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   215218篇
  免费   11811篇
  国内免费   2728篇
林业   10568篇
农学   9365篇
基础科学   3208篇
  26759篇
综合类   40245篇
农作物   8496篇
水产渔业   10305篇
畜牧兽医   102564篇
园艺   3072篇
植物保护   15175篇
  2019年   2197篇
  2018年   2874篇
  2017年   3382篇
  2016年   3205篇
  2015年   3146篇
  2014年   3551篇
  2013年   8352篇
  2012年   6348篇
  2011年   7562篇
  2010年   5359篇
  2009年   5315篇
  2008年   7351篇
  2007年   7165篇
  2006年   6856篇
  2005年   6348篇
  2004年   5598篇
  2003年   5640篇
  2002年   5298篇
  2001年   6338篇
  2000年   6290篇
  1999年   5343篇
  1998年   2533篇
  1997年   2493篇
  1996年   2329篇
  1995年   2788篇
  1994年   2490篇
  1993年   2359篇
  1992年   4366篇
  1991年   4510篇
  1990年   4548篇
  1989年   4446篇
  1988年   4109篇
  1987年   4123篇
  1986年   4166篇
  1985年   4000篇
  1984年   3433篇
  1983年   3037篇
  1982年   2095篇
  1979年   3080篇
  1978年   2498篇
  1977年   2219篇
  1976年   2017篇
  1975年   2147篇
  1974年   2795篇
  1973年   2840篇
  1972年   2726篇
  1971年   2485篇
  1970年   2346篇
  1969年   2218篇
  1967年   1981篇
排序方式: 共有10000条查询结果,搜索用时 687 毫秒
231.
Above ground production of a typical Cortaderia pilosa dominant sward in the Falkland Islands was estimated over one growing season by summing significant positive increments in DM production of various sward components and by sequential cutting to ground level. The standing crop was characterized by a high proportion of standing dead matter at all times of the year. Net herbage production was estimated as 2200 kg ha-1 and 1440 kg ha-1 by the 'difference' and "sequential culling" techniques, respectively. The pattern of leaf growth was determined on another site by regular linear measurements of a population of physiologically similar C. pilosa leaves. There was very little early season growth and most of the DM was produced in a short period in the middle part of the growing season. The poor early season growth was attributed to adverse climatic conditions. A significant increase in green matter digestibility in October and November was masked by the low digestibility and high contribution to the overall DM by the standing dead component of the herbage.
The implications of these findings are briefly discussed in relation to the increased utilization of C. pilosa by improved pasture management, including burning and possibly reseeding areas with improved species. Tissue turnover studies combined with more knowledge of sheep nutrition cycles would enable grazing systems to be developed on a sounder basis than previously.  相似文献   
232.
In a large pig breeding herd with high prevalence of post-parturient diseases of the sows, weaned sows of different parity groups with (n = 663) or without (n = 1125) recidiving mastitis metritis agalactia (MMA) in their previous history were subjected to ultrasonography. A total of 114 of 663 sows with recidiving MMA in their previous history and with ultrasonographic visible mammary gland changes, and 157 of sows without recidiving MMA in their previous history were culled and subjected to gross pathological and bacteriological examination of their mammary glands. The sows having suffered MMA had more (p < 0.001) hyperechogenic images in their mammary glands compared with the sow having suffered no recidiving MMA. Abdominal glands were more (p < 0.01) prone to pathological changes compared with the pectoral ones. Sows of high parity had more hyperechogenic images and gross pathological changes in their mammary glands compared with the sows of low parity.  相似文献   
233.
Canine distal renal tubular acidosis and urolithiasis   总被引:1,自引:0,他引:1  
Distal RTA is characterized by decreased distal renal tubular hydrogen ion secretion, decreased ability to acidify urine, hypercalciuria, hyperphosphaturia, hypocitraturia, and metabolic acidosis. Because of the resulting alterations in urine composition and pH, patients with distal RTA are predisposed to urolithiasis and renal calcification. Diagnosis of distal RTA is important because it is a potentially reversible disorder that, left untreated, may cause nephrocalcinosis, recurrent urolith formation, moderate to severe metabolic acidosis, and renal failure.  相似文献   
234.
Forty-one dairy cows were fed a low (LCa-13 g/d) and a high (HCa-83.5 g/d) calcium ration in the 8 weeks prior to parturition and the effect on the Ca mobilization rate around parturition was studied. Plasma Ca values were stable in the LCa group around parturition. In the older cows of the HCa group a very slight decrease in the mean plasma Ca was observed: 2.58 mmol/l at 12-36 h ante partum decreased to 2.38 mmol/l at parturition. Hypocalcaemia, which commonly occurs around parturition, did not occur in 40 of the cows. A subclinical hypocalcaemia (1.8 mmol/l) occurred in one cow (parity 10) from the HCa group. To assess the efficiency of Ca mobilization, a severe hypocalcaemia (1.0 mmol/l) with clinical signs was induced by means of Na2EDTA infusion (0.90 mmol/min), starting at 10 h post-partum. The older cows in the LCa group required more Na2EDTA than those in the HCa group. Higher urinary hydroxyproline excretion in the week before parturition in the LCa than in the HCa group suggested a higher bone turnover. Plasma PTH levels around parturition were not significantly different between the groups. The amount of colostrum milked out in the first 10 h post-partum did not influence Ca homeostasis around parturition. The results contradict those of many other experiments in which hypocalcaemia was observed in cows ingesting high levels of Ca. It is concluded that the restricted feed intake prepartum possibly had a favourable effect on Ca homeostasis.  相似文献   
235.
Success in veterinary dentistry lies in good home care and regular, thorough dental prophylaxis. The removal of supragingival and subgingival calculus must be accomplished before periodontal disease can be controlled. When gingival sulcus depths exceed 4 or 5 mm, the gingivae must be excised or reflected to allow proper treatment of the pocket area. Antibiotic therapy should be instituted in the extensively involved veterinary dental patient.  相似文献   
236.
237.
238.
239.
240.
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