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   105472篇
  免费   5522篇
  国内免费   52篇
林业   4309篇
农学   3038篇
基础科学   729篇
  11694篇
综合类   20721篇
农作物   4294篇
水产渔业   4620篇
畜牧兽医   54328篇
园艺   1195篇
植物保护   6118篇
  2018年   1204篇
  2017年   1333篇
  2016年   1285篇
  2015年   1091篇
  2014年   1260篇
  2013年   3931篇
  2012年   2564篇
  2011年   3185篇
  2010年   2078篇
  2009年   2054篇
  2008年   3205篇
  2007年   2938篇
  2006年   2802篇
  2005年   2638篇
  2004年   2744篇
  2003年   2678篇
  2002年   2623篇
  2001年   3272篇
  2000年   3288篇
  1999年   2440篇
  1998年   1070篇
  1997年   1081篇
  1995年   1251篇
  1994年   1164篇
  1993年   1121篇
  1992年   2362篇
  1991年   2530篇
  1990年   2438篇
  1989年   2493篇
  1988年   2272篇
  1987年   2349篇
  1986年   2484篇
  1985年   2394篇
  1984年   1919篇
  1983年   1773篇
  1982年   1165篇
  1981年   1123篇
  1980年   1025篇
  1979年   1762篇
  1978年   1424篇
  1977年   1146篇
  1976年   1130篇
  1975年   1165篇
  1974年   1524篇
  1973年   1610篇
  1972年   1584篇
  1971年   1515篇
  1970年   1408篇
  1969年   1262篇
  1967年   1094篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
Abstract Extract A 15-year-old pony mare developed firm irregular bony swellings on all four legs. These were most severe around the carpal and fetlock joints of the forelegs, restricting both flexion and extension. Over about 6 months the horse had periodic bouts of coughing. She showed moderate weight loss and a depressed demeanour.  相似文献   
892.
The relationship between fine-scale spatial patterns of forage abundance and the feeding patterns of large ungulates is not well known. We compared these patterns for areas grazed in winter by elk and bison in a sagebrush-grassland landscape in northern Yellowstone National Park. At a fine scale, the spatial distribution of mapped feeding stations in 30 m × 30 m sites was found to be random where there were no large patches devoid of vegetation. In areas similar to the mapped sites, the underlying spatial distribution pattern of biomass was also determined to be random. At a broad scale, forage biomass differed among communities across the northern range but forage quality did not. These results suggest that ungulates are feeding randomly within forage patches (fine scale) but may select feeding sites based upon forage abundance at broader, landscape scales. Contrary to what has been suggested in other systems, ungulates were not overmatching at finer scales.  相似文献   
893.
Lamb copper status is commonly assessed by measuring copper concentrations in four liver samples collected from lines of lambs sent to meat slaughtering premises. High liver copper concentrations were found in lambs examined in April and May. Five farms with two or more lamb liver copper concentrations greater than 3000 micromol/kg on a wet matter basis and two farms with adequate copper concentrations but less than 3000 micromol/kg in the autumn of 1992 were selected for a more detailed investigation into the factors affecting their lamb copper status in 1993. High liver copper status was associated with low pasture molybdenum, grazing paddocks recently topdressed with copper sulphate, supplementing with mineralised drenches and copperised salt licks, and the high copper content in chicory. Lamb liver copper concentrations were significantly (p<0.05) higher in the autumn than in the summer on two of three farms.  相似文献   
894.
Suspected photosensitisation occurred in three groups of lambs grazing birdsfoot trefoil (Lotus corniculatus c.v. Grasslands Goldie). In one group, sucking lambs aged about 10 weeks, grazing birdsfoot trefoil, developed skin lesions while lambs of a similar age and from the same flock grazing lucerne (Medicago sativa) or a mixed sward of both species showed no signs of photosensitisation. Affected lambs had lesions on their backs and ears. In a few animals the tips of the ears were shortened by 2-3 cm. In the affected lambs, serum liver enzymes (gamma-glutamyltransferase, glutamate dehydrogenase), bilirubin and serum Vitamin B12 levels were within the normal range. At necropsy, no significant pathological changes were detected in the liver and histological changes in the skin were consistent with primary photosensitisation. In the second group, three of 80 weaned lambs grazing the same birdsfoot trefoil at a restricted intake were affected in the same manner as the first group. In the third group, 15 animals from 28 sets of sucking twin lambs were also affected. In only two sets of twins were both lambs affected. None of the ewes grazing with the lambs in the first or third groups showed any clinical signs of photosensitisation.  相似文献   
895.
1. Pullets of 2 high‐producing commercial stocks (both brown‐egg layers) were exposed to 5 different lighting patterns between 18 and 72 weeks to test the hypothesis that photoperiods used in commercial lighting programmes early in the laying year may be unnecessarily long and, by accelerating the development of photorefractoriness, may contribute to the decline in egg production observed after the initial peak. Two rooms of 288 pullets were allocated to each treatment.

2. The rate of lay observed with a Step‐Up treatment which gave increases in photoperiod from 8L:16D at 18 weeks to 15L:9D at 27 weeks of age was not significantly different from that of treatments which held the birds on 11L:13D during peak egg production but gave increments up to 15L:9D later in the laying year.

3. A control group maintained on 11L:13D from 20 to 72 weeks laid 295 eggs per bird housed and a further group held on 8L:16D from 0 to 72 weeks laid 284 eggs per bird. These yields were lower than the Step‐Up treatment (299 eggs) but show the potential of modern hybrid stocks to lay prolifically even without light stimulation.

4. It is concluded that the stocks tested in this experiment showed no advantage when given lighting programmes in the first laying year which were designed to minimise the adverse effects of photorefractoriness.  相似文献   

896.
1. The effect of probiotic supplementation on egg production, on serum and yolk cholesterol and on egg shell thickness in 24 White Leghorn layers was studied from 28–38 weeks of age.

2. In 3 treatments the diet was supplemented with 0, 100 and 150 mg probiotic/kg food.

3. In the 100 mg probiotic group, egg production improved by 5%, and shell thickness improved slightly, with fewer thin‐shelled eggs than in the control (8.6% compared to 18.6%).

4. The initial serum cholesterol concentration of 170.2 mg/dl in control birds remained similar throughout the 10‐week experimental period, whereas in the 150 mg group the initial value of 176.5 mg/dl decreased to 114.3 mg by week 10.

5. Yolk cholesterol concentration was 14.69 mg in the control group and 11.28 and 11.37 mg/g in the 100 and 150 mg probiotic groups respectively. Overall mean total egg cholesterol was thus reduced by probiotic supplementation.  相似文献   

897.
Twenty-four Thoroughbred and twelve Standardbred racehorses aged between 2 and 6 years, presented for reported poor racing performance, underwent clinical exercise testing. During the last 10 s of exercise at each speed throughout an incremental speed exercise test on a treadmill inclined at a 10% slope, samples of arterial blood and expired gases were collected. Maximum oxygen uptake and the partial pressures of oxygen and carbon dioxide in arterial blood were determined. These values were compared between the two breeds of horses and also with reference to cytological findings of bronchoalveolar lavage samples, including neutrophil, erythrocyte and haemosiderophage percentage and the total nucleated cell concentration. The results revealed an inverse relationship (Spearman R = -0.45, p < 0.05) between the total nucleated cell count in bronchoalveolar lavage samples and arterial oxygen partial pressure during exercise at 11 m.s(-1). This result suggests that subclinical pulmonary disease may be a more important cause of poor racing performance than previously thought. Also of note was a positive correlation (Spearman R = 0.50, p < 0.05) between maximum oxygen uptake and the percentage of erythrocytes.  相似文献   
898.
Subsequent to a possum (Trichosurus vulpecula) poisoning operation purportedly using 1% phosphorus baits, six dogs with access to poisoned possums died of phosphorus poisoning. Two dogs survived, following treatment with oral copper sulphate and parenteral vitamin K. Clinical signs included depression, jaundice, vomiting and bloody diarrhoea. Post-mortem lesions included large areas of subcutaneous, interstitial and intermuscular haemorrhage, subserosal haemorrhage and liver degeneration. Free phosphorus was detected in the ingesta of three of the dogs which died up to 7 days after the last of the poison was laid. Analysis of two batches of baits used showed P levels of 1.17% and 1.24%.  相似文献   
899.
Two groups of six mature brushtail possums (Trichosurus vulpeculu) were housed in two respiration chambers, and their heat production, whole body conductance and lower critical temperatures were measured under a variety of simulated weather patterns. The possums were subjected to ambient temperatures of 30, 20 and 3 degrees C. At 20 and 3 degrees C, the animals were exposed to near still air and light winds (wind speed 0.8 and 6.7 km/h), both with, and without, simulated rain every 8 hours. The lower critical temperature in near still air lies between 7 and 10 degrees C. This temperature increases by about 2, 6 and 8 degrees C respectively for a wind velocity of 6.7 km/h, simulated rain and a combination of the two factors. Weather in New Zealand, especially in the cooler part of the year, will often produce conditions below the lower critical temperature of the thermoneutral zone of possums. This will necessitate significant increases in metabolic rate and hence food consumption or mobilisation of body fat reserves, which if not sustainable will result in the death of possums. Field studies have shown that this is often the case in the wild. It is proposed that this stress may be sufficient to decrease the resistance (especially cell-mediated immunity) of some possums and allow acceleration of the disease process in those infected with Mycobacterium bovis.  相似文献   
900.
The efficacy of an ivermectin jetting fluid against the sheep body louse, Bovicola (Damalinia) ovis, in long-woolled sheep was investigated in three field trials. In all trials, louse populations in hand-jetted sheep were reduced to nondetectable numbers within 2 weeks of treatment. Bv comparison, shorn and unshorn control sheep maintained detectable infestations for the duration of the trials.  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