Effect of timing of corn silage supplementation to Holstein dairy cows given limited daily access to pasture: intake and performance.

The timing in which supplements are provided in grazing systems can affect dry matter (DM) intake and productive performance. The objective of this study was to evaluate the effect of timing of corn silage supplementation on ingestive behaviour, DM intake, milk yield and composition in grazing dairy cows. In total, 33 Holstein dairy cows in a randomized block design grazed on a second-year mixed grass-legume pasture from 0900 to 1500 h and received 2.7 kg of a commercial supplement at each milking. Paddock sizes were adjusted to provide a daily herbage allowance of 15 kg DM/cow determined at ground level. The three treatments imposed each provided 3.8 kg DM/day of corn silage offered in a single meal at 0800 h (Treatment AM), equally distributed in two meals 0800 and 1700 h (Treatment AM-PM) or a single meal at 1700 h (Treatment PM). The experiment was carried out during the late autumn and early winter period, with 1 week of adaptation and 6 weeks of measurements. There were no differences between treatments in milk yield, but 4% fat-corrected milk yield tended to be greater in AM-PM than in AM cows, which did not differ from PM (23.7, 25.3 and 24.6±0.84 kg/day for AM, AM-PM and PM, respectively). Fat percentage and yield were greater for AM-PM than for AM cows and intermediate for PM cows (3.89 v. 3.66±0.072% and 1.00 v. 0.92±0.035 kg/day, respectively). Offering corn silage in two meals had an effect on herbage DM intake which was greater for AM-PM than AM cows and was intermediate in PM cows (8.5, 11.0 and 10.3±0.68 kg/day for AM, AM-PM and PM, respectively). During the 6-h period at pasture, the overall proportion of observations on which cows were grazing tended to be different between treatments and a clear grazing pattern along the grazing session (1-h observation period) was identified. During the time at pasture, the proportion of observations during which cows ruminated was positively correlated with the DM intake of corn silage immediately before turn out to pasture. The treatment effects on herbage DM intake did not sufficiently explain differences in productive performance. This suggests that the timing of the corn silage supplementation affected rumen kinetics and likewise the appearance of hunger and satiety signals as indicated by observed changes in temporal patterns of grazing and ruminating activities.

Evaluation of the effect of eprinomectin in young dairy heifers sub-clinically infected with gastrointestinal nematodes on grazing behaviour and diet selection.

Inappetence is commonly associated with parasitism and has been observed in both housed and pastured ruminants. In seeking a functional explanation for these observations, it has been hypothesised that parasitized animals may feed more selectively in order to proportionally increase the protein content of their diet and thus partially compensate for their reduced feed intake. Support for this theory is found principally in studies in housed animals under carefully controlled experimental conditions. Grazing animals face a far more heterogeneous environment and a multiplicity of potentially confounding factors that could influence diet selection. Controlled grazing of adjacent monocultures of grass and clover can mitigate some of these variables and was used in the current study to examine the dietary preference of dairy heifers with sub-clinical parasitic gastroenteritis when compared to those receiving regular anthelmintic treatments. Grazing behaviour and herbage intake rates were determined through the use of jaw-movement recorders, direct observation and short-term liveweight change. Consistent with previous observations and despite evidence that nematode burdens were low in the untreated control heifers, a reduction in daily grazing time of 56min (P=0.054) was observed in the control animals. There was, however, no evidence that the control heifers showed greater preference for clover compared with ryegrass: partial preference for clover was 73.0% in the untreated controls and 75.5% in the treated heifers. Furthermore control heifers were observed grazing the clover swards significantly (P=0.032) less frequently than the treated heifers. This study provides additional evidence in grazing cattle for parasite-induced inappetence, manifest as a reduction in grazing time and in subtle changes in ingestive behaviour. The observed partial preference for clover in both treated and control cattle was not significantly affected by the level of parasitism.

Effects of sequential treatments with eprinomectin on performance and grazing behaviour in dairy cattle under daily-paddock stocking management.

To evaluate the effect of gastrointestinal parasites on grazing behaviour, herbage intake and milk production in spring calving dairy cows, 12 naturally infected control cows were compared with 12 similar animals treated on three occasions (June, July and September) with eprinomectin. The cows were blocked according to calving date, parity, live weight and milk yield during week 2 after turnout and then allocated to the treatments. The grazing area was sub-divided into two sets of 12 replicated paddocks of equivalent size and topography. Pairs of either control or treated animals were randomly assigned to graze each paddock over the duration of the study. Within each plot, the pair of cows grazed a series of 1-day paddocks, of areas calculated to provide 72 kg of herbage dry matter measured to ground level. Faecal samples were collected from each cow in April, prior to allocation, and every 28 days thereafter. Samples were submitted for counts of nematode eggs (sensitivity 1 epg) and the presence of Dictyocaulus viviparus larvae. Additional faecal samples were taken on each occasion for culture and nematode identification. Pasture samples for direct larval counts were collected at the same time as faecal sampling. Behaviour measurements on all cows were made during three periods, once before the first treatment with eprinomectin and thence after the 2nd and 3rd treatments. During each behaviour measurement period, grazing and ruminating behaviour were recorded over two 24-h periods and measurements of components of short-term intake rate were made during a morning and a late afternoon grazing meal. Milk yield was recorded daily and milk quality was recorded weekly. Live weight and body condition score were recorded on the day of allocation, the day of initial treatment and thereafter at weekly intervals until the end of the trial. The parasitological results showed low levels of faecal egg output throughout the study with group arithmetic means ranging from 0 to 6.8 epg. Faecal culture yielded predominantly larvae of the genus Ostertagia, but the following genera were also identified: Cooperia, Oesophagostomum and Trichostrongylus. Pasture larval levels were also low with peak values of 135 and 58 L3/kg DM herbage (7 August) in the paddocks grazed by the control and treated cattle, respectively. Thereafter, larval counts on paddocks grazed by treated cows declined to undetectable levels by October, while control paddocks remained at approximately 40 L3/kg DM. There was no effect of treatment on components of grazing or ruminating behaviour recorded over 24 h or on short-term intake rates. There were significant differences between components of short-term intake rates measured during the morning and afternoon grazing meals. The overall milk yield response to treatment with eprinomectin was +1.68 kg/day solids-corrected milk (SCM) (P=0.026). The overall response included significant (P<0.050) increases in mean daily SCM yield following each of the three treatments, indicating a positive response to repeated treatments at several different stages of lactation. There were no significant differences in the overall percentages of fat, protein or lactose between control and treated groups. The differences in live weight were not significant, although there was a consistent pattern throughout for the treated cows to be heavier than the controls.

Elevating circulating leptin in prepubertal male rhesus monkeys (Macaca mulatta) does not elicit precocious gonadotropin-releasing hormone release, assessed indirectly.

The purpose of this study was to examine the hypothesis that the pubertal reaugmentation of pulsatile GnRH release in male primates is triggered by a rise in circulating leptin concentrations. Agonadal juvenile male rhesus monkeys (n = 7) were implanted with indwelling venous catheters and housed in specialized cages that allow continuous access to the venous circulation. GnRH release was monitored indirectly using LH secretion from the in situ pituitary sensitized to the LH releasing action of GnRH as a bioassay for the hypothalamic peptide. Infusion of recombinant human leptin (5 micro g/kg body weight.h for 16 d resulted in a marked square wave increment in circulating leptin concentration from approximately 2-20 ng/ml but did not elicit precocious GnRH release. GH secretion, however, was stimulated confirming that the heterologous leptin preparation was bioactive in the monkey. Parenthetically, recombinant human leptin was found to be immunogenic in the monkey and circulating antileptin IgG was demonstrable 22-35 d after the initial exposure to the human protein. These findings further support the view that circulating leptin is unlikely to provide the signal that triggers the onset of puberty in male primates.

Evidence that changes in luteinizing hormone secretion regulate the growth of the preovulatory follicle in the ewe.

The aim of the present study was to gain evidence that the level of LH secretion preceding the preovulatory LH surge is an important determinant of follicular maturation and corpus luteum function in the ewe. In addition it was hoped to establish whether the pattern of LH delivery to the ovary (pulsatile v. constant) is a critical factor in the maturation of a preovulatory follicle. To accomplish this, progesterone-primed anoestrous ewes were repeatedly injected i.v. with LH or luteinizing hormone-releasing hormone (LH-RH), or given an i.v. infusion of LH, over a 72 h period. These animals, together with the appropriate controls, were exposed to a sexually active ram so that oestrous activity could be recorded. All ewes were subjected to intensive blood sampling regimes so that the plasma levels of LH and progesterone could be determined and compared to those which occurred in the same breed of sheep during the oestrous cycle. Collectively the data suggest that the plasma levels of LH preceding the preovulatory LH surge are an important determinant of follicular maturation as judged by subsequent corpus luteum function. Moreover, they show that follicular maturation can be achieved with widely differing patterns of LH delivery to the ovary during the preovulatory period and that a strict pulsatile delivery of LH may not be an absolute requirement.

Effects of long-term treatment with LH on induction of cyclic ovarian activity in seasonally anoestrous ewes.

The aim of the present study was to establish whether cyclic ovarian activity could be induced and then maintained in anoestrous Romney ewes by the long-term administration of regular intravenous pulses of LH (10 micrograms ovine LH i.v. once every 1 or 2 h for 29-91 days). The LH pulse regimen was designed to generate plasma profiles of LH that were comparable to those experienced during the luteal and follicular phases of the oestrous cycle. The results showed that the LH treatments were capable of inducing cyclic ovarian activity, as assessed from the concentrations of progesterone in plasma, but that the treatments were inadequate for sustaining cyclic activity beyond two consecutive progestational phases. After 35-56 days of treatment, the plasma concentrations of FSH declined significantly (P less than 0.05) relative to those in the untreated animals. These data suggest that FSH supplementation as well as LH might be required for the long-term maintenance of cyclic ovarian activity in seasonally anoestrous ewes.

Accumulation of luteinizing hormone, oestradiol and androstenedione by sheep ovarian follicles in vivo.

The temporal relationship between the levels of LH in peripheral plasma and in follicular fluid of ovarian follicles in anaesthetized sheep were investigated for a 10-h period after a single i.m. injection of LH releasing hormone (LH-RH; 100 microgram). The ovarian secretion rates of oestradiol and androstenedione and the levels of these steroids accumulating in different sized follicles at varying time-intervals after the LH-RH injection were also compared. The data show that the rates at which pituitary LH enters and leaves the intrafollicular fluid-filled spaces are substantially slower than those of peripheral blood. Two hours after LH-RH injection the levels of LH in plasma had increased from 1 to 200 ng/ml, whereas in the follicle the levels remained at approximately 2ng/ml. Ten hours after the LH-RH injection, the levels of LH in plasma had returned to basal values (approximately 1.4 ng/ml) but in both small and large follicles the levels of LH (approximately 20 ng/ml) were comparable to those present in similar sized follicles 4h earlier. The data also indicate that more than 90% of the oestradiol produced by a large antral follicle (greater than or equal to 5 mm diameter) probably enters the bloodstream without first accumulating within the follicular antrum. Finally it is concluded that the clearance of the small amount of oestradiol which does accumulate in the follicular antrum is negligible compared with the clearance of this hormone from peripheral plasma.

Seasonal differences in ovarian activity in cows.

The plasma concentrations of LH and prolactin and various parameters of ovarian function were examined in cows on known days of the oestrous cycle during May and June (autumn and winter) and during October (spring). Luteinizing hormone peak frequency and plasma prolactin concentrations were significantly higher in October than during the May-June period (LH, P less than 0.05; prolactin, P less than 0.01). The mean diameters of large healthy follicles (greater than or equal to 8 mm diameter) and the dominant oestrogen-secreting follicles were significantly larger (P less than 0.01 for both follicle types) and each follicle contained more granulosa cells (both P less than 0.01) in May-June than in October. The LH responsiveness of theca interna with respect to androstenedione production and the levels of aromatase activity in granulosa cells did not differ with time of year. The corpora lutea were heavier (P less than 0.05) and secreted more progesterone (P less than 0.01) in May-June than in October. It is concluded that seasonal differences in ovarian activity exist in cows and that these differences are probably the consequence of seasonal differences in gonadotrophin secretion.

The role of neuropeptide Y (NPY) in the control of LH secretion in the ewe with respect to season, NPY receptor subtype and the site of action in the hypothalamus.

Neuropeptide Y1-36 (NPY1-36) acts through Y1 and Y2 receptors while the C-terminal NPY fragments NPY18-36 and N-acetyl[Leu28,31]pNPY24-36 act only through the Y2 receptor. We have investigated the effects of intracerebroventricular (i.c.v.) administration of NPY1-36, NPY18-36 and N-acetyl[Leu28,31]pNPY24-36 on LH secretion in the ovariectomised (OVX) ewe. These peptides were administered into a lateral ventricle (LV) or the third ventricle (3V) of OVX ewes during the non-breeding and breeding seasons. Microinjections of NPY were also made into the preoptic area (POA) during both seasons to investigate the effects of NPY at the level of the GnRH cell bodies. Tamed sheep were fitted with 19 gauge guide tubes into the LV, 3V or the septo-preoptic area (POA). Jugular venous blood samples were taken every 10 min for 3 h. Sheep were then given NPY1-36 (10 micrograms), NPY18-36 (100 micrograms) or saline vehicle into the LV; N-acetyl[Leu28,31]pNPY24-36 (100 micrograms), NPY1-36 (10 micrograms or 100 micrograms), NPY18-36 (10 micrograms or 100 micrograms) or saline vehicle into the 3V, or NPY1-36 (1 microgram, 5 micrograms, 10 micrograms) into the POA. Blood sampling continued for a further 3 h. LH was measured in plasma by radioimmunoassay. LV or 3V injection of 10 micrograms NPY1-36 caused a small but significant (P < 0.025) increase in the interval from the last pre-injection pulse of LH to the first post-injection LH pulse during the breeding season. Other LH pulse parameters were not significantly affected. NPY18-36 did not produce any significant change in LH pulsatility when injected into the LV, and neither peptide had any effect on plasma prolactin or GH levels. There was a significant (P < 0.01) reduction in LH pulse frequency after 3V injection of 10 micrograms and 100 micrograms NPY and 100 micrograms NPY18-36. Pulse amplitude was reduced by 3V administration of the Y2 agonist, N-acetyl[Leu28-31]pNPY24-36 and 100 micrograms NPY18-36. When the amplitude of the first post-injection LH pulse was analysed, 10 micrograms NPY also had a significant (P < 0.05) suppressive effect. During the non-breeding season, 100 micrograms NPY1-36 (but not 10 micrograms) decreased (P < 0.01) LH pulse frequency. LH pulse amplitude was significantly (P < 0.01) decreased by 100 micrograms NPY18-36. Doses of 10 micrograms NPY1-36 and 100 micrograms NPY18-36 had greater inhibitory effects on pulse frequency during the breeding season but the suppressive effect of 100 micrograms NPY was similar between seasons. Microinjections of NPY into the POA decreased (P < 0.01) average plasma LH levels during the non-breeding season at a dose of 10 micrograms but did not significantly affect pulse frequency or amplitude. We conclude that a substantial component of the inhibitory action of NPY on LH secretion in the absence of steroids is mediated by the Y2 receptor. This inhibition is probably exerted by way of a presynaptic action on GnRH terminals in the median eminence as NPY does not modulate the frequency or amplitude of LH pulses at the level of the GnRH cell bodies in the POA.

Effect of season on neuropeptide Y and galanin within the hypothalamus of the ewe in relation to plasma luteinizing hormone concentrations and the breeding season: an immunohistochemical analysis.

Within the hypothalamus, neurones that express neuropeptide Y (NPY) and galanin have been implicated in the regulation of gonadotropin-releasing hormone (GnRH) and gonadotropin secretion. We aimed to determine the extent to which the expression of these two neuronal systems is linked to the seasonal reproductive cycle, and the effect of chronic oestrogen treatment. Immunohistochemical analysis was used to examine changes between the breeding season and anestrus in ovariectomized (OVX) ewes with or without oestrogen treatment (s.c. implants for 2 weeks). Serial blood sampling established plasma luteinizing hormone (LH) profiles, and the ewes were subsequently killed and the brains perfused for immunohistochemistry. In OVX ewes, the amplitude of LH pulses was greater in the nonbreeding season than in the breeding season. Oestrogen treatment caused a marked reduction in plasma LH concentrations during anestrus, but not in the breeding season. The number of cells in the arcuate nucleus/median eminence region (ARC-ME) that stained for NPY was lower in ewes killed in anestrus (September) than in ewes killed in the breeding season (May), but there was no seasonal change in the number of galanin-stained cells. Within season, oestrogen treatment did not affect NPY- or galanin-cell number. There was no effect of season or oestrogen on the area of varicose fibres/terminals for either peptide in the ARC-ME, but galanin immunostaining was more intense during the breeding season. We conclude that the amount of NPY in cell bodies of the ARC-ME is lower in ewes in the nonbreeding season; this could reflect a steroid-independent effect of photoperiod. We also conclude that the long-term negative-feedback effect of oestrogen on GnRH/LH secretion does not appear to be mediated by NPY- or galanin-containing neurones in the ewe.

Seasonal changes in the expression of neuropeptide Y and pro-opiomelanocortin mRNA in the arcuate nucleus of the ovariectomized ewe: relationship to the seasonal appetite and breeding cycles.

Sheep experience well-documented seasonal changes in reproductive activity and voluntary food intake (VFI). Within the hypothalamus, neurones that express neuropeptide Y (NPY) and pro-opiomelanocortin (POMC) have been implicated in the regulation of reproduction and appetite. In this study, we aimed to determine the extent to which the expression of these two neuronal systems is linked to the seasonal reproductive cycle and/or the seasonal appetite cycle. VFI in our sheep reaches a nadir in August with no difference occurring between December and February. We examined the brains of ovariectomized (OVX) female sheep (n=5-7) that were killed during the breeding season (February) or during the early or late nonbreeding season (August and December, respectively). The brains of these animals were perfused with paraformaldehyde and processed for in situ hybridization histochemistry, using ribonucleotide probes labelled with 35S. The number of NPY and POMC cells and the number of silver grains per cell were counted using an image analysis system. For NPY, the number of cells counted in the arcuate nucleus/median eminence region and the number of silver grains per cell was significantly lower in animals killed during August than in animals killed in February or December. The number of grains per cell over NPY cells was also significantly lower in animals killed during August. For POMC, the number of cells was lower in February than in August and December. Similarly, the number of grains per cell for POMC were lower in February than in August and December. VFI was significantly lower in animals during August than at other times of the year. We conclude that in OVX ewes: (i) NPY gene expression is lower at the time of the year when VFI is reduced and (ii) POMC gene expression is greater at the time of the nonbreeding season than during the breeding season. Because these results were obtained in OVX animals, the changes appear to be independent of alterations in the secretion and/or action of ovarian steroids. Thus, the activity of NPY neurones appears to relate to changes in appetite whereas changes in POMC expression may be relevant to the seasonal breeding cycle.

Impact of eprinomectin on grazing behaviour and performance in dairy cattle with sub-clinical gastrointestinal nematode infections under continuous stocking management.

Forty spring-calving cows and heifers (20 of each) were allowed to acquire infection with gastrointestinal (GI) nematodes naturally during grazing. The control group (10 cows and 10 heifers) were compared with 20 similar animals treated with eprinomectin in order to evaluate the effect of GI nematodes on grazing behaviour, milk production, body condition score and live weight. The animals were paired according to parity and milk yield during the week prior to treatment, then within replicate pair randomly allocated to a different treatment group. The grazing area was sub-divided into 20 replicated paddocks of equivalent size and topography. Grazing pairs of either control or treated animals were randomly assigned to each paddock over the duration of the study (one pair per paddock). Grazing behaviour was recorded for both groups over a 10-day period commencing 4 days after treatment with eprinomectin. Milk yield was recorded daily and milk quality was recorded weekly. Live weight and body condition score were recorded on the day of allocation, the day of initial treatment and thereafter at weekly intervals until the end of the 4-week trial. Faecal samples were collected from each animal prior to, and after, allocation and submitted for counts of nematode eggs. Additional faecal samples were taken at the end of the study for culture and nematode identification. Individual faecal samples were also analysed for residual digestibility. Pasture samples for nematode larval counts were taken at the same time as faecal sampling. The parasitological results showed low levels of faecal nematode egg output throughout the study, with the heifers having higher counts than the cows. Faecal culture yielded species of Ostertagia, Cooperia, and Trichostrongylus. Pasture larval levels were very low throughout with no value exceeding 68 larvae/kg dry matter (DM) of herbage. There were significant (P < 0.05) effects of treatment on grazing time, eating time, total bites, total grazing jaw movements (TGJM), idling time and mean meal duration. Treated cows and heifers grazed for 47 and 50 min longer per day, respectively, than controls (P = 0.016). Mean meal duration was extended as a result of anthelmintic treatment by 11 and 38 min, in cows and heifers, respectively (P = 0.012). There were no significant (P > 0.05) treatment effects on ruminating time or residual faecal digestibility, but idling time was significantly reduced in both treated cows and heifers, by 50 and 110 min, respectively (P = 0.010). In the treated cattle, there was an increase in solids-corrected milk yield compared with the control cattle, which was significant (P < 0.05) in weeks 2 and 3 after treatment. The response was particularly marked in heifers, where the difference in yield between treated and controls was up to 2.35 kg/day. The differences in live weight gain and condition score over 28 days post-treatment were significant (P < 0.05) in both cows and heifers, in favour of the treated animals.

Evaluation of the effects of nematode parasitism on grazing behaviour, herbage intake and growth in young grazing cattle.

Parasitic nematodes, even in the absence of any clinical disease, can cause a reduction in voluntary feed intake in housed ruminants. This trial examined these effects on young cattle grazing pastures. Twenty dairy heifer calves, born in the previous autumn, were blocked according to liveweight and allocated to one of two groups: either untreated or dosed with an IVOMEC((R)) (ivermectin) SR Bolus 10 days prior to turnout on 1 May 1998. The groups grazed separately on two paddocks on predominately ryegrass swards. Liveweights were recorded every 28 days and faecal samples taken for worm egg counts at the same time. In mid-May and mid-July, for two 14-day periods, animals were paired within treatment groups and transferred to one of 10 replicate paddocks of similar sward height and herbage mass. Grazing behaviour and herbage intake were measured during these periods. In mid-May, 2 weeks after turnout, treated and untreated animals showed no difference in grazing behaviour or daily intake of grass. By mid-July, 10 weeks after turnout, the untreated heifers spent on average 105min less per day in grazing time and their daily herbage intake was 0.78kg dry matter (DM) per day lower than that of the treated animals. Residual sward height, mass and composition in the trial paddocks reflected these differences. There were no clinical signs of gastrointestinal parasitism in the untreated group up to this time; in July, the mean worm egg count in this group was 120 eggs per gram (e.p.g.) of faeces. Faecal samples from several untreated animals were positive for lungworm larvae by July, mild clinical signs of bronchitis were observed in two of these animals in mid-July. Over the period from turnout until mid-July, the untreated heifers showed a reduction in mean daily liveweight gain of 150g, compared to the treated animals.

A table tennis glove for tetraplegics.

A new table tennis glove to enable a tetraplegic to hold a table tennis bat is described and discussed. It is much easier to apply than to bandage the bat to the hand.

Plasma concentrations of FSH, LH and progesterone in sheep immunized against an androstenedione-protein conjugate.

A significant increase in the plasma concentrations of FSH (P less than 0.05) and LH (P less than 0.001) was observed during the luteal (Days 9-11) phase but not during the subsequent cloprostenol-induced follicular phase in androstenedione-immunized ewes compared to those in control ewes. Over the same time period the geometric mean (and 95% confidence limits) androstenedione antibody titres in the immunized ewes was 1/305 (1/158, 1/590) whereas they were not detectable in the controls. In the subsequent cycle, the ovulation rates were 1.6 +/- 0.2 for the immunized ewes and 1.1 +/- 0.1 for the control ewes (P less than 0.05) and the luteal progesterone concentrations were significantly higher in the immunized ewes compared to the controls (P less than 0.01). Collectively, these results suggest that active immunization against androstenedione leads to an increase in the plasma concentrations of both FSH and LH. The results are consistent with the hypothesis that FSH plays a central role in determining the ovulation rate in sheep.

Plasma concentrations of LH, prolactin, oestradiol and progesterone in female red deer (Cervus elaphus) during pregnancy.

Peripheral blood samples were collected throughout pregnancy from 11 red deer hinds. During the same period, 6 other hinds which mated but failed to produce calves were also sampled. Pretreatment of some of these hinds included synchronization of oestrus alone (N = 3) or with injection of 1000 i.u. PMSG (N = 9). During early and mid-pregnancy, LH and prolactin were frequently undetectable. Prolactin concentrations in pregnant and non-pregnant hinds were high (greater than 250 ng/ml) in December-January. The results of the hormone analyses suggested that the amount of progesterone in plasma correlates with the number of corpora lutea (CL) present. The concentrations of oestradiol and progesterone were low from mid-winter onwards in the non-pregnant hinds, suggesting a reduction in ovarian activity at this time. In pregnant animals, progesterone concentrations were high for the first 200 days of gestation. Oestradiol rose to peak values concomitant with declining progesterone concentrations just before parturition.