Adaptation of dairy cows to increasing degrees of incomplete milk removal during a single milking interval

Milk accumulation in the udder decreases milk secretion and this effect is explained as well by the effects of the quantity of milk stored in the udder as by the duration and repetition of periods of milk stasis. This experiment aimed to better understand the underlying mechanisms of decreased milk yield in response to the specific effects of the quantity of milk stored in the udder, independent from storage duration, on milk yield and composition. Sixteen Holstein cows were assigned to 4 blocks of 4 cows in a 4 × 4 Latin square design using 7-d periods, with a 4-d sampling period and a 3-d washout period. Cows were milked twice daily at approximately 0700 and 1630 h throughout the trial. Treatments consisted of 4 degrees of milk removal (100, 70, 40, and 0%) applied at one morning milking, designated M0. Effects of the quantity accumulated were studied in relation to udder distension, via measurements of the total distance between the ends of the 4 teats, and cisternal capacity, via the evaluation of cisternal area by ultrasonographic scan at 1 and 9 h after M0. The effect of the quantity accumulated was also evaluated in relation to mammary epithelium permeability by determining plasma lactose concentrations 1 h before and 4, 7, and 10 h after M0. Leaving milk in the udder at M0 decreased milk production during the M0-M1 interval in a negative curvilinear manner. As a result, M0+M1 milk yield decreased or tended to decrease significantly by -1.3, -5.3, and -12.8 kg for the 70, 40, and 0% treatments compared with the 100% treatment (41.7 ± 1.26 kg/d), respectively. Negative carry-over effects on milk yield were observed until the M3 milking only for the 40 and 0% treatments, and no differences were observed between the effects of these treatments. The total distance between teats increased significantly but to decreasing degrees during the M0-M1 interval. For the 40 and 0% treatments, cisternal area, which was increased 1 h after M0 milking, exhibited no further increase during the M0-M1 interval, suggesting cisternal distension was close to maximum. Simultaneously, lactose concentrations increased in blood plasma for only these 2 treatments, and this increase occurred earlier for the 0% treatment. It was also observed that cows presenting the earliest increases in plasma lactose concentrations during milk accumulation lost more milk in response to extended milking intervals.

Changes in mammary secretory tissue during lactation in ovariectomized dairy cows

In dairy animals, the milk yield (MY) changes during a lactation and is influenced by several physiological, livestock management and environmental factors. The MY produced by a mammary gland depends on synthetic activity of mammary epithelial cells (MECs) as well as MEC number and mammary secretory tissue organization. It has been suggested that ovarian steroids (estradiol and progesterone) have a negative effect on MY in lactating cows. In a previous study, we showed that the suppression of ovarian secretions by an ovariectomy improved lactation persistency in dairy cows. Here we were interested in the effects of ovariectomy on plasma estradiol and progesterone concentrations and on changes that occur in mammary secretory tissue during lactation. We demonstrated that the ovariectomy of lactating cows at the time of the lactation peak induced a rapid and dramatic drop in plasma progesterone and a smaller reduction in plasma estradiol. Interestingly, the study of the changes in mammary secretory tissue over time revealed that the improvement of MY measured in the ovariectomized cows was associated with a limited increase in estradiol receptivity in MECs, a reduced mammary tissue remodeling and reduced blood protein concentration in milk, in late lactation. These results suggest that ovarian secretions, particularly estradiol and progesterone, act to enhance processes for mammary gland involution in late-lactating dairy cows.

Composition of milk fat from cows selected for milk fat globule size and offered either fresh pasture or a corn silage-based diet

The objective of this study was to examine the synthesis and composition of milk produced by dairy cows that secrete either small milk fat globules (SMFG) or large milk fat globules (LMFG), and to study their response to diets known to alter milk composition. Four groups of 3 multiparous dairy cows were assigned to 2 isoenergetic feeding treatments: a corn silage treatment supplemented with soybean meal, and fresh pasture supplemented with cereal concentrate. The 4 groups comprised 2 groups of 3 dairy cows that produced SMFG (3.44 microm) and 2 groups of 3 dairy cows that produced LMFG (4.53 microm). The SMFG dairy cows produced higher yields of milk, protein, and calcium. Nevertheless, their milk had lower fat and protein contents. Both SMFG and LMFG cows secreted similar amounts of milk fat; therefore, higher globule membrane contents in milk fat were observed in SMFG cows. Higher calcium mineralization of the casein micelles in SMFG cows suggests that it may be possible to improve cheese-making properties even if the lower protein content may lead to lower cheese yields. The SMFG cows secrete milk fat with a higher concentration of monounsaturated fatty acids and a lower concentration of short-chain fatty acids. They also have a higher C18:1/C18:0 ratio than LMFG cows. This suggests that SMFG cows have more significant fatty acid elongation and desaturation. The pasture treatment led to an increase in milk and protein yields because of increased energy intake. It also resulted in lower milk fat yield and fat and protein contents. The pasture treatment led to a decrease in milk fat globule size and, as expected, an increase in monounsaturated and polyunsaturated fatty acid contents. However, it induced a decrease in the protein content, and in calcium mineralization of casein micelles, which suggests that this type of milk would be less suitable for making cheese. This study also shows that there is no correlation between the cows, based on milk fat globule size and diet. These results open up possibilities for improving milk fat quality based on milk fat globule size, and composition. The mechanisms involved in milk fat globule secretion are still to be determined.

Effect of weaning system on milk composition and distribution of milk fat within the udder of East Friesian dairy ewes

We investigated whether the inhibition of milk ejection during and/or between machine milkings is responsible for the low milk fat observed in commercial milk obtained from dairy ewes managed with a mixed system (MIX) of partial daily suckling (10 h) and once daily machine milking (after 14 h of udder filling). East Friesian crossbred dairy ewes were randomly allocated postpartum to the MIX system (n = 9) or to exclusive twice-daily machine milking (DY1, n = 8). Following wk 4, MIX ewes were permanently weaned from their lambs and milked twice daily. All ewes were injected with saline, oxytocin, or an oxytocin-receptor antagonist prior to three morning milkings during wk 2,4, and 6 of lactation to study cisternal and alveolar milk distribution. Overall milk yield (cisternal + alveolar) for MIX ewes was 42% greater than for DY1 ewes during wk 2 and 4, which demonstrates the beneficial effect of lamb suckling on milk production of dairy ewes. However, during normal machine milking, only the cisternal fraction was obtained from MIX ewes, confirming that milk ejection did not occur for as long as these ewes remained in partial daily contact with their lambs. Although the volume of milk stored within the cistern, and its concentration of milk protein was similar for the two weaning systems, milk of MIX ewes was significantly inferior in cisternal milk fat concentration and yield compared to DY1 ewes. This provides evidence that not only is there inhibition of milk ejection during machine milking of MIX ewes, there is additional inhibition of transfer of milk fat, but not milk protein, from the alveoli to the cistern during the evening when MIX ewes a reseparated from their lambs. Following weaning of MIX ewes, the majority of lactation traits studied were similar compared to DY1 ewes.

Effect of milking interval on alveolar versus cisternal milk accumulation and milk production and composition in dairy ewes

Cisternal and alveolar milk fractions were measured in East Friesian crossbred dairy ewes (n = 32) after 4, 8, 12, 16, 20, or 24 h of milk accumulation in a 6 x 6 Latin square design by administration of an oxytocin receptor antagonist for recuperation of cisternal milk followed by injection of oxytocin to remove the alveolar fraction. Less than half (38 to 47%) of the total milk yield was stored within the cistern for the first 12 h of udder filling compared with up to 57% after 24 h of udder filling. Subsequent milk yield was significantly reduced following the 16-, 20-, and 24-h treatments. Cisternal milk fat percentage, but not milk protein percentage, was lower than in alveolar milk (4.49 vs. 7.92% milk fat, respectively), indicating that casein micelles pass more freely from the alveoli to the cistern between milkings compared with fat globules. Alveolar compared to cisternal somatic cell count was higher for the 16-, 20-, and 24-h treatments. Significant increases in cisternal milk yield and milk composition observed for the 24-h compared with the 20-h treatment demonstrated the importance of the cistern as a storage space when the alveoli and small intramammary ducts became full. The main difference between cisternal and alveolar milk fractions is the poor fat content of cisternal milk, which is an important reason for the milk ejection reflex to be present during machine milking of dairy ewes. In a second experiment, milking every 16 h compared with every 12 h during mid- to late-lactation did not effect milk yield, milk composition, and quality, or lactation length; however, a 25% savings in labor was achieved with the longer milking interval.

Effect of presence or absence of corpora lutea on milk production in East Friesian dairy ewes

The potential luteal effects on milk production were examined in dairy ewes that were not superovulated in contrast to studies using superovulated ewes. Lactating East Friesian crossbred ewes (n = 24) were synchronized using intravaginal progesterone (controlled intravaginal drug-releasing device), PGF2alpha, and gonadotropins. After ovulation, corpora lutea (CL) were counted via laparoscopy on d 4 and 11. On d 5, ewes received either saline (CLYES, n = 12) or PGF2alpha (CLNO, n = 12) to allow CL persistence (2.4 +/- 0.3 CL on d 11) or regression (0 CL on d 11), respectively. Each ewe received two CIDR d 5 to 18 to maintain high concentrations of plasma progesterone (P4) and to suppress estradiol (E2). Each ewe received PGF2alpha on d 18. Data were collected during three periods (pretreatment: d 0 to 5; treatment: d 6 to 18; posttreatment: d 19 to 25). Milk yield and milking time were recorded daily, milk samples were obtained for analyses of fat and protein, and blood samples were collected for P4 and E2 immunoassay. During treatment, CLYES ewes had higher milk yield (1.56 vs. 1.44 +/- 0.01 kg/d), milk fat (92.2 vs. 81.1 +/- 1.3 g/d), and milk protein (83.7 vs. 77.5 +/- 0.8 g/d) compared with CLNO ewes, respectively. Differences were maintained posttreatment, despite luteolysis in CLYES ewes. Estradiol concentrations did not differ between treatments and were low after d 5. Milk production was increased in East Friesian ewes in the presence of an average of 2.4 corpora lutea, an effect independent of estradiol.

Comparison of two protocols with a progesterone antagonist aglepristone (RU534) to induce parturition in bitches

Parturition was induced in ten Beagle bitches by injecting them subcutaneously with 15 mg aglepristone kg-1 (Alizine) at day 58 of gestation and 24 h later and subsequently at 2 h intervals with either 0.08 mg alfaprostol kg-1 (Alfabedyl) (group 1; five bitches) or 0.15 iu oxytocin kg-1 (Ocytocine S) (group 2; five bitches). Blood samples were collected every 4 h until the end of parturition to assay plasma concentrations of progesterone, dihydro-keto prostaglandin F2 alpha (PGFM), oxytocin, prolactin and cortisol. Parturition occurred in all bitches. The mean time of onset of parturition for both groups was not significantly different (32.6 +/- 3.7 h for group 1 versus 31.6 +/- 3.6 h for group 2), although the mean expulsion time for bitches from group 2 (4.5 +/- 1.8 h) was significantly shorter than that of bitches in group 1 (9.1 +/- 2.0 h). At birth, 93% of the pups were alive in group 2 compared with 86% in group 1. Peripheral plasma concentrations of progesterone increased significantly after the administration of aglepristone, but direct or indirect luteolysis was not induced, and plasma concentrations of oxytocin or cortisol did not change during the first 24 h after administration of aglepristone. PGFM concentrations increased significantly after 4 h of aglepristone administration. During the first 20 h after aglepristone administration, prolactin concentrations increased significantly. At parturition, bitches in group 2, which had the shorter expulsion time of pups, were characterized by significantly higher concentrations of oxytocin and PGFM than bitches in group 1.

Hormonal variation in bitches after early or mid-pregnancy termination with aglepristone (RU534)

Seven bitches in early pregnancy (12.8 +/- 3.8 days after ovulation; group 1) and seven bitches in mid-pregnancy (32.0 +/- 1.53 days after ovulation; group 2) were used in this study. For each group, five bitches were treated with 0.10 mg aglepristone (Alizine) kg-1 and this dose was repeated 24 h later. Two control bitches received a placebo. Blood samples were collected at 6 h intervals to determine plasma concentrations of progesterone, dihydro-keto prostaglandin F2 alpha (PGFM), oxytocin, prolactin and cortisol. Parturition occurred in the four control bitches. All bitches treated with aglepristone aborted. In group 1, embryonic death occurred; in group 2, fetal expulsion occurred 60-132 h after administration of aglepristone. After pregnancy termination, the interoestrous interval of aglepristone-treated bitches was significantly shorter than that before treatment. Treatment with aglepristone did not modify plasma concentrations of progesterone, prostaglandin, oxytocin or cortisol within 24 h after its administration, but it induced, in mid-pregnancy (group 2) a discharge of prolactin within 12 h after its administration. As an abortifacient, aglepristone acted on the uterus and, therefore, did not have direct or immediate luteolytic properties. Termination of pregnancy occurred with high plasma progesterone concentrations. Fetal expulsion was characterized by an increase in the concentration of PGFM, but oxytocin and cortisol remained at basal concentrations.

The effect of a mixed-management system on the release of oxytocin, prolactin, and cortisol in ewes during suckling and machine milking

Milk yield and plasma oxytocin (OT), prolactin (PRL), and cortisol (CORT) during suckling and machine milking were measured in multiparous ewes subjected to a mixed management system of 3 sucklings and two daily milkings. Peak hormones were significantly increased and were similar during suckling and milking for PRL (181 vs. 163.3 ng x mL(-1)) and CORT (12.5 vs. 11.5 ng x mL(-1)). During the period of exclusive suckling, OT was always significantly released (90.3 pg x mL(-1)); however, during the period of mixed management, OT concentrations only increased during suckling compared to milking (91.7 vs. 13.1 pg x mL(-1)). The mean volume of milk obtained during suckling (632 mL) was significantly higher than during milking (255 mL). Thus, during a mixed management system, oxytocin and prolactin releases are not under similar central regulation. A mixed system, without OT release during milking, does not contribute to accelerate the conditioning of ewes for machine milking.

Isolated porcine ovarian follicles as a model for the study of hormone and growth factor action on ovarian secretory activity

The aim of our in vitro experiments with isolated porcine ovarian follicles was to study the effects of gonadotropins, GH, IGF-I and oxytocin (OT) on release of ovarian steroid, OT, IGF-I, insulin-like growth factor-binding protein-3 (IGFBP-3), prostaglandin F (PGF), prostaglandin E (PGE) and cAMP. It was found that quarters of ovarian follicles cultured for 8 days produced significant amounts of progesterone, estradiol-17 beta, OT and IGFBP-3 with peaks of accumulation from the 3rd to the 8th day of culture. Addition of serum promoted progesterone, estradiol and OT release, whilst accumulation of IGFBP-3 was maintained to a greater extent in serum-free medium. GH (10 ng/ml or above) was able to inhibit androstenedione, OT, PGF and IGFBP-3, to stimulate IGF-I and cAMP, and to alter testosterone and PGE release by follicles cultured in serum-supplemented and/or serum-free medium. IGF-I (10 ng/ml or more) inhibited androstenedione and PGF secretion, stimulated testosterone, estradiol, OT and cAMP production, but did not influence progesterone, IGFBP-3 or PGE output in these conditions. OT (100 ng/ml) was able to inhibit androstenedione and to stimulate testosterone, IGF-I, PGF and PGE, but not estradiol or IGFBP-3 release. A stimulatory effect of LH on progesterone and OT and an inhibitory influence of LH on estradiol secretion in the serum-supplemented medium were observed. FSH in these conditions stimulated OT, but not progesterone or estradiol secretion. The use of this experimental model suggests the involvement of gonadotropins, OT, GH and IGF-I in the control of ovarian steroid and nonapeptide hormone, growth factor, growth factor-binding protein, prostaglandin and cyclic nucleotide production. The stimulatory effect of GH on IGF-I, and the stimulatory influence of IGF-I on OT, as well as coincidence of the majority of effects of IGF-I and OT, suggest the existence of a GH-IGF-I-OT axis. On the other hand, the different patterns of action of GH and IGF-I on OT, estrogen and IGFBP-3 suggest that part of the GH effect on ovarian cells is IGF-I independent.

Intramammary pressure and luteal oxytocin after PGF2 alpha administration in cycling and early pregnant ewes

The aim of this work was to investigate whether luteal oxytocin released after non-luteolytic prostaglandin F2 alpha (PGF2 alpha) challenge could explain the intramammary pressure (IMP) rises previously described in pregnant ewes after the normal life span of a corpus luteum. Blood oxytocin levels and IMP after challenge were measured in cycling and pregnant lactating ewes until the response ceased (approximately 55 d post oestrus). Oxytocin release was not significantly different for cycling and pregnant ewes, and fell to its lowest levels on days 16-18 post oestrus in both groups, i.e. with or without luteolysis. However, although IMP response fell with luteolysis in cycling ewes, the response persisted until day 55 in pregnant ewes. This lack of correlation between oxytocin release and IMP response raised interesting questions including: is there an unknown oxytocic compound that can be released after PGF2 alpha stimulation of the corpora lutea?

Subpicogram determination of oxytocin by an enzyme immunoassay using acetylcholinesterase as label

The pure tetrameric form of Acetylcholinesterase (EC-3.1.1.7) from the electric eel electrophorus electricus has been covalently coupled to oxytocin. This conjugate has been used as tracer in a heterologous competitive immunoassay. Microtiter plates coated with a mouse monoclonal anti-rabbit immunoglobulin antibody were used to separate bound and free moieties of the tracer. Acetylcholinesterase activity bound to the solid phase was measured by a colorimetric assay. The minimum detectable concentration was 0.075 pg/well (ie 1.5 pg/ml) and precision was less than 8% at concentration above 0.15 pg/well. An extraction step improved sensitivity up to 10 times with good recoveries. To assess the validity of this assay, basal levels of oxytocin were measured during the oestrous cycle of a cow.

Instantaneous secretion rate of growth hormone in lambs: relationships with sleep, food intake, and posture

Relationships among sleep, feeding behavior, posture, and GH secretion were investigated in two groups of ruminant lambs in January (n = 6) and May (n = 3). Lambs were placed in individual cages and fed ad libitum. Behavioral features were obtained from continuous polygraphic recording. Blood was collected from undisturbed sheep every 3 min for 24 h via an indwelling catheter connected to a peristaltic pump. One month after the sampling session, ovine GH (oGH) was iv administered to evaluate oGH kinetic parameters. From GH plasma concentrations and oGH kinetic parameters, the instantaneous secretion rate of GH was reconstituted using a numerical deconvolution method. All lambs exhibited normal behavioral patterns. The clearance of oGH was similar for the two groups, and the daily production rates of GH were estimated at 14.60 +/- 7.99 micrograms/kg.24 h in January and 10.57 +/- 5.21 micrograms/kg.24 h in May. Analysis of concentration profiles indicate an episodic pattern of GH secretion into plasma. The mean number of peaks was 16.22 +/- 4.47/24 h, and the mean duration was 47.2 +/- 12.8 min for the nine sheep. When instantaneous secretion rates were taken into account, the number of identified peaks was similar, but the mean duration was reduced (32.9 +/- 9.8 min for the nine sheep). Significant relationships were not found between GH plasma concentration profiles and the state of vigilance, food behavior, or posture. Conversely, when the instantaneous secretion rates were taken into account, the highest GH production rate was detected during rest, i.e. slow wave sleep and rapid eye movement sleep, absence of food intake or rumination, and lying down. It is emphasized that the use of GH instantaneous secretion rate instead of GH concentration is of importance when evaluating the relationships between GH dynamics and short duration events. It is concluded that the influence of vigilance on GH secretion, which has already been demonstrated in humans, is likely to exist in other species.

Fenprostalene in cattle: Evaluation of oxytocic effects in ovariectomized cows and abortion potential in a 100-day pregnant cow

Four ovariectomized cows were used to compare the uterotonic (oxytocic) properties of the prostaglandins F2alpha analogue fenprostalene to cloprostenol and PGF2alpha-tromethamine salt (dinoprost). Uterine activity was measured by electromyography with the duration and magnitude of activity quantified by microcomputer. The administration of 1 mg of fenprostalene to estradiol primed animals significantly increased uterine motility for approximately 19 h. This was significantly longer than the duration observed for either cloprostenol (500 mug, i.m., 8.9 h) or dinoprost (25 mg, i.m., 7.7 h). However, the level of activity was similar for the 3 compounds tested, with postinjection levels of oxytocic effect averaging 369 % for treated animals compared to 100 % for controls. Therefore, the difference in effects for the three prostaglandins may be due more to pharmacokinetic properties rather than to different potencies of the three compounds. In addition, a pregnant cow (100 d gestation) was treated with fenprostalene (1 mg, s.c.). Fenprostalene treatment resulted in unchanged uterine activity for a 6-h period, followed by a four-fold increase in genital tract activity which lasted for 12 h. Thereafter, activity was inhibited for one day, followed by a sharp increase in uterine activity leading to abortion within 66 to 72 h after fenprostalene injection. The placenta was expelled 7 days after treatment.

In vivo pharmacological characterization of alpha adrenergic receptors in sheep myometrium and their physiological meaning

Utero-cervical responsiveness to alpha-adrenergic receptor stimulation was electromyographically evaluated in vivo in adult ovariectomized ewes. Spontaneous motility and drug-induced changes were quantified with a microcomputer. Our results suggested that the two activity patterns previously described on the ovine genital tract could be respectively controlled by alpha-2 and alpha-1 adrenergic mechanisms. Alpha-2 receptors could be assigned to a chronotropic function by activating a hypothetical zone triggering the outbreak of discrete episodes of motility (regular activity); this function is apparent at the cervix even in the absence of estrogen priming. On the other hand, alpha-1 receptors could be assigned to an inotropic function, regulating mainly the magnitude of estrogen-dependent utero-cervical motility (irregular activity).

Secretory profiles and production rate of growth hormone in ruminant lambs

Secretory profiles and production rates of growth hormone (GH) were determined in 6 ruminant lambs during winter. The mean GH concentrations (3.78 +/- 2.17 ng/ml) calculated were based upon blood sampling obtained every 3 min using a withdrawal pump. Body clearance (0.162 +/- 0.031 1/h/kg) was calculated from bolus intravenous oGH administration. The data were analysed by non-linear regression analysis; a bicompartmental model was selected to describe the data. production rate was 14.6 +/- 7.98 micrograms/kg/24 h. It has been emphasized that the experimental design used gave an accurate estimate of GH production rate.

Direction of uterine contractions during estrus in ewes: a reevaluation

In four ewes direction of propagation of uterine contractions was evaluated using an electromyographic technique during 15 entire estrous periods; 120,101 propagations were analyzed. When horns were considered separately 89.9% of all propagations were classified into three main modes: ascending (34.3%), descending (59.85%), and divergent (5.85%). When both horns were analyzed simultaneously, horn's synchronicity was observed in most instances; on this basis, eight modes of propagation were identified of which three accounted for two-thirds of all propagation: synchronous descending (24.9%), reciprocal propagation (descending on one horn and then ascending on contralateral horn) (22.1%), and isolated descending propagation (18.0%). A time-dependent pattern of propagation throughout estrus was clearly identified, the percentage of ascending propagations reaching a minimum (16.5%) and the percentage of descending propagation reaching a maximum (77%) at peak uterine motility level. By considering both direction of uterine propagation and cervical mechanical activity, a new hypothesis concerning two aspects of sperm transport (speed and mechanism) was formulated. It is hypothesized that the high prevalence of descending propagations is important to reduce sperm cell population by selecting the most vigorous spermatozoa; such selection is possible when the mechanical cervical activity is low or absent (cervix open); when cervical mechanical activity is high (cervix closed), it is suggested that both descending and ascending propagations participate in sperm transport by back and forth motion of luminal fluid within the uterine lumen.

Relative oxytocic properties of fenprostalene compared with cloprostenol, prostaglandin F2 alpha, and oxytocin in the ovariectomized ewe

The oxytocic effect of a prostaglandin F2 alpha analogue, fenprostalene, was assessed in 4 ovariectomized ewes fitted with electrodes in both uterine horns and in the cervix. In the absence of estradiol priming, significant motility changes were not elicited by fenprostalene. Conversely, when ewes were primed with 17-beta-estradiol, fenprostalene markedly increased the electrical activity in the uterus and cervix. After a single subcutaneous fenprostalene administration (5 micrograms/kg), activity values remained about twice that of the control values during 8.52 +/- 3.31 hours. When the same dosage was administered IM, similar post-injection activity values were obtained, but only during 5.88 +/- 0.72 hours. Oxytocic effects of fenprostalene were far longer than those elicited by a single IM administration of 50 micrograms of prostaglandin F2 alpha (tham salt)/kg (0.91 +/- 0.32 hours) or by a single IM administration of 1 microgram of cloprostenol/kg (1.88 +/- 0.81 hours). Using the dose-effect relationship curve obtained from the same ewes by continuous IV infusions of oxytocin (OXT), the postinjection activity values reached after a single subcutaneous administration of fenprostalene were equivalent to those of an IV infusion of OXT at an average dose of 4.09 ImU of OXT/kg/hr for 6 to 13 hours, according to the values of the particular ewe concerned. These long-lasting oxytocic properties, in addition to its luteolytic capabilities, would make fenprostalene a suitable drug for promoting effective evacuation of the uterus when required in daily veterinary practice.