days versus 98+/-7 days and 95+/-9 days versus 136+/-11 days; P<0.01 for each). Similarly, when compared with cows with normal ovarian cycles, those with anovulation had lower 100 days conception rate and pregnancy rate (50% versus 0%; P<0.05 and 42.1% versus 0%; P<0.01, respectively), and longer intervals to first AI and to conception ( 67+/-6 days versus 93+/-12 days; P<0.05 and 95+/-9 days versus 155+/-14 days; P<0.01, respectively). Survival analysis of the data for calving to conception interval showed that cows with prolonged luteal phase or anovulation were getting pregnant at a slower rate, and took longer to get pregnant than the cows with normal resumption of ovarian cyclicity postpartum. In conclusion, abnormal ovarian cycles during the pre-service period postpartum adversely affected reproductive performance, including AI submission rate, pregnancy rate, interval to first AI, and calving to conception interval in high-producing Holstein cows.     

Effect of eCG on early resumption of ovarian activity in postpartum dairy cows

The purpose of the present study was to hasten the resumption of ovarian activity early postpartum in lactating dairy cows, using equine chorionic gonadotropin (eCG), to enhance follicular growth, followed by hCG, to induce ovulation. Primiparous Holstein dairy cows (n=21) were assigned equally into eCG, eCG-hCG and Control groups. Cows in the eCG and eCG-hCG groups received an i.m. injection of eCG (500 IU Folligon?) on Day 6 postpartum. Cows in the eCG-hCG group were also given an i.m. injection of hCG (500 IU Chorulon?), once dominant follicle reached the diameter of 13-16 mm following eCG injection. Cows in Control group did not receive any treatment. Daily blood sampling and ultrasound examination were conducted, starting at Day 6 postpartum until confirming the third ovulation. Follicles ≥10 mm in diameter were detected on Day 11.5±1.48, 10.1±0.52 and 11.1±1.36 after calving in Control, eCG and eCG-hCG groups, respectively (P>0.05). The first wave dominant follicle ovulated in 71.4% of cows treated with eCG and eCG-hCG. In contrast, none of the first wave dominant follicles ovulated in Control cows. By Day 20 postpartum, all cows in eCG group, 6/7 cows in eCG-hCG group and none of the cows in Control group ovulated (P<0.05). Short estrous cycles (≤16 days) were detected in 2/7, 1/7 and 6/7 cows in eCG, eCG-hCG and control groups, respectively (P<0.05). In conclusion, injection of eCG on Day 6 postpartum could assist the early resumption of ovarian activity by enhancing ovarian follicle growth and early ovulation in postpartum cows. In this context, subsequent hCG injection may not provide any more beneficial effect.     

Influence of a CIDR prior to bull breeding on pregnancy rates and subsequent calving distribution

We determined whether insertion of a CIDR for 7 days prior to the breeding season enhanced pregnancy rates and altered the date of conception in suckled beef cows mated naturally. Suckled beef cows (n=2033) from 15 locations were randomly assigned to one of two treatments: (1) cows received a CIDR 7 days prior to the breeding season for 7 days (CIDR; n=999); (2) cows received no treatment (Control; n=1034). On the first day of the breeding season bulls were introduced to herds at a rate of 15-25 cows per yearling bull or 20-30 cows per mature bull. Pregnancy status and the date of conception were determined via transrectal ultrasonography at 56 and 120 days after initiation of the breeding season. Overall pregnancy rates ranged from 59.3 to 98.9% among the 15 locations. The percentage of cows becoming pregnant during the first 30 days of the breeding season was similar between CIDR (68.2%) and Control (66.7%) cows, and overall pregnancy rates were similar between CIDR (88.9%) and Control (88.6%) cows. The average day of conception after initiation of the breeding season was shorter (P<0.01) for CIDR (20.1+/-0.8 days) compared to Control cows (23.2+/-0.8 days). Of cows conceiving during the breeding season, more (P<0.05) CIDR cows (35.9%) conceived during the first 10 days of the breeding season than Control cows (30.8%). Neither body condition score and nor parity affected pregnancy rates or days to conception, whereas pregnancy rates and days to conception were affected (P<0.01) by location and days postpartum. Days to conception were greater for cows that calved within 40 days (31.6+/-1.2 days) of initiation of the breeding season compared to cows calving between 40 and 50 days (25.3+/-1.2 days) prior to initiation of the breeding season, which were greater than those cows calving between 50-60 days (20.0+/-0.8 days) and 60-70 days (21.3+/-1.0 days) prior to initiation of the breeding season. Cows calving greater than 70 days (17.3+/-1.5 days) from initiation of the breeding season had the shortest interval to conception. We concluded that insertion of a CIDR prior to the breeding season failed to increase overall pregnancy rates, but did influence the average day of conception.     

Influence of parity on follicular dynamics and resumption of ovarian cycle in postpartum dairy cows

The present study examined ovarian changes preceding the resumption of the ovarian cycle in postpartum dairy cows with different parities under similar body nutritional conditions. In postpartum primi- (n=6), bi- (n=4), and multiparous (n=6) Holstein dairy cows, ovarian ultrasonographic observations starting at 7 days after calving were performed every other day and then daily after the confirmation of clinical signs of oestrus for the detection of postpartum first ovulation. Blood samples were collected at the same time as ultrasonography and analyzed for oestradiol and progesterone to monitor ovarian activity. To evaluate the nutritional condition of the cows, body weight and body condition score (BCS, 1=emaciated to 5=obese) were measured weekly and blood samples for the analysis of glucose, insulin, and non-esterified fatty acid (NEFA) were collected at the same time until postpartum second ovulation. Dominant follicles (>8mm in diameter) of the first follicular wave were detected at 7 days after calving in all cows. The first wave follicle ovulated in five of six multiparous cows, whereas no first wave follicle ovulated in any of the primiparous cows. The days to first ovulation after calving in primiparous cows (31.8+/-8.3 days) were significantly greater (p<0.05) than those in multiparous cows (17.3+/-6.3 days), but were not significantly different from biparous cows (28.8+/-8.6 days). There was a significant relationship between parity and days to first ovulation after calving (p<0.05). BCS was maintained at a level of more than 2.5 during the postpartum period in all cows and there was no influence of parity on postpartum changes in BCS, glucose, insulin, or NEFA throughout the experiment. The present study demonstrated a negative relationship between parity and number of days from calving to first ovulation in dairy cows under similar body nutritional conditions. It is possible that the influence of parity on the resumption of ovarian cycle is modulated by the factors different from the nutrition-related changes during the postpartum period in dairy cows.     

Characteristics of temporal patterns of cortisol and luteinizing hormone in primiparous,postpartum, anovular,suckled, beef cows exposed acutely to bulls

Background  

The physiological mechanism by which bulls stimulate resumption of ovarian cycling activity in postpartum, anovular, suckled cows after calving may involve the concurrent activation of the hypothalamic-hypophyseal-ovarian (HPO) axis and hypothalamic-hypophyseal-adrenal (HPA) axis. Thus, the objectives of this experiment were to determine if characteristics of temporal patterns of cortisol and luteinizing hormone (LH) in postpartum, anovular, beef cows are influenced by acute exposure to bulls. The null hypotheses were that daily, temporal characteristics of cortisol and LH concentration patterns do not differ between cows exposed acutely to bulls or steers.     

Serum cholesterol and triglycerides in postpartum beef cows and their relationship to the resumption of ovulation

The variations in lipid metabolism according to the physiological stage and their relationship to the resumption of postpartum ovarian cyclicity were assessed in Limousine beef cows fed a grass diet over 3 yr. Weekly blood samples were collected from 59 cows beginning 10 wk before to 20 wk after calving to evaluate serum cholesterol and triglyceride concentrations and electrophoretic lipoprotein fractions. After parturition, progesterone concentrations were also measured at weekly intervals to determine time of resumption of ovulation. Cows were categorized by resumption of postpartum ovarian cyclicity into 3 groups: early (4 to 6 wk post partum, n = 36); mid (7 to 10 wk post partum, n = 46) and late (after 11 wk post partum, n = 38). Higher serum triglyceride values (P<0.05) were observed during the last 10 wk of pregnancy (0.36+/-0.15 g/L) than during the first 20 wk of suckling (0.29+/-0.09 g/L). Cholesterol values decreased significantly (P<0.05) at the end of pregnancy, were minimal (1.01+/-0.03 g/L) at parturition, and increased again up to 9 wk post calving. Increased cholesterolemia and low serum triglyceride values after calving could be linked to the increased bovine alpha-lipoprotein fraction and decreased beta fraction. Serum triglyceride concentrations were not related to the resumption of postpartum ovarian cyclicity. Higher serum cholesterol values were observed from 2 wk before to 4 wk after calving in cows with early rather than mid and late resumption of ovarian cyclicity. Therefore, modifications in lipid metabolism during the puerperium seem to be related to resumption of cyclicity during the early postpartum period.     

Reproductive function in beef cows calving in the spring or fall

Plasma progesterone was determined twice weekly for approximately 100 days postpartum in suckled purebred (Hereford) and crossbred (beef x dairy) cows that calved in the spring or late summer/fall season. The progestèrone profiles and occurrence of estrus were used to determine ovulation times and to monitor ovarian function. Postpartum ovulations occurred significantly earlier in crossbred than in purebred cows (38.1 ± 18.5 vs. 58.1 ± 21.8, P<0.01) and in fall compared to spring calving cows (32 ± 13.9 vs. 59.1 ± 20.3, P<0.001). Rations providing either 70 or 100% of requirements for metabolizable energy were fed from 30 days prepartum until the end of the subsequent rebreeding period. Cows receiving the 70% energy ration ovulated slightly earlier but there was no effect of ration on days to pregnancy. The minimal effect of energy ration was not unexpected in this trial since many of the cows were overconditioned during late gestation.The correlation between calendar date of calving and interval to first ovulation was significant for spring (r = ?0.38, P<0.01) but not for fall calving animals. Since cows were confined and received a balanced ration of stored feeds throughout the year, photoperiod and/or temperature rather than nutritional factors would be the probable cause of delayed ovarian activity in spring calvers.Reproductive performance was assessed during the period when samples were collected and the response for purebred (n=105) and crossbreds (n=142), respectively were: ovulated by day 60 postpartum, 57 vs. 87%; mated by day 100 without conception, 17 vs. 26%; ovulated before day 100 without detection and mating, 10 vs. 4%; anovulatory to day 100, 7 vs. 0%; pregnant by day 100, 67 vs. 70%. Similar comparisons for spring (n=133) and summer/fall (n=144) calvers, respectively, were: ovulated by 60 days postpartum, 56 vs. 96%; mated by day 100 without conception, 27 vs. 17%; ovulated by day 100 without detection and mating, 8 vs. 5%; anovulatory to day 100, 5 vs. 0%; pregnant by day 100, 60 vs. 78%. The difference with the greatest practical significance is that a higher proportion of the late summer/fall calving animals were pregnant by day 100 postpartum (P<0.01) which indicates that reproductive performance is superior in fall calving beef cows.     

Postpartum weight and body condition loss and performance of fall-calving cows

A total of 166 mature fall-calving Hereford cows were used to study the effects of weight and body condition losses on the reproduction of cows and on the performance of their calves. This study was conducted before and during the breeding season over a 3-yr period. The cows calved in a good body condition and were assigned to one of the following treatment groups: the MM group to maintain weight from calving (September - October) through breeding (December - January), the LM group to lose up to 10% of the postpartum weight from calving to the beginning of breeding and to maintain the weight during breeding, and the ML group to maintain weitht from calving to the beginning of breeding and to lose 10 - 15% of the postpartum weight during breeding. Weight losses were adjusted by altering daily amounts of protein supplement and forage. Cows in the LM treatment group lost 3, 17 and 6% of their postpartum weight before breeding in yr 1, 2, and 3, respectively, and they had a 10-d longer (P <0.05) interval from calving to first estrus than MM or ML group cows. The percentage of cows with ovarian activity at the beginning of breeding was reduced (37 vs 64%, P <0.01) for LM compared with MM cows. Treatment-by-year interactions were significant for pregnancy rate and ovarian activity at the end of breeding. In Year1, ML cows lost 14% of postpartum weight during breeding and had reduced (P <0.01) pregnancy rates compared with MM or LM cows (50 vs 79 and 88%). Only 41% of ML cows in Year 1 had luteal activity after estrus compared to 93% for MM and 79% for LM cows. During Year 2, LM cows lost 17% of postpartum weight before breeding, while ML cows lost 6% of their postpartum weight before breeding and 11% during breeding. Pregnancy rates tended to be reduced for both LM and ML cows compared to MM cows (53 and 65 vs 87%, P >0.10). The percentage of cows with ovarian activity at the end of breeding was reduced (P <0.05 for the LM treatment group and tended (P >0.10) to be lower for ML than for MM cows. In Year 3, pregnancy rates were greater (89, 84 and 85% for MM, LM and MM, respectively) for all groups. However, the percentage of cows with luteal activity after estrus was greater (P <0.05 for MM (94%) than for LM and ML (64 and 67%). Calf weaning weights (205 d in April) were not significantly influenced by treatment. We conclude that even if cows have adequate energy reserves at calving (good body condition), optimal reproductive efficiency of fall-calving range cows cannot be ensured.     

The time required for the presence of bulls to alter the interval from parturition to resumption of ovarian activity and reproductive performance in first-calf suckled beef cows

This experiment was designed to determine 1) if exposure of firstcalf suckled beef cows to mature bulls in the first 30 days, after 30 days, or continuously post partum reduces the postpartum anestrous period and 2) if exposure to bulls alters the first service pregnancy rate. Postpartum first-calf suckled crossbred (Angus x Hereford; Hereford x Angus) cows were randomly assigned to be 1) exposed continuously to mature, epididectomized bulls (BE; n = 18); 2) exposed to bulls for the first 30 days post partum (BE/NE; n = 17); 3) exposed to bulls after the first 30 days post partum (NE/BE; n = 16); or not exposed to bulls (NE; n = 18). Blood samples were collected weekly to be assayed for progesterone to determine resumption of ovarian cyclic activity. All the cows were bred for 21 days by AI while under their respective treatment regimens and were then exposed to fertile bulls for an additional 35 days. The postpartum interval to resumption of ovarian cyclic activity did not differ (P>0.10) among the 3 (BE, BE/NE, and NE/BE) treatment groups, but it was 15.4 d shorter (P<0.05) than for cows in the NE group. The overall pregnancy rates did not differ (P>0.10) among the treatment groups. The AI pregnancy rates for the BE/NE and NE/BE treatment groups were higher (P<0.05) than for the NE group. The AI pregnancy rates for the BE and NE treatment groups did not differ (P>0.10). The results showed that all three treatments (BE, BE/NE and NE/BE) similarly decrease the postpartum interval and that exposure to bulls may improve the reproductive performance of first-calf suckled beef cows.