Estrous Activity Research Articles

The role of dietary molybdenum on estrous activity, fertility, reproduction and molybdenum and copper enzyme activities of female rats

In this study, we investigated the effect of supplemental molybdenum (Mo) on estrous activity, fertility and reproduction, hepatic xanthine dehydrogenase/oxidase (XDH/OX), sulfite oxidase (SOX) and plasma ceruloplasmin (Cp) in female SD rats. Weanling female rats were assigned to five dietary treatment groups and fed an AIN-76A diet. They were given deionized water containing either no Mo or Mo at 5, 10, 50 and 100 mg/l. Mo significantly prolonged the estrous cycle when fed 10 mg/l or higher. Gestational weight gain was higher for the controls and the 5 mg/l, than for the 10–100 mg/l treatments. Histological data suggested that Mo supplemented at 10 mg/l or higher delayed fetal esophageal development, transfer of fetal hemopoiesis to bone marrow and myelination in the spinal cord. Intrauterine deaths were few, but the rate of fetal resorption increased with supplementation at 10 mg/l or higher. Mean hepatic XDH/XO, SOX and plasma Cu-Cp activity increased with Mo supplementation. This study suggests that supplemental Mo may influence estrous activity and embryogenesis. Hepatic XDH/OX, SOX and plasma ceruloplasmin may be affected differently in gestating and nongestating animals.

Effect of cohabitation with white-faced ewes on estrous activity of Hampshire and Suffolk ewes exposed to rams in June.

Two groups of 24 Hampshire and 26 Suffolk purebred ewes each were used to study effects of cohabitation with cyclic white-faced (WF) ewes on estrous activity in June. Ewes lambed in January, February and March and had been isolated from mature rams since the previous fall breeding. From June 1 to July 2, treated (T) ewes were exposed to vasectomized rams and to 65 WF ewes; control (C) ewes were exposed only to vasectomized rams. Ovulation was assessed with biweekly serum progesterone assays; crayon marks were used to detect estrus. Daily observations of ram behavior were conducted to assess sexual activity of rams joined with T and C ewes. Cohabitation with WF ewes increased (P less than .01) ovulation percentages from 46% in C (42% for Hampshires and 50% for Suffolks) to 76% in T ewes (79% for Hampshires and 73% for Suffolks). Mating percentage also was increased (P less than .05) by cohabitation with WF ewes from 14% for C ewes to 30% for T ewes. Rams with T + WF ewes spent more (P less than .05) time checking ewes for estrus than did rams with C ewes. Hence, cohabitation with cycling WF ewes increased ovulation and mating percentages. Many acyclic T ewes first ovulated after 10 or more days of teasing, possibly due to increased ram contact in the presence of WF ewes.

Physiological mechanisms controlling anestrus and infertility in postpartum beef cattle.

Postpartum infertility is caused by four factors: general infertility, lack of uterine involution, short estrous cycles and anestrus. The general infertility component is common to any estrous cycle and reduces potential fertility by 20 to 30%. Incomplete uterine involution prevents fertilization during the first 20 d after calving but is not related to anestrus. Short estrous cycles prevent fertility during the first 40 d after calving by causing the cow to return to estrus before pregnancy recognition occurs. Anestrus is the major component of postpartum infertility and is affected by several minor factors: season, breed, parity, dystocia, presence of a bull, uterine palpation and carryover effects from the previous pregnancy as well as two major factors: suckling and nutrition. These major factors have direct effects on anestrus but also interact with one or more other factors to control postpartum anestrus. Physiological mechanisms associated with anestrus involve blockage of the GnRH "pulse generator" in the hypothalamus, but other pathways also must be involved because bypassing the pulse generator is not an effective treatment for all cows. The primary cause of anestrus probably is different for different stages of anestrus. The mediating mechanisms for anestrus are not involved with prolactin, oxytocin, the adrenal or direct neural input from the mammary gland but are at least partially involved with blood glucose and the endogenous opioid peptide system. Management options to decrease the impact of anestrus and infertility include: 1) restrict breeding season to less than or equal to 45 d; 2) manage nutrition so body condition score is 5 to 7 before calving; 3) minimize effects of dystocia and stimulate estrous activity with a sterile bull and estrous synchronization; and 4) judicious use of complete, partial or short-term weaning.

Effects of intramuscular administration of recombinant bovine interferon-alpha I1 during the period of maternal recognition of pregnancy.

One hundred ewes were utilized to determine the effects of interferon supplementation on the number of ewes pregnant and embryonic survival. Ewes were checked twice daily (0700 and 1600) for estrus using fertile rams. On d 12 through 16, ewes received twice-daily i.m. injections of either recombinant bovine interferon alpha I1 (2 mg IFN) or vehicle. Ewes remained penned with rams and were observed for subsequent estrous activity for at least 35 d after mating. To determine the number of fetuses and corpora lutea, all ewes were subjected to one surgery during mid-pregnancy (d 45 to 80). More (P less than .05) ewes were pregnant after treatment with IFN vs vehicle (45 of 49, 92% vs 37 of 49, 76%, respectively). The interestrous interval for ewes that were treated with IFN and did not conceive was longer (P less than .05) than for ewes given vehicle (26 +/- 1 vs 17 +/- 2 d, respectively). Embryonic survival (98.2 vs 87.9%; P less than .05), calculated as the number of fetuses present at the time of laparotomy and expressed as a percentage of the ovulation rate, and percentage of ewes with 100% fetal survival (96 vs 76%; P less than .05) were greater after treatment with IFN. It was concluded that supplementation of IFN increased both the number of ewes pregnant and embryonic survival.

Computer modeling of sheep reproduction: I. An algorithm for quantifying anestrous cycles in ewes.

An algorithm that mathematically describes annual estrous activity in ewes is given. The function fitted as the ordinate is similar to a normal distribution function related to date as the abscissa. The data used were collected from literature on different breeds and from different locations of the world; all studies had been specifically conducted to study estrous activity in ewes in these locations. Parameters estimated in an iterative procedure from the algorithm were length of time during which estrous cycles occurred (SA), a measure of asymmetry (SD), day of mid-estrous (DE), highest incidence of estrus (MX) and lowest incidence of estrus (MN), day of lowest incidence of estrus (DN) and day of mid-anestrous season (DA). Incidence of estrus is defined as the proportion of ewes exhibiting an active estrus. These parameters were estimated for several breeds and locations. The data, as collected, were inadequately cross-classified to allow for unconfounded estimation. However, analyses of fitted parameters that describe the curves gave quantitative results about the influences of breed of sheep, location, latitude and altitude and indicated which parameters were associated with these factors. Breeds differed significantly for SA, DE, MX and DA. Locations differed for DE, MX and DA. Latitude was associated with differences in DE, DN and DA with altitude also associated with differences in DE and DA. There were no significant effects on SD even though breeds differed sizably in SD.

Effects of breed and time since lambing on spring estrous activity in mature ewes

Frequencies of ovulation in mature nonlactating Dorset and Suffolk ewes in Virginia (37° 15′N latitude) were compared over a 21-day period beginning 19 April. All Suffolk ewes ( n = 23) had lambed the preceding winter (January); Dorset ewes had last lambed in either the preceding winter ( n = 19) or the preceding fall (September and October; n = 39). Ewes had been isolated from mature rams since the previous breeding and remained isolated during the experimental period. Ovulations were detected from twice-weekly determinations of circulating progesterone levels. The frequency of ovulation did not differ between winter-lambing Dorsets (21 ± 9%) and Suffolks (30 + 10%) but was higher for fall-lambing Dorsets (59 ± 8%). Following the initial experimental period, Dorset ewes were exposed to intact rams for 43 days and twice-weekly progesterone determinations were continued for 14 days. Most Dorset ewes (93%) ovulated within 14 days of ram introduction. Previous lambing season (fall vs. winter) did not affect frequency of ovulation or lambing, lambing date or number of lambs born. Ewes that ovulated during ram isolation were not more likely to lamb (74% for both groups) but were more likely to conceive at the first ovulation following ram introduction (64 ± 10 vs. 24 ± 9%), lambed earlier (5 October ± 2.3 days vs. 12 October ± 1.9 days) and had fewer lambs born per ewe lambing (1.42 ± 0.13 vs. 1.88 ± 0.11). Ewes that conceived at the first ovulation of the breeding season lambed 16.6 days earlier but had fewer lambs (1.12 vs. 1.96 lambs/ewe lambing) regardless of whether or not the ewe ovulated during ram isolation. Thus, exposure of ewes to vasectomized rams for 14 to 21 days before the start of breeding appears useful, both to induce ovulation and to maximize subsequent lambing rates.

Effect of ram stimulation on estrous activity and lambing rate in two sheep breeds in the sierra of Peru

Two trials were conducted in the highlands of Peru to evaluate the ‘ram effect’ on ewes during the breeding season. Junin (trial 1) and Corriedale ewes (trial 2) were randomly assigned to a non-stimulated group that received no ram stimulation prior to the breeding season, or to a stimulated group that received 14 days of exposure to vasectomized rams prior to the start of the breeding season. Lambing rate by day of lambing season was used to evaluate the effect of ram stimulation prior to start of the breeding season (trial 1) while the number of ewes observed in estrus during the AI season was evaluated (trial 2). In both trials, ram stimulation resulted in a change in distribution of proportion of ewes lambing by day of lambing season (trial 1) or proportion of ewes observed in estrus by day of breeding season (trial 2), with more ewes lambing ( P < 0.01) or in estrus during the first 17 days of the season in the stimulated groups. These results show that ram stimulation can be used in the sheep populations studied in Peru to advance date of conception, increase lambing rate and to induce a more compact lambing pattern in the flock.

Effect of Season and Lactation on Postpartum Fertility of Polypay, Dorset, St. Croix and Targhee Ewes

The objective of this study was to compare rebreeding activities of spring- vs fall-lambing Polypay, Dorset, St. Croix and Targhee ewes that either suckled their lambs for 40 d or had lambs weaned at birth. Seasonal effects of male fertility were reduced by utilizing an excess number of fertile rams in the spring. Plasma concentrations of progesterone were monitored to assess days to the first normal ovulation, days to conception and estrous vs anestrous activity. Breed, season and lactation affected the rebreeding performance. Dorset ewes had similar conception rates between spring and fall but a shorter interval from lambing to first ovulation in the fall. Polypay and Targhee ewes were the opposite; they had higher conception rates in fall than in spring matings with no seasonal influence on postpartum interval. Postpartum ewes in the fall had higher conception rates, and fewer of these ewes became anestrous or had estrous cycles of abnormal duration than of those ewes lambing in the spring. Ewes that suckled for 40 d in the spring had delayed estrous activity, but when these ewes became estrual they had higher conception rates than ewes whose lambs were weaned at birth. Lactation had no inhibitory affect on the postpartum interval of fall lambing ewes. These data suggest that the response of different breeds to various components of postpartum fertility varies with season and management of the flock.

Endocrine responses and estrous activity in Holstein heifers fed supplemental beta-carotene

Three trials involving 56 Holstein heifers were conducted to determine the effect of feeding supplemental beta-carotene (BC) on several reproductive parameters. Heifers were maintained in a loose housing system and group fed year-old brome hay ad libitum. All heifers were individually fed 2.73 kg of a concentrate mix daily. Half of the heifers received 300 mg BC/d (Trials 1 and 2) or 600 mg BC/d (Trial 3) blended in the concentrate mix. After 4 wk of dietary treatment, estrus was synchronized to facilitate observation for estrous activity and collection of blood samples. Two weeks after the treatment was initiated, concentrations of BC in blood serum were greater in the heifers that were fed supplemental BC as compared to control heifers. This difference increased to 6 wk. However, feeding supplemental BC had no effect on any of the reproductive parameters measured, including incidence of estrus and changes in progesterone and luteinizing hormone (LH) concentrations after PGF 2α, intervals from PGF 2α to estrus and LH peak, estrual activity or first service conception rate. The results indicate that feeding supplemental BC does not alter reproductive performance in Holstein heifers.

LIMITED NURSING EFFECTS ON REPRODUCTIVE PERFORMANCE OF PRIMIPAROUS AND MULTIPAROUS COWS AND PREWEANING CALF PERFORMANCE

Effects of limited nursing and parity of the dam on cow reproductive and calf gain performance were determined in two experiments involving 97 beef cow-calf pairs. Fall-calving (exp. 1) and spring-calving (exp. 2) primiparous (P) and multiparous (M) cows were assigned to control (C) or limited nursing (L) treatments at 21–28 d postpartum. Cows and calves continued on nursing treatments until first estrus was detected, or until 42 d had elapsed. Nursing treatment and parity of the dam influenced estrous activity in exp. 1. Higher (P &lt; 0.05) percentages of multiparous dams and of L treatment cows were observed in estrus during the treatment period. A 5.6-d reduction (P &lt; 0.10) in postpartum interval to first estrus (PPI) was observed for L cows observed in estrus during treatment periods, and a 16.2-d reduction (P &lt; 0.01) occurred for L cows when all cows observed in estrus during or after the nursing treatment periods were compared to control cows in exp. 1. In exp. 2, nursing treatment and parity of the dam did not significantly affect PPI. Nursing treatments did not affect pregnancy rates in exp. 1, but M cows had higher (P &lt; 0.01) pregnancy rates than P cows. Average daily gains (ADG) were lower (P &lt; 0.10, exp. 1; P &lt; 0.01, exp. 2) for L calves compared with C calves during treatment periods. The L calves did not overcome ADG reductions suffered while on the L treatment by weaning time, and 205-d adjusted weights of L calves were 25 kg lower (P &lt; 0.05) than C calves in exp. 1. Potential improvement in reproductive efficiency resulting from limited nursing must be evaluated with regard to reduced preweaning calf performance. Key words: Cattle (beef), postpartum interval, estrus, once-daily nursing, weaning weight, first estrus

The postweaning rise of tonic luteinizing hormone secretion in anestrous cows is not prevented by chronic milking or the physical presence of the calf.

The objective of the following study was to examine the ability of frequent milking, the physical presence of the calf, and their combination to prevent a postweaning rise in tonic luteinizing hormone (LH) secretion, estrus, and ovulation. Thirty Hereford cows were allowed to suckle their calves ad libitum until 17-21 days post partum and confirmed as anestrus. They were then assigned alternately by order of calving to 1 of 5 treatment groups: (1) Suckled (S) ad libitum; (2) Nonsuckled (NS)--calf removed for 102 h; (3) Nonsuckled--calf present (NSC)--calf remained with cow, but muzzled to prevent suckling for 102 h; (4) Nonsuckled--milked 8 times a day (NSM)--calf removed for 102 h and cow hand-milked for 10 min every 2 h from 0700 to 2100 h; (5) Nonsuckled--calf present--milked 8 times a day (NSMC)--combination of 3 and 4. Luteinizing hormone secretion patterns, estrous activity, and ovulation were monitored throughout the experiment. Prior to treatment (Day 0), mean pulse frequency (pulses/6 h), mean concentrations (ng/ml), and median concentrations (ng/ml) of LH did not differ (p greater than 0.45) between groups, and were 0.7 +/- 0.15, 2.8 +/- 0.14, and 2.6 +/- 0.11, respectively. Marked rises (p less than 0.01--p less than 0.03) in LH pulse frequency were observed in all groups except S between 48 and 54 h after onset of treatment. Mean and median concentrations of LH were lower (p less than 0.02) in S cows than in all other groups at 48-54 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of elevating serum lipids on luteinizing hormone response to gonadotrophin releasing hormone challenge in energy-deficient anestrous heifers

A study was conducted to evaluate the effect of feeding a bypass fat on luteinizing hormone (LH) response to gonadotrophin releasing hormone (GnRH) in noncyclic Holstein heifers. Twelve cyclic Holstein heifers were fed a complete diet at 40% net energy for maintenance (NE m) until cessation of ovarian activity. Based on weights and condition scores, heifers were assigned to either a control or treatment diet containing 0.45 kg bypass fat and fed at an energy level of 85% NE m. Diet adjustments were made following weekly weighings. GnRH challenges were conducted at four periods: prior to initial energy deprivation, at termination of 40% NE m feeding, and twice more at 21-d intervals after 85% NE m feeding began. Blood was sampled via a jugular catheter every 15 min for 5 h, and GnRH was injected after the fourth sample. None of the heifers exhibited estrous activity after the initial energy deprivation. Heifers on the bypass fat diet continued to lose weight during the treatment period, while the control heifers gained a slight amount of weight. Baseline and peak concentrations of LH were not significantly affected by time or diet. Time to GnRH-induced LH peak was longer (53 vs 130 min, P < 0.01) after 40% NE m and remained greater at all times thereafter. Serum lipid levels increased 82.5% among heifers being fed the bypass fat. Energy restriction had no effect on the magnitude of LH response to GnRH but did delay response time.

Sexual activity of Assaf ewes after October and February lambings

The first ovulation in 20 Assaf ewes and the first estrus in 54 out of 77 Assaf ewes after the October lambings occurred at mean intervals of 26 ± 1.7 and 50 ± 1.5 d postpartum, respectively. After the February lambings, nine out of 20 examined ewes had their first ovulation and estrus 35 ± 2.5 and 71 ± 7.7 d postpartum, respectively, and resumed normal cyclic activity in April. Five animals showed considerable ovarian activity but only occasional estrous activity during spring, while the remaining six ewes had a 6-to 7-mo-long anestrous season during which only occasional ovulations were detected in three of them.

Effect of feed treatment and exogenous estrogen and progestogen on puberty and lambing rates in ewe lambs.

Four hundred seventy-four ewe lambs (5 to 6 mo of age) were assigned within breed to two postweaning feed treatments; 1 = alfalfa pellets ad libitum and 2 = alfalfa pellets plus 20% barley or wheat ad libitum. Ten days prior to the start of breeding, approximately one-half of the ewe lambs within each feed treatment were treated with a single injection of 2.5 mg of estradiol valerate and 1.5 mg of norgestomet and implanted with 3 mg of norgestomet for 8 d. At the start of breeding, fertile Suffolk rams were fitted with marking harnesses and penned with ewe lambs; evidence of estrus and breeding was determined on a weekly basis by visual examination for breeding marks. In the second year of the study, 7 to 13 d after recording estrous activity, all marked ewe lambs were bled; and blood was assayed for progesterone. In 1983, 60% of the ewe lambs showed estrus and 11% lambed compared with 48% exhibiting estrus and 30% lambing in 1984 (P less than .01). More (P less than .01) ewe lambs on feed treatment 1 displayed estrus, but more (P less than .05) lambed in the feed treatment 2 group. Steroid treatment resulted in more (P less than .01) ewe lambs showing estrus but fewer lambing (P less than .01). Examination of progesterone concentrations for evidence of ovulation and corpus luteum development indicated that treatment with steroids caused a large percentage of ewe lambs to exhibit estrus. However, many of these failed to develop a corpus luteum and become pregnant.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationships of Sexual Activities in Estrous Cows to Different Frequencies of Observation and Pedometer Measurements

Sexual activities were observed continuously for 120h in Holstein dairy cows during moderate climate. Pedometer measurements of motor activity were correlated with total activities initiated, total activities received, mounts received, and time in estrus. Twelve of 14 individual sexual activities also were correlated on a within-cow basis with pedometer-measured increases in motor activity; disoriented mounts and licking front were not correlated to pedometer increases. Although observations of four 1-h and four or six 30-min observations accounted for greater variation, pedometer increases in activity accounted for as much variation in total sexual activities as periodic observations of 30min or 1h for 1 to 3 ×/d. Combining periodic observations with pedometer measurements resulted in greater coefficients of determination for analyses of sexual activities than with each individual factor. Sexual activities and pedometer increases also were correlated positively with body condition score and day postpartum, and they were correlated negatively with milk production and body weight change since calving. Although mounting and total sexual activities were shifted significantly from expected on a quartile basis, shifts were probably not of practical significance and provided little evidence for more frequent occurrence of estrous activity during noctural periods. Rather, the most marked decrease in mounting and total sexual activities occurred at milking and feeding times, indicating that dairy farmers should be more concerned with disruptions of activities by management practices rather than nocturnal occurrence of estrus.

Induced puberty in prepuberal zebu heifers treated with norgestomet and pregnant mare serum gonadotropin

Three experiments were conducted to examine the progestogen plus PMSG treatment for its effectiveness in inducing synchronous puberty in prepuberal zebu heifers in three different seasons. In Experiment 1, ten Ongole heifers (age 21 months) were treated with Norgestomet implants for nine days and an intramuscular injection of 400 IU of PMSG two days before implant removal. Ten heifers (age 25 months) were kept as untreated controls. Animals were inseminated 12 h after detection of cyclic estrus (not bred at induced estrus) until all animals conceived. The proportion of treated animals showing estrous, ovulatory, and cyclic activity were 100%, 75% and 25% respectively, while the average age at first conception was significantly less (P < 0.05) than in the control group. In Experiment 2, 18 Ongole heifers (age 22 months) were divided into treatment and control groups. Fixed-time inseminations were done 48 and 72 h after implant removal and 12 h after being detected in heat at other times. Estrus was seen in all while 63% became pregnant (P < 0.05). At the end of the 100-day experiment, the percent pregnant were 33 and 63 in the control and treatment groups, respectively. In the third study, twenty-six Ongole heifers (age 22 months) were assigned to treatment and control groups. Eighty-eight percent of the animals exhibited estrus, 75% ovulated (P < 0.01) and 25% conceived to fixed-time insemination. The pregnancy rate at the end of the experiment was 10 and 56% (P < 0.01) respectively in control and treated groups. Estrous response and fertility were better in the cooler month (February) and the treatment imposed in the hotter month (May) resulted in a significantly higher (P < 0.05) age and body weight at conception.

Effects of Silastic progesterone implants on activity cycles and steroid levels in ovariectomized and intact female rats

We have previously shown that although Silastic implants of progesterone reduce the amount of running of animals living in activity wheels, progesterone-treated animals continue to show perodic fluctuations or peaks in activity. We hypothesized that although progesterone treatment inhibited estrous cycles, ovaries of animals treated with Silastic implants of progesterone continued to secrete estradiol in amounts adequate to stimulate moderate levels of running. In the present study we tested this hypothesis by removing ovaries from progesterone-treated animals and comparing their running behavior and steroid levels to progesterone-treated animals who received sham ovariectomies. Although progesterone treatment significantly inhibited running activity, removal of ovaries in progesterone-treated animals further suppressed running activity. In addition, both estradiol and progesterone levels were significantly reduced following removal of ovaries in progesterone-treated animals. We conclude that although Silastic progesterone implants inhibit normal ovarian and estrous activity cycles, ovaries produce sufficient estradiol to stimulate running behavior.

Sexual Activity of Holstein Cows: Seasonal Effects

Continuous observations of Holstein dairy cows during natural and prostaglandin-induced estrus were conducted to determine effects of cold and hot weather on the degree and pattern of sexual activities, circadian profile of activities, and location of activities. Mounts with pelvic thrusts and mounts plus 12 other estrous-related activities for 73 cows were monitored for 96h in cold weather and 120h in hot weather. Eleven cows in hot weather with more than one mount were in estrus longer with less intensity of mounting than 16 cows in cold weather; climate did not affect mounts received or total activities initiated per cow. Number of cows in standing estrus affected mounts received per cow, total activities received per cow, and time in estrus but not activities initiated per animal. Two estrual cows in cold weather and 6 cows in hot weather received less than 2 mounts. Cows in cold weather had more (28%) total estrous activity as mounting activity than cows in hot weather (14%); cows in hot weather interacted more by rubbing and licking than cows in cold weather. Cows in cold weather had more mounting activity from 0600 to 1759h (73%) than from 1800 to 0559h (27%) and had more mounts from 1200 to 1759h than cows in hot weather. During both cold and hot weather, sexual activities decreased at milking time. In the freestall housing with drylot, 80% of the mounting activity in both climates occurred in an outdoor drylot and feed manger area, which had best footing. These data indicate that climate affected mounting and total sexual activity of lactating dairy cows.

Effects of Climate on Reproduction in Cattle

Animal environment is affected by climatic factors that include temperature, humidity, radiation, and wind. Extremes in climate alter energy transfer between the animal and its environment and can affect deleteriously reproduction. Seasonal variation of environment, nutrition, and management alters estrous activity and duration of estrus. Conception rates are reduced under stress of heat and cold. Endocrine functions are altered by climatic extremes. In hyperthermia, adrenal function is reduced, and this may allow the animal to cope with the environment because of the lower calorigenic actions of glucocorticoids. Estrogens are lower during the proestrus to metestrus period of the estrous cycle and during late gestation and appear to manifest their physiological actions through shorter duration of estrus and lower calf birth weights, respectively. Season alters endocrine profiles and influences fertility of males. Spermatogenesis is impaired, and testosterone is lower during early exposure to hyperthermia. Environmental modifications can alleviate stress of heat and cold to some extent. Experimentation using indices of environmental measures is needed to assess interactive effects of environment on reproduction.

Growth and Estrous Activity of Rats Fed Adequate and Deficient Levels of Phosphorus

Female Sprague-Dawley rats were fed two levels of phosphorus ad libitum and in a paired feeding arrangement to yield the following three treatments: 1) adequate phosphorus (0.40%) ad libitum intake, 2) low phosphorus (0.04%) ad libitum intake and 3) adequate phosphorus pair-fed with the low phosphorus group. Feed intake, growth rate and feed conversion in rats fed 0.04% phosphorus ad libitum and in pair-fed control rats were lower (P < 0.01) than in rats fed 0.40% phosphorus ad libitum. Seventy-five percent of the rats exhibited normal estrous cycles, as indicated by vaginal smears, throughout the 105-d period regardless of dietary phosphorus level. Breeding efficiency and number of pups per litter were not altered by dietary treatment. However, compared to either phosphorus-adequate group, dams fed 0.04% phosphorus weaned fewer (P < 0.05) pups and pup survival to 45 d of age was lower (P < 0.05). Average daily gain postweaning was 67% lower for pups in the phosphorus-deficient group than for those in either phosphorus-adequate group. Phosphorus deficiency in rats does not alter estrous cycles but reduces pup survivability postnatally.