首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   284501篇
  免费   16338篇
  国内免费   1128篇
林业   24516篇
农学   14781篇
基础科学   3289篇
  46349篇
综合类   29080篇
农作物   17664篇
水产渔业   17894篇
畜牧兽医   116773篇
园艺   6578篇
植物保护   25043篇
  2021年   2738篇
  2020年   3135篇
  2019年   3907篇
  2018年   4470篇
  2017年   4927篇
  2016年   5157篇
  2015年   4655篇
  2014年   6037篇
  2013年   16688篇
  2012年   7836篇
  2011年   9968篇
  2010年   8168篇
  2009年   8606篇
  2008年   9201篇
  2007年   8394篇
  2006年   8716篇
  2005年   7907篇
  2004年   7649篇
  2003年   7560篇
  2002年   6792篇
  2001年   7671篇
  2000年   7368篇
  1999年   6538篇
  1998年   4314篇
  1997年   4355篇
  1996年   4091篇
  1995年   4648篇
  1994年   4038篇
  1993年   3728篇
  1992年   5025篇
  1991年   5246篇
  1990年   5054篇
  1989年   5057篇
  1988年   4462篇
  1987年   4501篇
  1986年   4352篇
  1985年   4536篇
  1984年   4118篇
  1983年   3826篇
  1982年   3064篇
  1981年   2919篇
  1980年   2864篇
  1979年   3494篇
  1978年   3048篇
  1977年   2815篇
  1976年   2631篇
  1975年   2503篇
  1974年   2678篇
  1973年   2756篇
  1972年   2449篇
排序方式: 共有10000条查询结果,搜索用时 288 毫秒
981.
In 72 (46%) of 155 gilts discarded for genetic reasons after performance testing and housed under fattening conditions no heat could have been detected during the first 30 days. The gilts were assigned alternatingly to a control group and four different treatments of delayed puberty. The induction of puberty was carried out by injections of 1000 iu PMSG, 400 iu HCG and 2 mg oestradiol benzoate, 400 iu PMSG and 200 iu HCG and 800 iu PMSG and 400 iu HCG. If there was no estrus gilts were slaughtered 12 days later for examination of the ovaries. Those coming into estrus were slaughtered 8 days after disappearance of estrus. Estrus could be induced in 69 to 94% of the gilts, whereas 40% of the untreated showed estrus signs. After treatment with PMSG and HCG in 40 and 87% of the gilts cysts were found whereas none of the untreated and 26 and 29% of those treated with PMSG und HCG + oestradiol benzoate revealed ovarian cysts. In addition, those gilts that had come into estrus during the first 30 days were given injections of either 1000 iu PMSG or 800 iu PMSG and 400 iu HCG. The injections were made either on the 5th, 10th or 15th day of cycle. In both latter groups significantly more gilts showed standing heat than after treatment at cycle day 5. The results of inspection of the ovaries at slaughter and steroid hormones could not be assigned to a defined stage of the physiological ovarian cycle.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
982.
Rosette inhibition tests for the detection of early pregnancy factor (EPF) were performed on naturally ovulated and superovulated mice from day 2 of pregnancy up to 4 days after parturition. In both groups of mice, the rosette inhibition titre (RIT) increased on day 2 of pregnancy, and persisted at high levels until day 15. Thereafter, the RITs of both groups of mice decreased to the non-pregnancy range. No significant differences of the mean RITs between these two groups were observed during the high RIT period. These results showed that the superovulatory treatment did not cause any changes or interference in the detection of EPF. In order to investigate the initial time of appearance of EPF in the maternal circulation in relation to the stage of fertilization, measurement of RIT and examination of the fertilization stage were carried out on superovulated mice 1 day after mating. The mean RIT of mice with pronucleus stage ova was significantly (p less than 0.01) higher than that of mice with sperm-penetrated ova. EPF was considered to appear in the maternal peripheral blood at the pronucleus stage.  相似文献   
983.
To determine whether prolactin has luteolytic properties during the first part of the luteal period, hysterectomy was performed in four dogs, in which prolactin had been chronically suppressed by bromocriptine administration. The concentration of progesterone in the peripheral blood decreased upon hysterectomy during the first part of the luteal phase and regained normal values after about seven days. The progesterone patterns during the perisurgical period in these dogs were similar to those patterns observed in dogs hysterectomised without bromocriptine treatment. It is concluded therefore that, in the dog, luteolytic properties can not be attributed to prolactin.  相似文献   
984.
Lymphocytes, monocytes, granulocytes, B-lymphocytes and CD8-positive T-lymphocytes of non-infected mink and mink infected with Aleutian disease virus (ADV) were measured by flow cytometry. The gammaglobulin levels of the sera were also measured. Besides development of hypergammaglobulinaemia in the infected mink, the most pronounced finding was that the number of CD8-positive lymphocytes doubled on average during development of Aleutian disease, while the number of B-lymphocytes did not change dramatically. The enhanced CD8 frequency was still apparent 6 months after initial ADV infection of the mink. The present experiments contribute to a better understanding of the immune deficiency stage seen in mink infected with ADV.  相似文献   
985.
Gastroendoscopy was performed on 111 horses (1 to 22 years old) that had signs of abdominal discomfort of variable duration and severity. At least 1 episode of colic had been observed within 48 hours of examination in 31 horses. Recurrent episodes of colic were observed in 28 horses within 2 to 10 days of examination, 31 horses within 11 to 30 days, 12 horses within 31 to 60 days, and in 9 horses at more than 60 days after the initial examination. Gastric ulceration was found in 91 of 111 horses examined. Other abnormalities involving the gastrointestinal tract or other abdominal viscera were not found on examination in 57 of 91 horses with gastric ulcers. The most frequent concurrent abnormalities found in the remaining 34 horses with gastric ulcers were impaction of the large colon (n = 6), colonic tympany (n = 6), peritonitis (n = 6), gastric impaction (n = 4), ileocecal intussusception (n = 3), small-colon impaction (n = 4), and proximal enteritis (n = 2). Thirteen horses with gastric ulceration underwent abdominal surgery, and in 5 horses, lesions were not found at surgery. Gastric ulceration was determined to be the primary cause of colic in 31 horses on the basis of the lack of other abnormalities, clinical response to treatment with histamine type-2 receptor (H2) antagonists, and confirmation of improvement or resolution of gastric ulceration via endoscopy. Gastric ulceration was the suspected cause of colic in 26 other horses on the basis of the lack of other abnormalities, severity of lesions, and clinical response to treatment with H2 antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
986.
A postparturient period is characterized by low basal secretion of adenohypophysis gonadotropins with the following appropriate changes in ovarian hormones and their response to the morphology of vaginal epithelium. In this study the dynamics of the cytological picture of vaginal swabs and ovarian hormones 17 beta-oestradiol (E2) and progesterone was investigated in the puerpery of ewes. The objective was to obtain and extend the knowledge of cytological changes in vaginal epithelium and levels of ovarian hormones of ewes after parturition and of their relationships from the first several days after lambing until the 51st day of the period of observation. Vaginal swabs for vaginal cytology were taken from nine ewes on days 1, 4, 7, 14, 17, 21, 25, 34, 42 and 51 after parturition. These swabs were fixed in ether-alcohol 1:1, stained according to the Faltínová-Zidovsky method, embedded in Canada balsam and evaluated by differentiation of cells according to Luksh (1953). Blood samples for E2 and P4 determinations were taken from the jugular vein in the same intervals as vaginal swabs. The serum was centrifuged and stored at -18 degrees C until use. E2 and P4 concentrations were determined radioimmunologically, using kits RIA-test ESTRA and RIA-test PROG from URVJT Kosice. A statistically significant decline (P < 0.05) of percentual representation of basal and parabasal cells (Fig. 1, Tab. I) on day 7 after lambing was replaced by their multiplication from day 14 reaching the values of 66.07 +/- 3.95 on day 42. A statistically significant decrease in intermediary flat cells (Fig. 2, Tab. II) was observed on days 14 (P < 0.001), 34 and 42 (P < 0.01; P < 0.001), in comparison with the first day after lambing. An evaluation of intermediary convoluted cells revealed their highest percentage on days 1 and 17 after parturition (34.65 +/- 4.77-20.62 +/- 12.57) and their decline to values in the range of 6.77 +/- 1.46-7.66 +/- 2.25 on the remaining days of the period of observation. Percent occurrence of superficial flat cells (Fig. 3, Tab. I) ranged from 3.9 +/- 1.10 to 10.63 +/- 7.23 from day 1 to day 51 after lambing. The lowest percentual representation (1.32 +/- 0.79-4.10 +/- 1.89) was recorded for superficial convoluted cells. Multiplication of the evaluated cells was observed, reaching the highest but insignificant representation (P > 0.05) on day 25 of postparturient investigation: 4.10 +/- 1.89 (Fig. 3, Tab. I). 17 beta-oestradiol (E2) concentrations were compared to the -1st day before parturition, when its values varied at the level of 2.45 +/- 0.64 nmol/l serum.(ABSTRACT TRUNCATED AT 400 WORDS)  相似文献   
987.
A haematological disorder in a dog characterised by a massive leucocytosis, mainly composed of eosinophils and their precursors is reported here. The normal composition of cells in the bone marrow was displaced in favour of the eosinophils and their precursors. No apparent cause for the pronounced eosinophilia could be determined by clinical, haematological, clinical-chemical, radiological or pathological examinations. A diagnosis of eosinophilic leukaemoid reaction was suggested as the criteria for the diagnosis of eosinophilic leukaemia in the dog but this was not firmly established.  相似文献   
988.
Mixed cropping of leguminous underseed species and cereal crops may increase productivity and serve some additional purposes. Two leguminous weed species which are widely distributed in annual crops of the Ethiopian highlands have been studied with varying coverage in greenhouse trials in Ethiopia with regard to their suitability as underseeds and with wheat ( Triticum turgidum [L.] Thell. var. durum [Desf.] MacKey) as a main crop. The competitive effects of Scorpiurus muricatus L. on the grain yield of wheat were smaller than those of Medicago polymorpha L. (-14.1 % and -23.6 % respectively, compared with the underseed free control) which is mainly ascribed to differences in their speed of development and shoot height. The residual effects of the underseed's root masses on the grain yield of the successive teff crop (Eragrostis tef [Zucc] Trotter) were significantly higher with Scorpiurus muricatus (+99.3 %) than with Medicago polymorpha (+63.6 %). The results are discussed with regard to the specific site conditions in the cereal-dominated cropping systems of the Ethiopian highlands.  相似文献   
989.
40 in-calf heifers were observed to examine the effect of timbers placing in passage area on their use of cubicle beds. Prior to the experiment preliminary observation was taken on these 40 heifers and they were divided into two even groups (cubicle user(or control) group versus cubicle refusal (or experimental) group by whether they properly used the cubicle beds or not. The timbers were placed in passage and feeding areas for the cubicle refusal group to force the animls to lie in the cubicle beds. The results of this experiment showed that feeding behaviour was unaffected by the presence of the timbers, but lying acitvity was about 8% higher for the experimental group, which led to a mean of almost 2 hours longer lying time over 24 h. After the timbers were removed, there was a significant decline in lying by the experimental group. The diurnal pattern of lying behaviour in cubicles indicated that the major effect on lying activity exerted by the presence of the timbers was seen during midnight to 10:00 a  相似文献   
990.
The discriminating ability of 15 parameters alone or in combinations, including results from analysis of plasma endotoxin, the Nycomed plasma D-Dimer test and phospholipase A2, were analyzed to predict morbidity and mortality in equine gastrointestinal colic. Endotoxaemia was a characteristic feature of the colic horses. The problem of adequately predicting non-survivors among colic horses required several parameters to be included in the logistic model: if the “classical parameters”, (heart rate, respiratory rate, PCV, anion gap) were included in the model, addition of plasma D-dimer, phospholipase A2, and Cl- significantly improved the predictive value of the logistic model. Increasing heart rate and D-dimer together with decreasing chloride was a risk factor for nonsurvival. The sensitivity of this three-parameter logistic model to predict nonsurvival was 78% and specificity 77%. The Nycomed D-Dimer test is recommended as a horse-site test to predict disseminated intravascular coagulation and nonsurvival in equine colic.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号