Triennial Lactation Symposium: Effects of stress on postpartum reproduction in dairy cows

The objective was to review the effects of production stressors on reproductive performance of dairy cows. It has been well documented that genetic selection for milk yield over the last 50 yr has been associated with reduced fertility. In addition to negative associations between yield and conception rate, there is also an association between milk production and expression of behavioral estrus. Stress caused by production diseases in high-yielding dairy cows also contributes to the problems of poor fertility. Lameness results in reduced intensity of estrus and can contribute to ovulation failure, which is largely due to reduced pre-ovulatory estradiol secretion and failure of the LH surge. Mastitis has been associated with prolonged intervals to dominant follicle selection, and in animals with uterine infection the dominant follicle grows slower and produces less estradiol. In a recent study, we identified that milk yield was associated with an increased incidence of uterine infection, which is known to contribute to reduced fertility and prolonged calving-to-conception intervals. The incidence of uterine disease was 73% in high-yielding, compared with 45% in low-yielding cows. As well as effects at the ovary, various models of stress have also been shown to perturb endocrine secretion in the hypothalamus and anterior pituitary. In conclusion, the adverse effects on fertility associated with genetic selection for yield in dairy cows is, in part, associated with increased incidences of production disease-induced stress but is also associated with high milk yield.     

Nutrition and reproduction in dairy cows

Diagnosis of nutritional effects on herd fertility requires a systematic analysis of reproductive records in addition to quantifying the feeding program on the farm. Corroborative laboratory tests for specific deficiencies may be useful.     

Effects of nutrition on reproduction in dairy cows

It is clear that nutrition is closely related to reproduction in the dairy cow. In addition to classic nutrient deficiency and excess, dry cow body condition and postpartum energy balance have a major impact on fertility. Dry cow rations must be designed to prevent milk fever, dystocia, retained placenta, other calving problems, and metritis. Most importantly, milk yield and reproductive performance should be regularly monitored to detect nutritionally associated health and performance changes that precede impairment of reproductive function.     

Heat synchronization in dairy cows

Dairy cows were heat synchronized with two injections of cloprostenol (PG1/PG2) with an interval of 13 days and inseminated 72 hours after PG2. The total pregnancy rate obtained was 42% The probability of achieving luteal phase at PG2, the probability of undergoing luteolysis after PG2 and the probability of achieving pregnancy were all equal whether the cows were in luteal phase, follicular phase, had follicular cysts or small ovaries at PG1. Close correlations were found between the signs of heat at the insemination and the interval from calving to PG1 to the pregnancy rates.     

The effect of stress on udder health of dairy cows

The appropriate literature has been reviewed for the purpose of defining the phenomenon of stress in lactating dairy cattle, establishing a baseline concept of lactation stress and emphasizing the most significant aspects of the natural mammary defence mechanisms. Data on the general adaptation syndrome (GAS) make it clear that stress is essentially the rate of wear and tear of the biological system affected by a stressor either eliciting stress of the organism as a whole or partly so. Owing to the variety of stressors which may affect the dairy cow at physiological and pathological levels, a definition of stress in the broad sense is indicated. This is essential from the point of view of the anti-homeostatic effects (metabolic and immunological) of lactation stress, aggravated by anti-homeostatic effects elicited by superimposed other types of stress (e.g. heat stress). The lactating cow, as a ruminant in a state of sustained stress, requires a special profile of hormonal mediators. In high yielding cows, for example, acute and sustained heat stress promotes increased activities of prolactin, progesterone and catecholamines. Compared with the mainly glycogenic/glycogenolytic metabolism of non-ruminant mammals, the lipogenic/lipolytic and glycogenic/glycogenolytic metabolism of the dairy cow depends on hormonal mediators which differ from those of the former not so much in their nature but in their magnitude and ratios. Stressors induce the development of GAS reactions in the dairy cow. These enable the cow to create and maintain homeostasis of its integrated 3 main physio-pathological systems and thus to endure the stressor(s). The cow's compensating adjustments to a stressor are therefore the effects of stress. This means that natural lactation is the effect of the lactation stress induced by the cow's progeny (i.e. the natural lactation stressor). Artificial lactation stressors (e.g. removal of milk by hand and machine) may affect the lactation stress in magnitude but not necessarily in nature. Likewise, a range of behavioural, physiological, lactational and lacteal changes related to other stressors are the effects of different types of stress. Lactation stress, like other types of stress, shows 3 stages of development, i.e., an overcompensating alarm phase (= lactogenesis), resistance phase (= galactopoiesis) and exhaustion phase (= regression). They facilitate adjustments of the cow's homeostasis from the level of involutional homeostasis (= no lactational activity) to that of lactational homeostasis. Like other tissues in a state of stress, the lactating mammary epithelium requires a greatly increased supply of glucose.(ABSTRACT TRUNCATED AT 400 WORDS)     

The effect of eprinomectin treatment at calving on reproduction parameters in adult dairy cows in Canada

A clinical trial was carried out in two provinces of Canada to investigate the effect of treatment with eprinomectin at calving on production parameters in adult dairy cattle in 1999-2000. One of the objectives of this study was to evaluate the impact of treatment on reproductive performance as measured by: calving-to-conception interval, calving-to-first service interval and number of services per conception. The ability of an indirect ELISA using a crude adult Ostertagia ostertagi antigen to predict response to treatment also was evaluated. All lactating cows in 20 dairy herds were allocated randomly to receive either eprinomectin pour-on or placebo at calving. Information on reproductive parameters was obtained from computerised cow records. Survival models were used to evaluate the effect of treatment on the two intervals and a Poisson model was used to evaluate the number of services to conception. A total of 549 cows were included in these analyses. A marginally significant treatment effect on calving-to-conception interval was observed (hazard ratio=1.24, P=0.06) but not on calving-to-first service interval. A significant reduction in the number of breedings to conception for treated animals also was observed with a longer effect in cows with short interval to first service. Milk samples from a subset of 109 late-lactation cows were tested for antibodies against O. ostertagi. The ELISA optical-density ratio (ODR) values obtained between 120 days before calving and drying off were categorised as high ODR (>or=0.5) and low ODR (<0.5). Among untreated animals, the hazard of conception was lower (hazard ratio=0.38, 95% CI=[0.19,0.75]) for high-ODR cows compared to low ODR cows suggesting that higher parasite burdens had an adverse effect on reproductive performance. Treated high-ODR cows had a hazard of conception equivalent to the hazard for all cows in the low-ODR group (indicating that treatment prevented the negative effect associated with these higher parasite burdens).     

The effect of season on reproduction in dairy cattle

A survey was carried out in North West Tasmania to compare the breeding performance of 2,068 cows in 27 dairy herds--15 herds calving in spring and 12 in autumn. The percentage of cows submitted to artificial insemination for the first time in the first 24 days of the mating period was 87% for the spring calving group and 75% for the autumn calving group. Figures for non-return-to-service rate over the subsequent 21 days were 72% and 55%, respectively. The difference in breeding performance associated with time of calving was not attributable to age, calving to mating interval or on cow condition or daily milk fat yield at 3 weeks (+/- 1 week) after the start of mating.     

Oxidative stress in dairy cows seropositives for Neospora caninum

Bovine neosporosis is caused by the protozoan Neospora caninum and is one of the major causes of abortion in cows. Cattle are intermediate hosts of this parasite and may have asymptomatic or symptomatic infections. Therefore, the aim of this study was to evaluate oxidative stress marker reactive oxygen species (ROS), thiobarbituric reactive acid substances (TBARS) levels, glutathione S-transferase (GST), adenosine deaminase (ADA), and butyrylcholinesterase (BChE) activities in dairy cows seropositives for N. caninum (asymptomatic or symptomatic). Dairy cows (n = 90) were tested by immunofluorescent antibody assay (IFA) for N. caninum and divided accordingly into three groups: the group A (seronegatives, n = 30), the group B (seropositives and asymptomatic, n = 30), and the group C (seropositives and symptomatic, n = 30). It was observed increased levels of TBARS and reduced (P < 0.05) BChE activity in seropositives either asymptomatic or symptomatic animals. ROS levels and ADA activity increased, and GST activity decreased (P < 0.05) only in seropositives symptomatic dairy cows (the group C) compared to seronegatives dairy cows (the group A). Based on these results, it was observed that seropositive animals showed cell damage associated with oxidative stress and inflammation, mainly in those with symptomatic infections. Increased seric ROS levels and BChE activity may have influenced N. caninum pathogenesis in symptomatic animals due to increased cell damage and exacerbated inflammatory response, leading to the development of clinical signs.     

Adrenocortical function testing in dairy cows and its effect on milk yield.

Administration of 6IU synthetic ACTH1-24 intravenously to six Holstein-Friesian cows resulted in a cortisol peak concentration after 1 hour of 148 +/- 34.2 ng/ml. Basal plasma cortisol concentration (4.84 +/- 0.83 ng/ml) was reached 5 hours after ACTH injection. Until 7 days after ACTH administration no effect on milk yield was recorded. So it is concluded that a dose of 6 IU ACTH1-24 is sufficient for a conspicuous release of cortisol without any alteration in milk production. This dose can be used as a standard test for the evaluation of adrenocortical function in lactating cows when administered intravenously at 9 a.m. and when plasma samples for cortisol assay are collected just prior to administration and at 10 a.m.     

Anthelmintic treatment of dairy cows and its effect on milk production.

The results of more than 80 experiments on gastrointestinal parasitism and the impact of anthelmintic treatment on milk production in dairy cattle were reviewed. Abattoir surveys of culled dairy cows, faecal egg counts in milking cows, and serological tests and worm counts in cull cows in milk production studies were collated to assess the level of parasitism in dairy herds. The studies were divided into four general categories: induced infections in previously uninfected cattle; naturally infected cattle treated in mid-lactation; naturally infected cattle treated one to three times during the dry period and/or just before or just after parturition; and naturally infected cattle treated repeatedly from early lactation or given strategic treatments throughout the year. In most studies, the milk production of anthelmintic-treated cattle was compared with that of untreated controls. The anthelmintics investigated included members of the organophosphate, benzimidazole, imidazothiazole and macrocyclic lactone groups. The number of experiments in which the medicated (or uninfected) group had a higher milk yield was compared with the number of experiments in which the control (or infected) group had a higher yield. Overall, the studies demonstrated that grazing dairy cattle are likely to be infected with gastrointestinal nematode parasites, usually Ostertagia ostertagi and Cooperia species. These infections may be present as inhibited larvae, and a periparturient or spring rise is associated with their emergence. There is, at present, no reliable means of determining whether a cow or a herd may be parasitised subclinically at a level sufficient to interfere with milk production. In 70 of 87 experiments (80 per cent) there was an increase in milk production (P < 0.001) after anthelmintic treatment, with a median increase of 0.63 kg/day. In each of the four trial categories, a majority of the studies showed that anthelmintic treatment increased milk production. The yield of milk fat by the medicated cows was greater than by the controls in 26 of the 35 experiments in which that variable was studied (P < 0.01).     

Alterations in biochemical profiles and reproduction performance in postpartum dairy cows with metritis

The aim of the present study was to evaluate the biochemical profile and reproductive performance in postpartum dairy cows with clinical metritis. The research was carried out using performed on 213 pluriparous lactating cows from two dairy herds in southeast Brazil. Dairy cows were assigned into a group of healthy cows (n = 147) and cows with metritis (n = 66). Blood samples to assess serum albumin, cholesterol, fibrinogen, calcium, gamma-glutammyltransferase (GGT), aspartate aminotransferase (AST) and β-hydroxybutyrate (BHBA) concentrations were performed on day 8 ± 2 postpartum. Metritic cows had lower (p < .05) serum albumin, cholesterol and calcium concentration, and higher (p < .05) GGT and AST activity, and BHBA and fibrinogen concentrations when compared to the healthy group. The calving-to-conception interval and services per pregnancy were higher in metritic cows (p < .01) than in healthy cows and the conception rate at first service was lower in metric group (p < .01). Variable importance in projection plots demonstrated that cholesterol was the main discriminator between metritic and healthy cows. Our results indicated alterations in the biochemical profile and impaired reproduction performance in metritic cows.     

奶牛的热应激反应及其对策

所谓奶牛的热应激反应，指的就是奶牛由于受到炎热的刺激而引起的一系列的反应。这是因为奶牛体内热量的散发主要靠呼吸器官，而奶牛的呼吸特别恒热，易于发生热应激反应，大量的饲养实践证明，在每年的６～８月间，由于环境气温的升高，奶牛自身肌体的热调节机能开始失调，正常的功能及其激素平衡等遭到破坏，并易发生皮肤代谢受阻，体内物质代谢发生障碍、呼吸急促，心跳加快、体温上升、食欲下降、体重减轻、发情机制紊乱、配种受胎率下降、健康水平低下、抗病能力减弱、犊牛生长缓慢及死亡率增加等症状，而且大量的统计学数字还表明，在…     

高产奶牛热应激营养调控效果的研究

将24只荷斯坦奶牛按胎次、产奶量、泌乳天数、膘情、体型等因素相近原则分成3组,分别饲喂基础日粮(对照组)、基础日粮+抗热应激预混料(试验Ⅰ组)、基础日粮+过瘤胃脂肪+抗热应激预混料(试验Ⅱ组),且自由采食玉米秸秆,研究其对奶牛采食量、生产性能、呼吸率、肛温以及血液生化指标的影响。结果表明:试验Ⅱ组日粮组合可以降低热应激奶牛直肠温度,提高精料、干物质采食量,增进奶牛食欲,显著提高产奶量,改善乳汁品质。     

Relationship between oxidant stress and milk productivity in dairy cows

An imbalance between formation and detoxification of oxygen radicals leads to oxidant stress that may increase in more intense oxidative metabolism caused by a high intake of metabolizable energy to provide metabolic intermediates for the milk synthesis and secretion. This hypothesis was tested using dairy cows and the concentration of hydroperoxides in lipids (LHP) extracted from circulative lipoprotein particles of low and very low density (LDL and VLDL/chylomicrons) as oxidant stress indicator. The particles were prepared by ultracentrifugation of serum obtained by coccygeal bleeding (13 cows, 1. parity, n=8 and 2. parity, n=5, lactation stage, 53 +/- 1.4 days post partum) and purified by precipitation. Concentrations of LHP-LDL/mg Lipoprotein correlated significantly with daily milk yield (r = 0.73, P = 0.004) or daily milk energy output (r = 0.77, P = 0.003) in contrast to LHP of VLDL/chylomicron particles. Thus, some evidence was obtained for an almost linear, positive relationship between milk productivity and oxidant stress occurring in LDL.     

Effects of feeding rumen-protected choline on incidence of diseases and reproduction of dairy cows

The objectives of this study were to determine the effects of feeding rumen-protected choline (RPC) to dairy cows on the incidence of disease in early lactation, and on their fertility as measured by return to ovarian cyclicity and pregnancies per insemination (P/AI). In the first experiment, 369 cows were fed 15 g/day of RPC between 25 days pre-calving and 80 days post calving. In the second experiment, 578 primigravid cows were fed 15 g/day of RPC for the last 21 days of gestation only. In both experiments, P/AI were evaluated for the first and second inseminations, and health disorders were monitored daily until 90 days after calving. Return to cyclicity was evaluated in experiment 1 only, on days 51 and 61 post calving. In the first experiment, feeding RPC reduced the incidence of clinical ketosis, mastitis, and morbidity, and the number of cases of mastitis per cow. Feeding RPC did not influence cyclicity and P/AI. In the second experiment, cows fed RPC tended to have greater morbidity than controls because of an increased incidence of metritis and fever, although the incidence of retained fetal membranes was lower. The P/AI for first and second inseminations were similar between treatments. Supplementing the diets of dairy cattle with RPC from before until after calving improved the health of early lactation cows. However, in primigravid cows feeding RPC before calving only had mixed effects on health. Feeding RPC had no beneficial effects on reproduction in either experiment.     

The effect of mastitis on milk progesterone concentration in dairy cows

A within cow comparison was made between milk progesterone levels in healthy and mastitic quarters. Material was collected from cows with mastitis induced by bacterial inoculation, or by inoculation with bacterial endotoxin. Furthermore material from cows with spontaneous subclinical mastitis was used. Milk progesterone levels were lowered due to mastitis. However, the decrease was not large enough to cause misinterpretation of where in the oestrous cycle (luteal phase or non-luteal phase) the samples were taken.     

益生菌对奶牛亚急性瘤胃酸中毒的保护作用

亚急性瘤胃酸中毒对奶牛的生产性能有很大影响,使用益生菌来稳定奶牛分娩后-产奶过渡期瘤胃pH可以减轻这种代谢紊乱的症状。因此,本试验选择了体重(741±55)kg,产奶期在(212±19.5)d的奶牛4头,试验采用4×4拉丁方设计,共4种日粮:对照组饲喂基础日粮,处理1组为日粮添加0.5 g/d米曲菌,处理2组为日粮添加2.5 g/d米曲菌,处理3组为日粮添加2 g/d粪肠球菌和酿酒酵母菌的混合物。每组经过3 w的适应期、4 d酸中毒期以及3 d恢复期。结果显示:瘤胃最大pH范围在5.6～6.0,占比最高,与适应期和中毒期的结果相似。中毒期间,其他pH范围表现为显著差异(P <0.05)。中毒期,处理3组较对照组显著提高了瘤胃pH(P <0.05),同时处理3组瘤胃pH在5.6～6.0的占比也显著高于对照组(P <0.05)。日粮添加2.5 g/d米曲菌显著降低了产奶量(P <0.05)。奶牛遭受瘤胃酸中毒后,随着时间的推移,丙酸、丁酸和戊酸含量显著升高(P <0.05),而乙酸、异丁酸和异戊酸含量显著降低(P <0.05)。除了处理3组外,其他组瘤胃乳酸含量随时间的增加而升高(P <0.05)。饲喂12 d后,处理3组较对照组显著提高了乳酸含量(P <0.05)。瘤胃白球菌和大肠杆菌含量在饲喂2 h后显著升高(P <0.05),之后在6 h达到稳定。结论 :粪肠球菌和酿酒酵母菌复合物可以缓解奶牛亚急性瘤胃酸中毒的症状,米曲菌可以调控瘤胃pH,但高剂量米曲菌添加水平对瘤胃pH无显著影响。     

热应激对荷斯坦牛与荷-娟F1牛抗氧化能力的影响

临床健康的荷斯坦乳牛和荷斯坦牛-娟姗牛杂交F1牛(荷-娟F1)各20头,分为2组,分别于冬季12月份(牛舍日平均气温为10.35℃,THI为47.24)和次年夏季7月份(牛舍日平均气温为38.07℃,THI为82.61)条件下,研究热应激对血清中丙二醛(MDA)含量,总超氧化物歧化酶(T-SOD)活力,总抗氧化能力(T-AOC)的影响。结果表明:热应激期间荷-娟F1血清中MDA含量显著低于荷斯坦牛(P<0.05);T-SOD活力显著高于荷斯坦牛(P<0.05);T-AOC有升高的趋势但差异不显著(P>0.05)。在非热应激期间,荷-娟F1与荷斯坦牛相比血清中的MDA含量、T-SOD活力和T-AOC均无显著差异(P>0.05)。荷-娟F1和荷斯坦牛在热应激期间的血清中MDA含量、T-SOD活力与非热应激期差异均极显著(P<0.01),荷斯坦牛血清中T-AOC差异显著(P<0.05),而荷-娟F1血清中T-AOC差异不显著(P>0.05)。由此表明,热应激对乳牛的抗氧化能力产生显著的影响;热应激期间荷-娟F1的抗氧化能力高于荷斯坦牛,更能适应热带和亚热带地区高温高湿的气候条件。