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   157341篇
  免费   8606篇
  国内免费   373篇
林业   6476篇
农学   5220篇
基础科学   1080篇
  20472篇
综合类   25436篇
农作物   6433篇
水产渔业   7920篇
畜牧兽医   80395篇
园艺   2129篇
植物保护   10759篇
  2020年   1426篇
  2019年   1670篇
  2018年   2451篇
  2017年   2803篇
  2016年   2531篇
  2015年   2124篇
  2014年   2684篇
  2013年   6461篇
  2012年   4873篇
  2011年   5971篇
  2010年   3887篇
  2009年   4039篇
  2008年   5824篇
  2007年   5458篇
  2006年   5102篇
  2005年   4741篇
  2004年   4490篇
  2003年   4616篇
  2002年   4112篇
  2001年   4916篇
  2000年   4980篇
  1999年   3935篇
  1998年   1728篇
  1997年   1674篇
  1996年   1506篇
  1995年   1780篇
  1994年   1589篇
  1993年   1437篇
  1992年   3011篇
  1991年   3240篇
  1990年   3044篇
  1989年   3124篇
  1988年   2823篇
  1987年   2772篇
  1986年   2886篇
  1985年   2736篇
  1984年   2205篇
  1983年   1969篇
  1979年   2137篇
  1978年   1731篇
  1977年   1414篇
  1976年   1465篇
  1975年   1557篇
  1974年   1836篇
  1973年   1856篇
  1972年   1754篇
  1971年   1555篇
  1970年   1523篇
  1969年   1574篇
  1967年   1394篇
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
91.
Data from the official milk recording scheme of the Austrian Simmental population were analysed to estimate variance components due to genomic imprinting. Traits regarded were milk yield, fat and protein content, persistency, days open (first, second and third lactation) and herd life. All dairy traits were preadjusted for BLUP herd‐year effects. After applying some data restrictions, the number of records ranged from 3391 (persistency, third lactation) to 33 993 (days open, first lactation). Two different estimation approaches were used: (i) estimation of the maternal and paternal gametic component by a dam and sire model, respectively, and (ii) estimation of the animal and an additional gametic component (maternal or paternal) by treating gametes as homozygous diploid individuals. All models also accounted for year of first calving and the cytoplasmic effect of maternal lineages. In tendency, the results of both approaches were fairly well in line with each other. For the majority of the investigated traits, no substantial imprinting effects were detected. Significant evidence of genomic imprinting on a multiple type I error ≤ 0.10 was only found for protein content and days open. For protein content in the second and third lactation, the proportions of variance due to the paternal gametic effect exceeded those of the maternal by 0.096 and 0.152, respectively. For days open in the second lactation, however, the proportion of variance of the maternal gamete effect exceeded that of the paternal by 0.036. In tendency, indication of paternal imprinting was found for all fitness‐related traits. For true and functional herd life, significant differences (pairwise type I error ≤ 0.05) of 0.040 and 0.032 were found between the proportions of variance due to the maternal and paternal gamete effect. Significant variance components of cytoplasmic effects (multiple type I error ≤ 0.01) were found for first lactation of milk yield (0.020), for first, second and third lataction of persistency (0.026, 0.035 and 0.033) and of days open (0.029, 0.016 and 0.022) and for true and functional herd life (0.019 and 0.029).  相似文献   
92.
The cervical patency of six domestic female cats was monitored under sedation by infusion of contrast medium (Omnipaque) into the cranial vagina during early oestrus, mid‐oestrus, late oestrus and interoestrus or a radiopharmaceutical (99mTc‐HSA) during mid‐ and interoestrus in a non‐ovulatory oestrous cycle. The transport of the contrast medium or the radiopharmaceutical through the cervix and within the uterine horns was observed under fluoroscopy and with the aid of scintigraphy. In three of the queens, transcervical transport of contrast medium was demonstrated in all stages of oestrus, in one queen during mid‐oestrus, late oestrus and 1 day after oestrus, and in two queens only during late oestrus. The relations between the cervical patency to the contrast medium and the oestrous behaviour, cornification of the vaginal cells and the serum oestradiol‐17β concentration were evaluated, and a relationship was found between the cervical patency and the degree of vaginal cornification. Transcervical transport of the radiopharmaceutical was observed in three queens during mid‐oestrus. When the cervix was open, hysterography under a fluoroscope and hysteroscintigraphy were performed. The fluoroscopic and scintigraphic recordings revealed the patterns of the uterine contractions during oestrus in both ascending and descending directions, and the movement of the uterine contents back and forth between the uterine horns. The hysterograms were classified according to the shape of the uterine horns and the appearance of the endometrial lining. Spiral‐shaped uterine horns with a smooth inner contour were observed in two queens, and a corkscrew appearance with irregular filling defects in the uterine lumen was shown in two queens that had developed subclinical cystic endometrial hyperplasia. These findings demonstrated that fluids or particles deposited in the cranial vagina of the cat can be transported into the uterus during some stages of the oestrous cycle. The fluoroscopic and scintigraphic techniques developed in this study may be further modified to permit more detailed studies of uterine contractile patterns and sperm transport in the feline female reproductive tract. Hysterography proved useful to diagnose uterine disease. The information on cervical patency is of value also for the development of techniques for artificial insemination in this species, and should be studied also in the ovulatory cycle.  相似文献   
93.
An eight-year-old neutered female Persian cat with a four-year history of relapsing skin disease, characterised by ulcerated nodules with a yellow granular discharge, was examined. A diagnosis of granulomatous dermatitis and panniculitis (pseudomycetoma) caused by Microsporum canis was made on the basis of history, clinical signs, histopathological examination of skin biopsy specimens and culture of macerated tissue. Because previous therapy, comprising repeated surgical excision and the administration of griseofulvin and itraconazole, had not been successful, terbinafine was administered for an eight-month period. However, lesions progressed despite this treatment and the cat was euthanased.  相似文献   
94.
Abstract. Nitrous oxide (N2O) is involved in both ozone destruction and global warming. In agricultural soils it is produced by nitrification and denitrification mainly after fertilization. Nitrification inhibitors have been proposed as one of the management tools for the reduction of the potential hazards of fertilizer-derived N2O. Addition of nitrification inhibitors to fertilizers maintains soil N in ammonium form, thereby gaseous N losses by nitrification and denitrification are less likely to occur and there is increased N utilization by the sward. We present a study aimed to evaluate the effectiveness of the nitrification inhibitor dicyandiamide (DCD) and of the slurry additive Actilith F2 on N2O emissions following application of calcium ammonium nitrate or cattle slurry to a mixed clover/ryegrass sward in the Basque Country. The results indicate that large differences in N2O emission occur depending on fertilizer type and the presence or absence of a nitrification inhibitor. There is considerable scope for immediate reduction of emissions by applying DCD with calcium ammonium nitrate or cattle slurry. DCD, applied at 25 kg ha–1, reduced the amount of N lost as N2O by 60% and 42% when applied with cattle slurry and calcium ammonium nitrate, respectively. Actilith F2 did not reduce N2O emissions and it produced a long lasting mineralization of previously immobilized added N.  相似文献   
95.
96.
Radiopaque uroliths and nonradiopaque (water density) uroliths are filling defects encountered in the urinary tracts of dogs and cats. Other free luminal and attached soft tissue density filling defects encountered during uroradiographic special procedures include blood clots, air bubbles, hematomas, granulomas, abscesses, inflammatory and neoplastic polyps. Nonradiopaque uroliths cannot be identified on survey radiographs from other soft tissue dense structures. Gray scale ultrasonography can be used to differentiate nonradiopaque (water dense) uroliths from other soft tissue attached or free luminal filling defects of the excretory pathway. The differential radiographic features of filling defects encountered during cystography and urethrography are described and illustrated.  相似文献   
97.
Pregnant goats were inoculated intravenously or in uterine arteries with Brucella abortus, and tissues from the uterus and placenta were examined by electron microscopy. Identification of B. abortus in placentae was with antibody-coated colloidal gold. B. abortus was first seen in phagosomes of erythrophagocytic trophoblasts and in the rough endoplasmic reticulum of chorioallantoic trophoblasts. Subsequently, trophoblast necrosis and ulceration of chorioallantoic membranes were present. Coincidently, B. abortus was present in the lumen of placental capillaries. In late stages of infection, placental vasculitis was present, and placentomal trophoblasts were separated from maternal syncytial epithelium. In lesions with vasculitis, large numbers of B. abortus were in connective tissue of chorionic villi. Within the placentome, trophoblasts that lined chorionic villi contained no intracellular bacteria and were separated from B. abortus by intact basement membranes. These results suggest that bacteremic B. abortus is endocytosed by erythrophagocytic trophoblasts and that B. abortus replicates in the rough endoplasmic reticulum of chorioallantoic trophoblasts. Replication of brucellae in trophoblastic rough endoplasmic reticulum is unique; we believe that B. abortus may utilize endoplasmic reticulum for synthesis and glycosylation of bacterial membrane proteins or that B. abortus catabolizes trophoblast secretory proteins.  相似文献   
98.
Components of the energy and heat balances were examined in two maize hybrids grown at three different plant densities (40, 70 and 100 thousand plants per hectare). One of the hybrids was drought tolerant, while the other was bred for cultivation under irrigated conditions. An increase in plant density influenced not only the size of the leaf area, but also the distribution of the leaves at various plant heights. The extinction coefficient, which provides a quantification of radiation penetration, was higher in the irrigated treatments. By contrast to the other two treatments, the plant canopy in the thinly sown stands remained open throughout the vegetation period, and thus behaved quite differently to the closed stands, making it impossible to compare them. Smaller albedo values were recorded for the hybrid bred for irrigation and in thinly sown stands. The low plant density allowed more energy to reach the soil, from which it was reflected, making a considerable contribution to the final temperature in the stand. The latent heat, in keeping with the quantity of water transpired, was the greatest in the densely sown stands. There was little difference between the latent heat values of the normal and dense stands in either hybrid, indicating that they both had a similar sensitivity to increased stand density. If sufficient water is available it would appear that the stand density could be increased even for the drought-tolerant hybrid.  相似文献   
99.
Abstract. Soil samples from a 32-year grassland field experiment were taken from 0–5, 5–10, and 10–15 cm soil depths in February 2002. Plots received annual treatments of unamended control, mineral fertilizer, three rates of pig slurry and three rates of cow slurry, each with six replicates. Samples were analysed for cation exchange capacity (CEC), exchangeable cations (Na+, K+, Ca2+, Mg2+), pH and Olsen P. Exchangeable sodium percentage (ESP) was calculated as a sodicity indicator. Mean ESP was generally greater for slurry treatments than the control, with a trend of increasing ESP with application rate. This was particularly marked for cow slurry. At 0–5 cm depth ESP increased from 1.18 in the control to 1.75 at the highest rate of pig slurry and 5.60 at the highest rate of cow slurry. Similar trends were shown for CEC, exchangeable Na+, K+ and Mg2+, Ca2+ and Olsen P. The build-up of soil P due to slurry applications, together with this combination of physical and chemical factors, may increase the risk of P loss to surface waters, particularly from soils receiving high rates of cow slurry.  相似文献   
100.
This study tested different tags for each size of farm-reared gilthead sea bream, Sparus aurata L. juveniles, used in enhancement programmes. The following tags were tested: dye mark, fingerling tag, opercular tag, anchor tag A (filament diameter of 1.20 mm) and anchor tag B (0.85 mm). Tag/mark retention, wounds produced by the tag, ease of application, ease of detection and cost were examined. Epidermal dye marks had poor retention and detection rates. Of the tags tested, retention was greater for the anchor tag A (80–100%) after 7 months; anchor tag B showed a retention rate of only 20% after 3 months. The fingerling tag had a retention rate of 27.5% after 12 months. The opercular tag had a retention rate of 44% after 20 days but fractured the opercular bone in many cases. Tag incisions and wounds persisted in some fish for the duration of the experiment.  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