Regulation of estrous cycles in dairy cattle: A review.

Artificial insemination has allowed rapid genetic progress in dairy cattle through the use of semen from genetically superior sires. Detection of estrus and proper timing of insemination remain as major problems limiting reproductive efficiency in dairy herds. Numerous programs for regulating estrous cycles have been examined in this review. While several programs appear promising, results remain variable and the need for accurate detection of estrus has not been eliminated. Nulliparous heifers seem to respond more consistently than lactating cows. Further research is required to develop a successful program that will consistently produce a precise synchrony of estrus and ovulation with normal fertility and be cost effective.

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 PGF2alpha, intervals from PGF2alpha 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.

Effect of beta-carotene supplementation on reproductive performance of dairy heifers.

Three trials were conducted to determine the effect of feeding supplemental beta-carotene on reproductive performance of Holstein heifers. In each trial, the animals were randomly assigned to either a control or treatment group. Animals in the treatment groups received 300 mg supplemental beta-carotene daily for the test periods which varied from 6 to 8 weeks in length. To facilitate sample collections and observations, estrus was synchronized with two injections of 25 mg PGF(2alpha) at 11 day intervals. The first injection was after 2 weeks of feeding supplemental beta-carotene. Blood serum beta-carotene concentrations were higher in the heifers fed supplemental beta-carotene as compared to concentrations in control heifers after 1 week of feeding and this difference increased throughout the test periods. The interval from the second injection of PGF(2alpha) to onset of estrus was shorter in the control heifers as compared to that interval in heifers supplemented with beta-carotene (trial 1,44.0 vs 56.0 hr; trial 2, 51.3 vs 70.8 hr; trial 3, 40.7 vs 62.5 hr, respectively). The intervals from PGF(2alpha) administration to the preovulatory LH peak (43.3 vs 61.5 hr) and ovulation (69.3 vs 85.9 hr) were also shorter in the control heifers in trial 3. No significant differences were found among treatments in the number of heifers that expressed estrus, the blood serum progesterone concentrations or the conception rates in any of the three trials.

Avulsion of the bovine prepuce from its attachment to the penile integument during semen collection with an artificial vagina.

Avulsion of the prepuce (AP) from the penile integument during intromission into an artificial vagina (AV) is the most common injury involving the penis and prepuce of bulls collected for the production of frozen semen at artificial insemination (AI) centers. We observed clinical cases at two AI centers. Because temporary loss of seminal collections from genetically superior bulls occurs and the possibility of impotency exists, the economical loss from AP is significant. Surgery, rather than a nonsurgical procedure, is the preferred method for repair of AP if the diagnosis occurs on the same day as the injury. After injury, seminal collection is avoided. The bull can then begin a conditioning program prior to returning to a normal semen collection schedule. A specific etiology for AP remains undetermined; however, many variables potentially involved in its occurrence are known. Bulls with good libido following 4 to 5 yr of sexual inactivity are particularly susceptible to AP. No genetic predisposition was observed. At present, we believe that AP is caused by many factors and that implementing preventive measures does help.

Recent advances in the chemistry of spinosyns.

The spinosyns are a new class of fermentation-derived insect control agents that are effective against a variety of chewing insect pests. The successful introduction of spinosad into the agricultural marketplace represents an important milestone in the use of natural products for commercial pest control. The development of a natural product presents additional limitations relative to a synthetic material. While the latter affords some degree of control in building appropriate physical attributes such as photostability, a natural product, designed to function in a different environment, is often less suited for traditional spray applications. Despite its intrinsic photolability, spinosad is stable enough to perform under field conditions. In an effort to generate analogs with improved physical characteristics, we have developed a variety of conditions for selectively modifying different portions of the molecule, and we have discovered analogs with greater activity against a broader spectrum of pests. The inability to translate improved greenhouse activity to actual field conditions resulted in a detailed study of the effects of formulations and crystallinity on biological activity. Through this effort, measurably improved field performance of synthetic spinosyn analogs relative to the natural product have now been observed.

Precipitin response of cattle to commercial wart vaccine.

Fifty young bulls were given commercial wart vaccine upon their arrival at a bull stud establishment. The bulls were given 2 additional vaccinations 2 and 24 weeks later. The precipitin antibody response of these bulls to bovine papilloma virus was monitored. Antibody levels increased significantly, particularly after the 3rd vaccination was given at 24 weeks, but decreased markedly within 20 weeks after the last vaccination was given. These results demonstrated that formalin-inactivated bovine papilloma virus was capable of inducing precipitin antibody similar to infective bovine papilloma virus. Multiple, repeated vaccination provided the best serologic response.

Relationship of the estrous cycle to milk composition.

The relationship of the estrous cycle to milk composition was examined in 15 Holstein cows in first lactation which were approximately 45 days postpartum. Composite milk samples were collected twice daily and analyzed for progesterone, fat, protein, total solids, sodium, potassium, magnesium, calcium, and somatic cells. Milk progesterone was minimal (less than ng/ml) from day -1 to day 2 (day 0 = estrus) of the estrous cycle and then increased at a relatively constant rate through day 15. Milk yield and sodium and magnesium concentrations of the milk varied with days of the estrous cycle. Mean milk yield was highest and sodium and magnesium concentrations were lowest on day 1. However, sodium concentration of the milk was the only component that varied significantly during the 3 days centered on estrus (days -1, 0, and 1). No other milk component changed significantly during the estrous cycle. Although milk yield and composition varied during the estrous cycle, none of these components appeared to be a practical indicator of estrus.

Vitamin A supplementation of Holsteins at high concentrations: progesterone and reproductive responses.

Five trials involving 168 Holstein cows and 20 heifers were conducted to determine the effect of vitamin A supplementation at high concentrations, 1 or 2 million IU, compared with 100,000 IU/d, on peripheral concentrations of progesterone and on several measures of reproductive performance. Supplemental vitamin A at high concentrations had no effect on circulating concentrations of progesterone in either cyclic or pregnant cows or on most measures of reproductive performance examined in trials. Thus, it was concluded that providing vitamin A at these high concentrations is not warranted.

Effect of dietary protein and exogenous gonadotropin-releasing hormone on circulating progesterone concentrations and performance of Holstein cows.

Sixty-three Holstein cows were used in a 2 x 2 factorial arrangement of treatments to determine the effects of dietary protein and exogenous gonadotropin-releasing hormone on performance postpartum and on circulating concentrations of progesterone. The experimental diets, 14 or 20% CP (dry basis), were fed from parturition to 14 wk postpartum. Blood serum urea N was elevated in the cows fed the 20% versus 14% CP diet. Feed intake was increased, but yield and composition of milk were not affected by the higher protein diet. Circulating concentrations of progesterone were lower in cows fed 20% than 14% protein on d 12 of both the synchronized and subsequent estrous cycles. Exogenous gonadotropin-releasing hormone administered on d 12 of the preceding estrous cycle did not increase the concentration of progesterone on d 12 of the subsequent estrous cycle. Luteal phase concentrations of progesterone were reduced by high dietary protein, but were not affected by gonadotropin-releasing hormone given on d 12 of the preceding cycle.

Effect of supplemental beta-carotene on luteinizing hormone released in response to gonadotropin-releasing hormone challenge in ovariectomized Holstein cows.

Thirteen lactating Holstein cows were assigned randomly to either a control or beta-carotene (600 mg/d) treatment group to determine the effect of long-term beta-carotene supplementation on releasable luteinizing hormone in response to gonadotropin-releasing hormone challenge. The experimental period was 28 wk. Milking was terminated at wk 16, ovariectomy performed at wk 20, and response to gonadotropin-releasing hormone determined at wk 27. Serum beta-carotene concentrations reflected dietary intake and were higher in the cows fed beta-carotene after 2 wk of supplementation and remained higher for the duration of the trial. Feeding supplemental beta-carotene had no effect on circulating progesterone concentration, corpora lutea size or corpora lutea progesterone concentration, or basal concentrations of luteinizing hormone, frequency, and amplitude of luteinizing hormone pulses, or the release of luteinizing hormone in response to gonadotropin-releasing hormone. Thus, pituitary responsiveness to exogenous gonadotropin-releasing hormone was not affected by feeding supplemental beta-carotene.

Effect of beta-carotene supplementation on reproductive performance of lactating Holstein cows.

Fifty-four lactating Holstein cows were assigned by parity and calving date to a 2 X 3 factorial arrangement of a randomized block design to determine effects of beta-carotene supplementation and ration fiber content on reproduction. Because ration fiber treatments did not differ in their effects on reproductive performance, data for beta-carotene analyses were pooled over the three rations. From 3 to 98 d postpartum, cows were individually fed either 0 or 300 mg supplemental beta-carotene daily by adding it on top of their complete mixed ration. By wk 3 postpartum, concentrations of beta-carotene in blood serum were higher in cows fed supplemental beta-carotene and remained higher throughout the experimental period. Feeding supplemental beta-carotene had no effect on the interval from the time of parturition to uterine involution, ovulation, first observed estrus, or conception. Feeding beta-carotene did not influence incidence of cystic follicles based on palpation, ovarian cyclicity based on blood progesterone patterns, peak progesterone concentrations, or first service conception rate. Cows fed supplemental beta-carotene required fewer treatments for clinical mastitis. Reproductive performance in lactating Holstein cows was not improved by feeding supplemental beta-carotene.