Reproductive activity of Pelibuey and Suffolk ewes at 19 degrees north latitude.

Under photoperiod regulated conditions, some Pelibuey ewes appear to express estrous cycles continuously throughout the year. Ovulatory activity of Suffolk ewes has not being determined at 19 degrees N. The aim of the present study was to describe the annual reproductive cycle of Pelibuey and Suffolk ewes under natural conditions at 19 degrees N. Ten adult ewes of each breed were maintained under natural environmental conditions, on a constant plane of nutrition and isolated from rams. The ovulatory activity of both breeds was monitored over 2 years (2003 and 2004) by quantifying progesterone concentrations in blood samples taken twice a week. For both breeds, the proportion of ovulating ewes per month and year was analyzed by chi(2). The duration of anestrus was compared between breeds and years by analysis of variance. Correlation of ambient temperature and weight changes with the ovulatory activity was evaluated by regression procedure. Ovulatory activity was different (P<0.05) between breeds from February to July. Pelibuey sheep (60%, 6 of 10) ovulated continuously during the entire study; one animal was eliminated in the second year, due to ovarian follicular cysts and three ewes had anestrous periods from 65+/-46 to 70+/-36 days. Suffolk ewes displayed reproductive seasonality similar to that observed in sheep of the same breed in latitudes greater than 35 degrees . Within breed, reproductive behavior of sheep was similar between years, with the only difference (P<0.05) observed in February for Pelibuey and in July for Suffolk relative to other months. Ambient temperature was not correlated with ovulatory activity. The body weight of Pelibuey ewes was constant during the entire study and was not correlated with ovulatory activity. There was, however, an increase on the body weight of Suffolk ewes, but this variation was not related to ovulatory activity. These findings lead to the conclusion that at 19 degrees N, a high proportion of Pelibuey ewes is capable of ovulating throughout the year, whereas Suffolk ewes undergo seasonal anestrous periods similar to those observed in individuals of the same breed at latitudes greater than 35 degrees . Identification of continuous ovulatory Pelibuey ewes may be the first step for developing a selection program against seasonality to increase lambing frequency.

Influence of energy balance and body condition on estrus and estrous cycles in Holstein heifers.

The objective was to determine effects of energy balance and body condition on estrous behavior and estrous cycles in Holstein heifers. Before the experiment heifers were fed so body condition remained moderate or they became fat. During the 2 x 2 factorial experiment, moderate and fat heifers were in positive or negative energy balance. Heifers were fed individually twice daily, and energy balance was calculated daily. Heifers were observed for 30 min every 3 h, and all standing and mounting events were recorded for three consecutive estrous cycles. When progesterone in serum was less than 1 ng/ml, standing and mounting activity were profiled. Area, peak, and duration of activity profiles were examined among groups of heifers. Negative energy balance or fat body condition did not reduce peak or duration of estrous behavior and thus did not reduce detectability of estrus. Onset of diestrus was delayed in heifers that were fat and in negative energy balance relative to other heifers. We suggest that fat body condition, coincident with negative energy balance, may reduce accuracy of timing artificial insemination relative to ovulation and may consequently reduce fertility in cattle.

Energy balance and body condition influence luteal function in Holstein heifers.

A factorial experiment was conducted to determine influence of energy balance (EB) and body condition (BC) on luteal function in heifers. Heifers with moderate (MBC) or fat (FBC) BC were fed individually to sustain positive EB (PEB) or to cause negative EB (NEB). Intake of feed was measured daily and body weight weekly. Progesterone was quantified daily in serum for 3.5 estrous cycles. On days 9, 10, or 11 after fourth estrus, blood was sampled every 15 min for 12 hr to quantify luteinizing hormone (LH), growth hormone (GH), insulin and non-esterified fatty acids (NEFA). The next day, luteal cells were incubated and proportions of small to large cells were determined. After fourth estrus, area of progesterone profiles in serum for 10 days postestrus was reduced in all heifers relative to MBC-PEB heifers. But, luteal weight from FBC-PEB and MBC-NEB heifers was less than MBC-PEB heifers and FBC-NEB heifers were intermediate. Secretion of progesterone in vitro was increased by LH for PEB but not NEB heifers. MBC-NEB heifers had increased ratios of small to large luteal cells. Independent of BC, NEB decreased concentrations of insulin and increased GH and NEFA. Secretion of progesterone was not associated with LH, GH or insulin, but was correlated negatively with NEFA. We conclude that reduced concentrations of progesterone in serum of FBC-PEB and MBC-NEB heifers is due to impaired luteal development. But, reduced concentrations of progesterone in serum of NEB heifers is due also to reduced basal (MBC) and LH-induced (MBC and FBC) secretion of progesterone by luteal cells. Body condition at onset of NEB may determine when effects of NEB on progesterone are detected.

Association between energy balance and luteal function in lactating dairy cows.

Our objective was to determine the relationship between energy balance and secretion of progesterone in lactating dairy cows. Eight primiparous and 24 multiparous lactating Holstein cows were studied from parturition to 100 d postpartum or conception. Cows calved normally and remained healthy throughout the study. All cows were fed ad libitum a total mixed diet formulated to satisfy requirements for maintenance and lactation. Intake of feed and production of milk per cow were measured twice daily. Body weight was determined weekly. Daily energy balance was determined by subtracting energy required for maintenance and lactation from intake of energy. Concentrations of progesterone were determined in milk sampled every 3rd d. For at least 4 successive d postpartum, 81% of cows were in negative energy balance. Variation in energy balance was explained largely by intake of energy. Duration of luteal phases was not associated with energy balance. Energy balance within 9 d postpartum was correlated positively with concentration of progesterone within second and third postpartum luteal phase. Postpartum interval to nadir and magnitude of nadir of energy balance interacted to reduce progesterone within second and third postpartum estrous cycles. Thus, in lactating cows, secretion of progesterone is reduced by spontaneous caloric deficit and is modulated by timing and magnitude of maximal caloric deficit. Spontaneous caloric deficit is a potential source of infertility in lactating dairy cows.

Destruction of bovine ovarian follicles: effects on the pulsatile release of luteinizing hormone and prostaglandin F2 alpha-induced luteal regression.

Destruction of ovarian follicles during diestrus prolongs the lifespan of corpora lutea in cows, but the site(s) of action is unclear. Thus, ovarian follicles were destroyed in 10 beifers (X-IRRAD) on Day 9 postestrus, while 10 additional beifers (SHAM) served as a control group. To investigate changes in luteotropic support resulting from destruction of ovarian follicles, pulses of luteinizing hormone (LH) were characterized on Days 8, 13, and 15 postestrus. To study the interaction between products from ovarian follicles and prostaglandin F2 alpha (PGF2 alpha) in luteolysis, changes in serum concentrations of progesterone were monitored after an injection of saline or PGF2 alpha on Day 14 postestrus. Frequency and amplitude of pulses of LH increased by Day 13 in X-IRRAD beifers. An increase of similar magnitude in amplitude but not frequency of pulses of LH occurred between Day 13 and Day 15 postestrus in SHAM beifers. Exogenous PGF2 alpha was significantly less efficacious in causing luteolysis in X-IRRAD animals. We suggest that increased luteotropic support may be involved in but is not the only cause for lengthening the lifespan of corpora lutea following destruction of ovarian follicles. Additionally, we suggest that regression of bovine corpora lutea involves a synergistic action between products from ovarian follicles and PGF2 alpha.

Enhanced precision of estrus and luteinizing hormone after progesterone and prostaglandin in heifers.

Objectives were to determine if low progesterone and increased basal luteinizing hormone for 3 d after injecting prostaglandin F2 alpha would affect precision of preovulatory surges of luteinizing hormone and onset of estrus in Holstein heifers. In experiments 1 and 2, heifers received progesterone devices for 10 d and prostaglandin F2 alpha on the 7th d after inserting progesterone device. For controls, progesterone devices were removed when prostaglandin F2 alpha was injected, and control devices were installed for 3 d. In experiment 1, jugular blood was sampled every 2 or 4 h from injection of prostaglandin F2 alpha until 108 h after removing progesterone to determine basal and peak secretion of luteinizing hormone. In treatment heifers, basal luteinizing hormone tended to increase, and intervals to peak secretion of luteinizing hormone were shorter and more precise than for controls. In experiment 2, heifers were observed for estrus for 30 min every 4 h for 96 h after removing progesterone devices. Intervals to onset of estrus for experimental heifers were shorter and more precise than for controls. After exogenous prostaglandin F2 alpha, 1 to 2 ng of progesterone per milliliter of serum for 2 to 3 d allows increased basal luteinizing hormone and is associated with increased precision of preovulatory secretion of luteinizing hormone and onset of estrus. This may be an acceptable method to control ovulation precisely for fixed time insemination of cattle.

Effect of ovarian follicles on luteal regression in heifers.

Our objectives were to determine whether or not ovarian follicles contribute to spontaneous luteal regression in heifers and, if so, when during diestrus do follicles exert their effect. Thirty-one Holstein heifers having displayed at least one estrous cycle (19 to 21 d) were assigned, as available, to randomized blocks for a factorial experiment. Reproductive organs were exposed through a midventral incision on d 9, 12 or 15 postestrus (estrus = d 0). Visible follicles were electrocauterized and both ovaries were x-irradiated (1,500 rads) in treated heifers, whereas ovaries of controls were exteriorized but follicles were not destroyed and ovaries were not x-irradiated. In two additional heifers, the ovary containing the corpus luteum was exteriorized and x-irradiated on d 15 postestrus, but follicles were not electrocauterized. Jugular blood was collected before and every 8 h after surgery until d 24 postestrus. All heifers were ovariectomized on d 24 postestrus to inventory follicles and to weigh corpora lutea. No follicles (greater than or equal to 1 mm diameter) were observed in ovaries from treated animals and concentrations of estradiol-17 beta did not change over time, whereas different numbers of follicles were observed in ovaries from controls and concentrations of estradiol-17 beta increased (P less than .05) during proestrus. Hence, treatment destroyed follicles and prevented follicular development. On d 24 postestrus, corpora lutea from treated heifers (5.5 +/- .5 g) were heavier (P less than .001) than corpora lutea from controls (1.1 +/- .1 g), independent of day when follicles were destroyed.(ABSTRACT TRUNCATED AT 250 WORDS)