The effect of a diet containing excess quickly degradable nitrogen (QDN) on reproductive and metabolic hormonal profiles of lactating dairy cows.

The objective of this experiment was to examine the effects of an excess intake of quickly degradable nitrogen (QDN) on metabolic and reproductive parameters in lactating dairy cows. Twenty-two lactating dairy cows were fed a total mixed ration once daily. The control diet was a typical ration for high producing cows in the UK (CP = 17.5%, ME = 11.8 MJ/kg DM). The cows were randomly divided into two groups, control diet (control; n = 12) and excess QDN diet (QDN; n = 10). The QDN group was fed an additional 250 g of urea per cow per day, from 10 days before insemination (day 0) until the end of the experiment, 17 days after the second insemination. Ten days before insemination, a synchronized oestrus was induced and the cows inseminated twice, 48 and 72 h after synchronization, with commercial frozen semen from a single sire. Ovaries were scanned using B-mode ultrasonography 10 days before insemination and then daily from 3 days before insemination. Eighteen of the cows (9 control and 9 QDN) were sampled more intensively to determine the pulsatile pattern of secretion of luteinizing hormone (LH) and growth hormone (GH). Cows were slaughtered 17 days after insemination, the reproductive tracts recovered and flushed to retrieve embryos. The excess QDN diet resulted in elevated (P < 0.05) plasma urea concentrations 3 days after starting urea feeding and these were maintained until the end of the experiment. However, the excess QDN diet did not significantly affect daily milk production or plasma concentrations of insulin and IGF-I. The QDN treatment did not significantly affect pulsatile patterns of secretion of LH and GH or the number of small (< 0.5 cm diameter) and medium to large follicles (> 0.5 cm diameter). Twenty cows ovulated following synchronization (control 11/12; QDN 9/10). There were no significant differences between the control and the QDN groups in the peak concentrations of oestradiol during the follicular phase or in the post-ovulatory pattern of plasma and milk progesterone secretion. Embryos and/or foetal membranes were recovered from 10 cows (5 control and 5 QDN). The results of the current study show that feeding excess QDN, as urea, for 27 days commencing 10 days before insemination had no effect on reproductive or metabolic hormonal parameters. Ovulation and the formation and function of the post-ovulatory corpus luteum were also unaffected by excess QDN. These data suggest that the harmful effects of excess intakes of QDN on fertility occur after 17 days following ovulation.

Effect of timing of urea feeding on the yield and quality of embryos in lactating dairy cows.

High protein diets, which lead to excess production of nonprotein nitrogen such as ammonia and urea, have been associated with reduced fertility in dairy cows. In this study we test the hypothesis that diets containing high levels of quickly degradable urea nitrogen (QDN) compromise embryo development. Lactating dairy cows were fed mixed silage and concentrates twice daily. At 60 days postpartum, a synchronized estrus was induced and the cows were subsequently superovulated and inseminated using a standard protocol. On Day 7 after insemination, the uteri were flushed and embryos retrieved. At the start of treatment, cows were randomly allocated into three nutritional groups: control (CONT, n = 8), long (L-) QDN (n = 8) and short (S-) QDN (n = 9). The L-QDN cows were fed a supplement of urea from 10 days before insemination, and the S-QDN cows were fed the supplement from insemination until embryo collection. Both L- and S-QDN diets produced significant increases in plasma ammonia and urea 3 h post-feeding. The S-QDN but not the L-QDN diet was associated with a significant reduction in embryo yield. Embryo quality was also significantly reduced in the S-QDN cows. This study indicates that there is no deleterious effect on the yield and quality of embryos recovered 7 days after breeding when QDN feeding is initiated during the previous midluteal phase. However, introduction of a similar diet 10 days later, at the time of insemination, was deleterious. We suggest that QDN is toxic to embryos but cows can adjust within 10 days.

Development of a gonadotrophin-releasing hormone and prostaglandin regimen for the planned breeding of dairy cows.

Four studies were carried out to determine the ovarian responses of dairy cows undergoing natural oestrous cycles to sequential injections of gonadotrophin-releasing hormone (GnRH), followed seven days later by prostaglandin and, 48 to 72 hours later, by a second injection of GnRH. In study 1, of 60 cows so treated, 47 were in the intended periovulatory phase when a fixed-time insemination was given 72 hours after the prostaglandin. In study 2, detailed observations were made in 32 cows treated as in study 1, using ultrasound to determine the optimum time to administer the second dose of GnRH. Ovulation was most effectively synchronised by giving GnRH 56 to 60 hours after the prostaglandin. Study 3 investigated the timing of ovulation when no initial dose of GnRH was given. Six cows were injected with prostaglandin on day 12 of the oestrous cycle, followed by GnRH 60 hours later. Five of the six cows ovulated 24 to 36 hours after GnRH, an equivalent timing and synchrony to that in study 2, in which a dose of GnRH had been given seven days before prostaglandin. In study 4, an initial dose of GnRH was given to six cows late (day 17) in the oestrous cycle, and prostaglandin seven days later. The GnRH treatment delayed luteolysis in five of the cows so that they were responsive to the prostaglandin and ovulated 24 to 36 hours after the second dose of GnRH. The use of GnRH (day 0) - prostaglandin (day 7) - GnRH (day 9.5) appears to be an effective means of synchronising ovulation in most cows.

Dietary protein and the reproductive performance of cows.

Increasing a cow's intake of dietary protein intake can increase its milk production, but can also reduce its fertility. This paper reviews the effects of increasing dietary protein on the fertility of the dairy cow, and the mechanisms that may produce them. The effects vary widely, but all stages of the reproductive cycle from the return to cyclicity after parturition, to the survival of the embryo, may be affected. However, the underlying cause of the link between protein intake and fertility is unclear. Fertility could be reduced by a direct toxic effect of protein breakdown products, but alternatively the increased energy demand for their metabolism could be responsible. The effect of protein degradability is also uncertain. Excess rumen degradable protein is commonly associated with reduced fertility, but similar effects are produced by diets that contain excess rumen undegradable protein. Increasing the intake of protein of all degradabilities has significantly different effects on blood biochemistry than a reduction in the intake of energy, suggesting that not all the effects of protein are due to energy imbalance. The primary site of action of the effect is also unclear. Limited evidence suggests that it is localised to the reproductive system, but effects on the pituitary and hypothalamus, as well as the ovary and uterus, have all been postulated. It is also uncertain what toxic principle is involved. Ammonia, nitrate and urea have all been suggested, but there is no conclusive evidence. Although a high protein intake has been postulated to have an effect on fertility for over 30 years, the evidence remains inconclusive, and the aetiology and pathogenesis of the effect remain obscure.

Effect of buserelin on pregnancy rates in dairy cows.

Three field trials were carried out to assess the effect of buserelin on the fertility of dairy cows. In the first, 10 micrograms of buserelin was injected on the day of insemination; there were no significant effects on fertility in comparison with untreated control cows. In the second study the cows were injected 12 days after insemination; the mean pregnancy rates to first insemination were 53.4 and 65.4 per cent for the control and treated cows, respectively (P < 0.01) and the mean pregnancy rates to repeat inseminations were 52.9 and 59.4 per cent for the control and treated cows. The mean calving to conception intervals were 91.4 and 85.3 days (P < 0.01) and the incidences of barren cows were 10.2 and 5.2 per cent. In the third study the cows were injected with buserelin either eight days or 10 days after insemination; there were no significant effects on fertility in comparison with untreated control cows.

Experimental and practical approaches to the establishment and maintenance of pregnancy.

A series of three large field trials was carried out to assess the effect of buserelin on fertility in dairy cows. In the first, 10 micrograms buserelin was injected on the day of insemination. There were no significant effects on fertility parameters compared to untreated controls. In the second trial cows were injected on day 12 after insemination. Mean pregnancy rates to first insemination were 53.4 and 65.4% for control and treated cows respectively (P < 0.01). Mean pregnancy rates to repeat inseminations were 52.9 and 59.4% for control and treated cows (NS). Mean calving to conception intervals were 91.4 and 85.3 days (P < 0.01) and the incidence of barren cows was 10.2 and 5.3% (NS). Overall the economic benefit of buserelin injection on day 12 was calculated to be 27.43 pounds per cow treated excluding the cost of the treatment. In trial 3 cows were injected with buserelin either on day 8 or 10 after insemination. There were no significant effects on fertility parameters compared to untreated control cows. In a fourth trial ewes were injected with 4 micrograms buserelin on day 12 after service. There were indications that both pregnancy rate and lambing percentage could be increased by buserelin treatment. Daily blood samples were collected from 5 dairy cows during a control cycle and a cycle in which 10 microgram buserelin was injected on days 11 and 13. Cycle length was unaffected by treatment and the concentration and pattern of progesterone secretion did not differ between control and treated cycles. Plasma oestradiol concentrations were similar in the control and treated cycles before day 11. However from day 12 to 16, equivalent to the time of maternal recognition of pregnancy, the mean concentration of oestradiol was significantly reduced in the treated cycle. As oestradiol stimulates both the development of uterine oxytocin receptors and the secretion of PGF2 alpha we suggest that any improvement in pregnancy rate after buserelin is due to a weakened luteolytic mechanism, resulting from a lower plasma oestradiol concentration.

Effect of progesterone treatment on the calving-to-conception interval of Friesian dairy cows.

A total of 615 autumn calving Friesian dairy cows in seven herds were allocated to treatment or control groups according to date of previous calving, parity and milk yield. All cows were scored for body condition at the start of the breeding season. A silastic coil impregnated with progesterone with a capsule containing oestradiol benzoate attached was inserted into the vaginas of the 310 treated cows. The coils were removed after 12 days and cows inseminated 48 and 72 hours later. Cows more than 47 days post partum were inseminated on the first day of the breeding season. Thereafter, cows were inseminated on a weekly basis when between 47 to 54 days post partum. The 305 control cows were observed from 42 days post partum and inseminated when observed to be in oestrus. In both groups, cows returning to service were reinseminated at observed oestrus. Pregnancy was diagnosed by milk progesterone assay and by rectal palpation. The mean intervals to first service were 74.8 days for the treated and 90.4 days for control cows. No significant differences were found in the pregnancy rates to first service, services per conception or barren percentage for treated and control groups respectively. The mean calving-to-conception intervals differed significantly, 93.1 days for treated and 107.3 days for control cows. There was no significant association between condition scores and either pregnancy rates to first service, barren percentage or calving-to-conception intervals.

Fertility of norgestomet treated suckler cows.

Four studies are described involving 3050 single suckled mainly Hereford-Friesian cross beef cows treated with norgestomet and oestradiol valerate to control the ovarian cycle. In study 1 the effect of insemination timing on pregnancy rate was studied. A greater proportion of cows inseminated 48 and 72 hours after treatment became pregnant than after a single insemination at 48 hours. In study 2 large variations in fertility among farms were recorded in field trials of norgestomet and oestradiol valerate. In study 3 the effect on fertility of nutrition, weight and body condition was examined. In study 4 57.9 per cent of 1113 treated cows became pregnant to insemination at 48 and 72 hours. Cows were fed a balanced ration designed to provide for a predicted daily milk yield of 9.0 kg for a 12-week period starting six weeks before insemination. The range of fertility was from 36.7 per cent to 94.9 per cent.

Fertility of norgestomet treated dairy heifers.

Four studies are described involving 2573 Friesian heifers treated with norgestomet and oestradiol valerate to control the ovarian cycle. All treated animals were inseminated at fixed times following treatment. In study 1 insemination 48 and/or 60 hours after treatment resulted in a non-significant greater proportion of heifers becoming pregnant than at 48 X 72 hours. Large variations in fertility among farms were recorded in field trials of norgestomet (Searle) and oestradiol valerate. In study III the effect on fertility of nutrition, weight and body condition was studied. In section IV a pregnancy rate to fixed time insemination of 66-8 per cent of 500 heifers inseminated at 48 and 60 or at 48 and 72 hours after norgestomet/oestradiol valerate treatment was obtained. Heifers in study IV were fed a balanced ration designed to provide a predicted daily liveweight gain of 0-7 kg for a 12-week period starting six weeks prior to the date of insemination. The range of fertility was from 59-0 per cent to 85-7 per cent.

PIP: A total of 2630 heifers were used in 4 controlled breeding studies using norgestomet and estradiol valerate to control the ovarian cycle. In Study 1, alternative times of insemination were compared; in Study 2, the 2 most successful insemination timings in the 1st study were used in heifers throughout the United Kingdom; in Study 3, the effect of nutrition level, weight, and condition on treated heifers was considered, and in Study 4 the findings were implemented. A special polymer implant containing 6 mg norgestomet was implanted sc in the outer surface of the ear and at time of implantation a 2 ml injection of 5 mg estradiol valerate and 3 mg norgestomet dissolved in sesame oil was given im. The implant was removed after 9 days. 95% of treated heifers were in estrus within 5 days after implant removal while 71.9% of controls were in estrus over a 4-week period. 73.2% of the treated animals and 52.6% of the controls became pregnant. Insemination at 48 and 60 hours after removal produced as great a pregnancy rate as insemination at 48 and 65 or 48 and 75 hours. In field trials, this method had widely varying results depending on farm. On an average 55% became pregnant. Of those nonpregnant, running with bulls proved more effective than a 2nd artificial insemination. It was found that nutritional status played a significant role in fertility.