Postpartum Anestrus Research Articles

Economic values for reproduction traits in beef suckler herds based on a calving distribution model

Economic values for the genetic improvement of gestation length, post partum anoestrus interval, conception rates at first or later post partum oestrus and calving day (relative to the first day of the calving season) in seasonal calving beef suckler herds were estimated. The approach used combines partial budgeting of the economic cost of a barren cow with a model of the herd calving distribution which is driven by assumed levels of reproductive parameters. Economic values for all traits were shown to be highly variable across farms and are also likely to vary across years. For an increase in gestation length and equivalently in post partum anoestrus interval, estimates of economic values ranged between −0.2 and −2.77 £/day with higher values apparent in herds with a wide calving spread. Economic values for conception rates were higher at later (0.25 to 1.28 £ per % increase) than at first (0.1 to 0.7 £ per % increase) post partum oestruses. For calving day, economic values were approximately half of those derived using an alternative method from the literature which assumes calving day to be normally distributed. Implications for establishing selection criteria to choose terminal beef sires and suckler beef cows are discussed.

Effects of chromium supplementation on early lactation performance of Holstein cows

Two experiments were conducted to evaluate the effects of chromium (Cr) supplementation on early lactation performance of Holstein cows. Experiment 1 involved 12 primiparous (PP) and 22 multiparous (MP) cows, while exp. 2 involved 18 PP and 22 MP cows. In each trial, cows were fed total mixed rations (TMR) and assigned to one of two treatments: control or 0.5 mg kg−1 supplemental chelated Cr from week 6 prepartum to week 16 postpartum The cortisol response to synthetic adrenocorticotropin (ACTH) intramuscular (i.m.) and the luteinizing hormone (LH) response to synthetic gonadotropin-releasing hormone (GnRH) i.m. were evaluated as well, during weeks 2 and 6 postpartum in exp. 2 During the first 16 wk of lactation, supplemental Cr tended to increase milk yields in PP cows by 13% (P = 0.06) and increased milk production by 7% (P < 0.05) in exps. 1 and 2, respectively, but had no effect in MP cows. Despite increased milk yield, Cr supplementation had no adverse effect on milk composition, reproductive performance, or health status. Furthermore, Cr supplementation tended to increase the milk lactose content in PP cows. Supplemental Cr reduced serum β-hydroxybutyrate levels and may reduce subclinical ketosis in MP cows. In addition, in overconditioned, aged cows, Cr supplementation may reduce the incidence of certain other metabolic disorders. Peak values and production rates of cortisol in Cr-supplemented cows tended to be greater (P < 0.10) than in control cows at week 6 Basal LH concentrations did not differ between the two treatments for either week However, the Cr-supplemented cows released less (P < 0.05) LH than did control cows at week 6. Results from hormonestimulation tests suggest that the Cr-supplemented cows may have experienced a greater stress with their increased milk production. However the time in postpartum anestrus and the time to conception were similar, suggesting that Cr supplementation may have mitigated the slower return to fertility usually seen in higher producing PP dairy cows. Further study is needed to determine the effects of Cr supplementation on reproductive performance, health status, and metabolic disorders in dairy cows. Key words: Chromium, milk production, luteinizing hormone, cortisol, reproduction, dairy cows

Post-partum anoestrus in dairy cows: a review.

In modern high-yielding dairy herds fertility is of major economic importance. In order to gain maximum profit, calving intervals should not exceed 365 days. The achievement of a 365-day calving interval requires an early resumption of ovarian activity, an excellent oestrus detection, and a high first-service conception rate. Especially the inability to detect oestrus and to mate the cows by 60 to 80 days after calving is a common problem among dairy farmers nowadays. In this article a review is given about the occurrence, causes, treatment, and prevention of post-partum anoestrus in dairy cows.

Controlling reproduction to meet future market requirements

The profitability of suckled calf production is closely linked to the number and value of calves weaned. Hence, herd fertility is a crucial measure of efficiency. For the commercial suckled calf producer fertility can be defined as the production of a viable, quality calf when required. While there are many factors influencing herd/individual conception rates, three of the most important are time, nutrition and calving difficulties.Conception rates to first oestrus post-partum are low, particularly where post-partum anoestrus periods are short. This effect can be clearly seen in trials monitoring over 1500 suckler cows where fertility was depressed if the interval between calving and mating was less than 30 days, ie the calving period extended over 50 days. Where however the post-partum anoestrus period is extended the depression in conception rate to first oestrus is less marked.

Influence of fenceline bull exposure on duration of postpartum anoestrus and pregnancy rate in beef cows

Two experiments (1991 and 1992) were conducted to test the hypotheses that fenceline exposure of cows to bulls following parturition shortens duration of postpartum anoestrus and improves pregnancy rate to artificial insemination (AI). In Experiments 1 and 2, 75 and 125 postpartum crossbred beef cows, respectively, were stratified by calving date and while maintaining equivalent primiparous cows in each treatment group were randomly assigned to be exposed to bulls through the fenceline (BE) or not exposed to bulls (NE). In each experiment, twice weekly blood samples were collected for 13 weeks beginning 1 to 4 weeks postpartum. These samples were used to assess concentrations of progesterone and determine time of onset of ovarian luteal function following calving. In Experiment 2, a 23 day program of AI was included in the study in which signs of behavioral oestrus were detected twice daily and cows were artificially inseminated 12 h following detection of oestrus. Pregnancy rate to AI was determined by concentrations of progesterone, detection of pregnancy by rectal palpation, and confirmed by calving date. Primiparous cows in the BE group had shorter durations of postpartum anoestrus in each experiment than primiparous cows in the NE group (Experiment 1: BE = 78 days, NE = 92 days, P < 0.05; 2: BE = 109 days, NE = 117 days, P < 0.10). There was no influence of fenceline bull exposure on the duration of postpartum anoestrus of multiparous cows in either experiment. Pregnancy rate to AI in Experiment 2 was not improved by fenceline bull exposure. We conclude that fenceline bull exposure was effective in shortening length of postpartum anoestrus in primiparous cows but this improvement did not translate into improved pregnancy rates during the subsequent breeding season.

Controlling reproduction to meet future market requirements

The profitability of suckled calf production is closely linked to the number and value of calves weaned. Hence, herd fertility is a crucial measure of efficiency. For the commercial suckled calf producer fertility can be defined as the production of a viable, quality calf when required. While there are many factors influencing herd/individual conception rates, three of the most important are time, nutrition and calving difficulties.Conception rates to first oestrus post-partum are low, particularly where post-partum anoestrus periods are short. This effect can be clearly seen in trials monitoring over 1500 suckler cows where fertility was depressed if the interval between calving and mating was less than 30 days, ie the calving period extended over 50 days. Where however the post-partum anoestrus period is extended the depression in conception rate to first oestrus is less marked.

Creep feeding and prepartum supplementation effects on growth and fertility of Brahman-cross cattle in the dry tropics

Post-partum anoestrus is a primary contributor to low branding rates in Bos indicus cattle herds in the dry tropics of northern Australia [Entwistle, K. W. (1983). Australian Meat Research Committee Review No. 431. To increase branding rates, it was hypothesised that creep feeding for a short period in mid-late lactation during the latter half of the growing season may trigger an earlier onset of post-partum oestrus cycling, just as short-term, high-level, prepartum supplementation can achieve. Two experiments were conducted using Fn Brahman-cross cows (1/2, 5/8 and 3/4 crosses with Beef Shorthorn) which calved from late October to late Januarv. Cows were mated from mid-late Januarv to mid-April. Calves in one treatment in both experiments had ad libitum access to creep feed (calf pellets: 16% crude protein, 10 MJ ME/kg) for 4042 days from late February to early April. In experiment 2, the effects on cow growth and fertility due to supplementation with either cottonseed meal (1.5 kg/day) or molasses with 7.4% (w/w) urea for 49 days late in the dry season before calving ('spike' feeding) were also evaluated. Control cattle were unsupplemented. Creep feed was only consumed at 0.1 and 0.4 kg/day in experiments 1 and 2, respectively. Short-term creep feeding had no consistent effects on cow liveweights, condition, or fertility, or on calf growth and temperaments under extensive grazing conditions during the tropical wet season. Spike feeding reduced weight loss by 0.2-0.4 kg/day (P&lt;0.01). The effects on liveweights did not persist into the wet season. There were no effects on cow fertility in this year of extreme weather conditions, when 4 months of nutritional and climatic stress followed supplementation.

Reducing post-partum anoestrous interval in first-calf Bos indicus crossbred beef heifers. III. Effect of nutrition on responses to weaning and associated variation in metabolic hormone levels

Interactions between effects of undernutrition and weaning on the duration of post-partum anoestrus and associated variation in milk yield, suckling behaviour, and metabolic hormone levels in Droughtmaster [Bos indicus x Bos taurus (518, 3/8)] first-calf cows were studied in 2 successive years. Low body condition score at calving (3.5f 0.1, scale 1-9) was associated with prolonged post-partum anoestrous intervals in suckled cows, but provided cows were maintaining liveweight, ovarian cyclicity resumed within 50 days if calves were weaned 70 days post-partum. This response to weaning was totally inhibited in cows that calved in low body condition that lost liveweight post-partum (Year 1). In contrast, marked levels of post-partum liveweight loss (23%) did not affect resumption of ovarian cyclicity in response to weaning (at 50 days post-parturn), or time to conception, among cows that calved with high body condition scores (5.8 � 0.2, Year 2). Reduced milk yields and calf weight gains among underfed cows were associated with higher suckling frequencies and durations ( P &lt; 0.05). Acyclic interval after weaning was negatively related to body condition score at weaning (P &lt; 0.01), and positively related to preweaning suckling intensity (P &lt; 0.01). The time taken to resume ovarian cyclicity after weaning increased by a factor of 1.2 for every half unit decrease in body condition score at weaning, and by a factor of 1.3 for every 30-min increase in time suckled per day prior to weaning. Within level of nutrition, higher frequencies and durations of suckling were associated with higher plasma prolactin levels. Plasma IGF-1 levels were consistently depressed in underfed cows, whereas plasma insulin and GH levels were less affected by nutritional treatments. Effects of undernutrition in suckled cows appear to operate at least partly through interactions with milk yield and suckling intensity. Management of post-partum anoestrus in Bos indicus cows should focus on the conservation of cow body condition and on the strategic use of early weaning.

Calf presence and milking twice daily prolongs postpartum anestrus (1996)

Four treatments were initiated approximately 15 days after calving: 1) calf was weaned permanently from its dam (CW; n=6); 2) calf was present continuously with its dam (CPO; n=5); 3) calf was weaned permanently from its dam + dam was milked twice daily (CWM; n=6); 4) calf was present continuously with its dam but contact with the udder was prohibited + dam was milked twice daily (CRM; n=5). During the 4-week treatment period, cows in the CRM treatment produced about twice as much milk, milk fat, milk protein, milk lactose, and milk solids-not-fat (SNF) than CWM cows. After completion of treatments, calves were returned to their dams and allowed to suckle ad libitum. After calves had been reunited with their dams for 1 week, cows in the CRM treatment produced similar amounts of milk, milk fat, milk protein, milk lactose, and milk SNF as CPO cows, but about twice as much as CWM cows. Cows weaned and milked twice daily had their first postpartum ovulation about 2 weeks after weaning, similar to cows weaned but not milked, whereas cows milked in the presence of their own restricted calf first ovulated in about 4 weeks and calf-present cows in about 5 weeks. We conclude that both milk removal (either mechanically or by a calf) and a cow-calf bond are essential to prolong postpartum anestrus.

Effects of infusion of GnRH pulses and level of body condition on ovarian function in postpartum beef cows

An experiment was conducted to test the hypothesis that postpartum anoestrus in beef cows is prolonged in cows in low body condition (BC) because they have a reduced LH pulse frequency compared with cows in high BC. Thirty-six multiparous Blue-Grey (White Shorthorn × Galloway) cows were fed so that they calved in either low (L) (BC score 2.07; SE 0.05; n = 24) or high (H) (BC score 2.81; SE 0.08; n = 12) body condition. They were then fed to maintain BC after calving. Twelve L cows were infused (i.v.) with 2 μg GnRH in 2 ml saline every 2 h from 5 to 7 weeks postpartum (LG) while the remaining L cows and all H cows were infused with saline only (LS and HS). Ovulations, as indicated by the presence of a morphologically normal corpus luteum, were recorded in one, one and ten of the cows of the LS, HS and LG groups, respectively. Mean LH concentrations and pulse frequencies were not affected by either GnRH treatment or BC but mean LH pulse amplitudes were lower ( P < 0.05) in LG and LS cows than in HS cows at Week 5 and in LG cows at Week 6. At Week 7 postpartum, the numbers of small (3–7.9 mm diameter) and large (≥ 8 mm diameter) ovarian follicles, mean granulosa cell numbers per follicle and mean concentrations of LH receptors (pg per mg thecal and granulosa tissue) were not affected by GnRH treatment or BC. Granulosa cells from oestrogen active follicles of HS and LG cows secreted more oestradiol in vitro ( P < 0.01) than cells from LS cows. However, there were no significant differences with treatment in the intrafollicular concentrations of oestradiol, testosterone or insulin-like growth factor-1. It was concluded that, since infusion of GnRH pulses enhanced both follicular steroidogenesis and the incidence of ovulation in low BC cows, the frequency of GnRH pulses is one determinant of follicle development in the postpartum cow. While H cows also exhibited a degree of enhancement of oestradiol synthesis by the granulosa cells of oestrogenic follicles, compared with L cows there was no difference in the LH pulse frequency or in the incidence of ovulation. It is concluded that there may be a threshold level of oestrogen synthesis by granulosa cells below which the final stages of follicle maturation and ovulation cannot be initiated, or that a high rate of oestradiol synthesis by this tissue is not the only factor mediating the effects of body condition on follicle development in the postpartum cow.

Reproductive performance and productivity of Menz sheep in the Ethiopian highlands

Ewe lambs attain puberty at 10 months and 16.9 ± 0.1 kg (56% of mature weight) and first lamb at 15 months. Postpartum anoestrus is 76 days, lambing interval is 8.4 months and 65% of ewes lamb three times in 2 years with a gestation period of 149 days. Ewes cycle year-round but oestrus activity declines from June to September due to increased silent ovulations. Fertilisation rates are high (⩾90%) but lambing rate is 72%, suggesting moderate embryonic mortality. Conception rates to first, second and third oestrus postpartum are 78%, 18% and 4%, respectively. Litter size (prolificacy) is 1.12 but can be increased by 10–40% through premating ewe nutrition management or treatment with gonadotropins. Ewe oestrus activity can be synchronised by ram introduction after a period of isolation and with progesterone sponges or prostaglandin F 2α therapy. Lambs average 2.0 ± 0.1 kg at birth but preweaning mortality (15%) and slow growth, resulting in 8.6 kg weaning weight, still limit overall productivity index to 11.0 ± 0.87 kg per ewe per year, and the annual reproductive rate to 1.4 lambs per ewe. In the traditional management system, where most animals are raised, further studies are required into better nutrition and control of diseases (endoparasitism) in young animals to increase reproductive efficiency, flock productivity and number of animals for finishing and slaughter. This could serve as a model for interventions in other ruminants in the agroecological zone.

Cyclic reproductive activity and efficiency of reproduction in Serrana goats

Reproductive efficiency was studied in mature Serrana goats in three commercial units with ovarian activity being assessed from plasma progesterone profiles between October and May. Over a 2 year period, the mean fertility was 76.2% and the prolificacy was 1.73. Parturitions were recorded between 1 October and 28 February, with 50% of them occurring before the end of October. Goats kidding in October had a significantly shorter ( P<0.01) postpartum anoestrous period in the first year (33.0±9.4 days; n = 20) than in the second year of this experiment (50.3±19.7 days; n = 20). Ovarian activity, assessed by plasma progesterone profiles, was low in October (postpartum anoestrus), increased until early January, then declined suddenly to less than 3% and remained low until the first week of May (seasonal anoestrus) when the breeding period commenced. The data suggest that Serrana goats have a period of anoestrus from January to May. Fertility in the beginning of the breeding period (May) could be improved by adequate management techniques so that most parturitions occur in October/November.

Follicular development in long-term anoestrous suckler beef cows fed two levels of energy postpartum

Follicular growth patterns were monitored daily by ultrasonography, from calving until completion of the first ovarian cycle postpartum in beef cows fed either a low (L; n = 11) or high (H; n = 12) energy diet. Mean cow body condition score at calving was 2.1 (range 1.75–2.25). The mean (± standard error of the mean) number of days postpartum to detection of the first follicle wave was 10.4±0.9 and 9.6±0.9 days for the L and H groups respectively ( P>0.10). L diet cows tended to have more ( P=0.09) medium-dominant follicle waves before first ovulation than H diet cows. Because the first two follicle waves postpartum consisted of the medium follicles, this resulted in a slightly longer ( P>0.10) interval to detection of the first dominant follicle wave in the L cows than in the H cows. The total number of follicle waves before first ovulation was 10.6±1.2 anand 6.8±1.2 ( P<0.05) for L and H cows, respectively. The growth rate, maximum diameter, persistence and regression rate of anovulatory dominant or medium-dominant follicles were not affected by level of diet ( P>0.10) but did change ( P<0.05) as wave number postpartum increased. Peripheral concentrations of oestrogen were low and were not related to follicular growth during the non-ovulatory period. The oestrogen concentrations were significantly higher immediately before first and second ovulations, and paralleled the growth of the first and second ovulatory follicles. The mean interval from calving to first ovulation was longer ( P<0.05) for the L cows (95 ± 12 days) than for H (70 ± 12 days) cows. All of the cows on the L diet (11/11) had a short cycle following first ovulation while 8/12 of the H cows had a short cycle, 3/12 had a normal cycle and 1/12 had a long cycle after first ovulation. In summary, beef suckler cows with a prolonged postpartum interval to first ovulation have an early resumption of the development of dominant follicles whose growth characteristics are largely unaffected by plane of nutrition; these dominant follicles undergo recurrent development and atresia. In conclusion, the problem of prolonged postpartum anoestrus in beef cows is due to failure of ovulation of dominant follicles rather than a lack of development of dominant follicles.

Ad libitum suckling by a foster calf in the presence or absence of the cow's own calf prolongs postpartum anestrus to first ovarian cycle (1995)

Five treatments were initiated approximately 15 days after calving: 1) calf was weaned from its dam (CW); 2) calf was present continually with its own dam (CP-O); 3) calf was present continually with its own dam but contact with the udder was restricted (CR); 4) foster calf was pre sent continually but the cow's own calf was absent (CP-F); and 5) foster calf was present continually, and the dam's own calf was present but restricted (CR+F). Cows weaned at 15 days (CW) cycled in about 2 weeks, whereas cows in the CR treatment cycled 1 week later, and cows in the CP-O treatment did not cycle for about 5 weeks. Cows fostering calves in the presence (CR+F) or absence (CP-F) of their own calves had extended anestrus periods similar to those in cows nursing their own calve s (CP-O). If a cow bonds with a foster calf (as in the CP-F treatment), then the duration of anestrus is lengthened. We conclude that anestrus is prolonged only when milk is removed by a calf (her own or a foster calf) to which the cow is bonded.

Aspects of reproduction in female Bos indicus cattle: a review.

Differences occur between female Bos indicus and Bos taurus cattle in various aspects of reproductive physiology and behaviour. These may be associated with different natural and human selection pressures, compounded by strong genotype-environment interactions. B indicus cattle are better adapted for tropical environments (despite overall poor cattle reproductive rates in these regions) which tend to be more stressful for B taurus genotypes. Conversely, B taurus cattle generally show superior reproductive and productive traits under more favoured, temperate conditions. Despite genotype-environment effects, B indicus females are generally considered to take longer to achieve puberty and to have longer gestation lengths, exhibit prolonged postpartum anoestrus, show greater seasonality of reproductive traits (tending to be long-day breeders), display a shorter, less overt oestrus as well as less tendency to allow riding behaviour by subordinate females. Some groups appear to have increased losses both during pregnancy and in the neonatal period. On the positive side, B indicus females respond well to managerial and nutritional interventions, tend to have greater reproductive longevity and they generally exhibit strong maternal traits. Culling of infertile females and selection for greater male scrotal circumference and sex-drive, in conjunction with the use of target weights, body condition scoring and weaning stratagems can improve reproductive rates in B indicus females.

Postpartum anestrus in French beef cattle: An epidemiological study

Postpartum anestrus of lactating beef cows was studied by means of an epidemiological study carried out on 878 lactating beef cows in 60 French herds. The cows calved between October 1992 and March 1993 and were housed 2 mo after calving, when the anestrus status was determined by progesterone radioimmunoassays. Data analysis was performed using a multiple logistic model in order to adjust for confounding and interaction. Fifty-one percent of the primiparous and 23% of the multiparous cows were found to be in anestrus. Factors significantly related to anestrus were parity (primiparous); breed (Charolais); housing type (tie housing); suckling (compared to weaning at birth); and, among those that were under the control of the farmer, calving conditions (manual exploration of the birth canal); body condition score at calving (3 or less, on a 5-point scale); and loss in body condition score after calving (1 point or more within 2 mo). Previous reproductive performance for multiparous cows such as a long calving interval and induced estrus in the previous year also appeared to be related to anestrus.

Influence of dietary supplementation and partial suckling on body weight and on lactation and reproductive performance of primiparous Boran (Bos indicus) cows in Ethiopia

SummaryThis study was conducted in the central highlands of Ethiopia during 1990–92 to examine the effects of diet and partial suckling on lactation and reproductive performance of Boran (Bos indicus) cows. Primiparous Boran cows were allocated to dietary supplementation or no supplementation with either milking or partial suckling based on body weight at calving. All cows received grass hay ad libitum and the supplemented groups were provided with additional concentrate feed which supplied 222 g/kg DM crude protein and 10 MJ/kg DM energy. Calves were weaned at 8 weeks of age. Mean (±S.E.) dam calving and calf birth weights were 314±11·4 and 25±0·6 kg. Body weights at first postpartum oestrus and at conception were not affected by suckling. Supplemented cows however, were heavier at first oestrus (P = 0·06) and at conception (P &lt; 0·01) than non-supplemented cows. Lactation length was longer (P &lt; 0·01) and mean daily milk yield higher (P &lt; 0·001) in suckled than in milked cows. Diet, however, did not influence either lactation length or daily milk yield. In suckled cows, mean daily milk yield declined sharply after weaning (week 9) and was lower by 38 and 43% of week 8 production in the supplemented and non-supplemented groups, respectively. The overall mean lactation yield was 212·4±64·6 litres; with a significant (P &lt; 0·05) diet × suckling interaction effect. Concentrations of milk fat, total protein and total solids were not affected by either diet or suckling treatment. The postpartum anoestrus interval was longer (P &lt; 0·01) in non-supplemented than in supplemented cows, but did not differ between suckled and milked cows. The percentage of cows cycling by 150 days postpartum was higher (P &lt; 0·05) in supplemented than in nonsupplemented and in milked than in suckled (P &gt; 0·05) cows. By this interval, a higher (P &lt; 0·05) percentage of supplemented-milked (90%) cows cycled than in the other three treatment groups (48·5%). Pregnancy from the first insemination ranged from 45·5 to 63·6% in the four treatment groups. It was concluded that although partial suckling extended the postpartum anoestrus interval, lactation length and milk yield were improved. The negative effects of partial suckling on postpartum reproduction could be overcome with the appropriate dietary supplementation. The economic tradeoff between dietary supplementation, milk yield and reproduction has to be determined in partially suckled cows.

Risk factors for post-partum anoestrus in Charolais beef cows in France

Data from 248 Charolais cows from 20 herds of the Puisaye region (Yonne, France) were collected during winter 1991–1992 to investigate risk factors affecting post-partum anoestrus. Blood was collected 60 days after calving; diagnosis of cyclicity was established by progesterone radioimmunoassay. Herd variables were housing type, lighting of herd accomodation, use of nutritional flushing and nutritional variables (protein and energy before and after calving), and individual variables were parity, ease of calving, body-condition score and body weight at calving and at Day 60, exposure to bulls and calving period. Sixty days post-partum, 62.1% of cows were cyclic. Logistic regression was used to calculate odds ratio (OR). A random effects model was used to take into account the two levels of variables (animal and herd). Although it was defined subjectively, lighting was the only significant herd-level risk factor. Odds of anoestrus were higher in dark than in light accomodation (OR = 6.2, CI 1.2–33.3). Primiparous cows had a higher anoestrus risk than multiparous cows (OR = 10.2, CI 2.9–36.5). Compared with light cows, heavy or medium weight cows at calving had a lower anoestrus risk (OR = 0.25, CI 0.07–0.91, and OR = 0.20, CI 0.07–0.61, respectively.

The effect of exogenous melatonin and the plane of nutrition after weaning on ovulation rate in salz ewes lambing in the seasonal anoestrus

Induction of oestrus and pregnancy in the seasonal anoestrus permits lambing during the breeding season and mating in a short period of time after weaning, but after spring lambing postpartum anoestrus is longer than it is in autumn lambing, and no breeds are able to resume cyclic activity two months after lambing. Nutrient status has been shown to be a major factor affecting the onset of ovarian cyclicity in postpartum ewes. On the other hand, melatonin implants were demonstrated to be efficient in advancing the annual breeding season, but the efficiency of this treatment could be modified by the nutritional status of the animals. The aim of this experiment was to determine the effect of exogenous melatonin, connected with the plane of nutrition after weaning, on the resumption of sexual activity and ovulation rate (OR) in a synthetic Spanish sheep breed lambing in spring.

Serum luteinizing hormone, 13,14-dihydro-15-keto-prostaglandin F2α and cortisol profiles during postpartum anestrus in Brahman and Angus cows

Pluriparous suckled Brahman and Angus cows were utilized to evaluate the effect of breed, day after calving and endogenous opioid peptides (EOP) on hormonal profiles during postpartum anestrus. On Days 17 and 34 after calving, blood samples with and without heparin were collected at 15- and 30-min intervals, respectively, for a 7-h period via jugular cannula. Two hours after the start of blood sampling, cows of each breed were administered either 1 mg/kg iv naloxone or saline. Three hours later, all animals received 10 ng/kg iv GnRH. On Day 34 after calving cows received 0.2 IU/kg iv ACTH. Mean LH, basal LH and area under the LH curve increased (P < 0.01) from Day 17 to Day 34 after calving. Height of LH pulses increased (P < 0.05) by Day 34 after calving. Brahman cows had higher (P < 0.05) mean LH, basal LH, LH pulse frequency and area under the LH curve than Angus cows. Naloxone increased postchallenge area under the LH curve in treated cows above that of control cows (P < 0.06). Naloxone also increased the postchallenge area under the LH curve above that of the prechallenge level (P < 0.01). No breed differences in the response to the naloxone challenge were observed. The LH response to naloxone challenge occurred earlier on Day 34 than on Day 17 after calving but the amount of LH released was similar between days. The GnRH-induced LH release was greater in Brahman than in Angus cows (P < 0.04). Mean cortisol concentrations and area under the cortisol curve decreased (P < 0.05) between Day 17 and Day 34 after calving. Mean cortisol concentrations and area under the cortisol curve were lower (P < 0.01) in Brahman than in Angus cows. Cortisol secretion after ACTH treatment was similar between Brahman and Angus cows. The cortisol response after ACTH challenge was positively correlated (r=0.68; P < 0.001) to the prechallenge area under the cortisol curve. Under optimal environmental conditions Brahman cows have a greater LH release and their anterior hypophysis is more sensitive to GnRH challenge than the Angus cows.