收费全文 | 1700篇 |
免费 | 117篇 |
林业 | 49篇 |
农学 | 21篇 |
基础科学 | 5篇 |
151篇 | |
综合类 | 520篇 |
农作物 | 54篇 |
水产渔业 | 139篇 |
畜牧兽医 | 792篇 |
园艺 | 32篇 |
植物保护 | 54篇 |
2023年 | 19篇 |
2021年 | 35篇 |
2020年 | 33篇 |
2019年 | 43篇 |
2018年 | 26篇 |
2017年 | 33篇 |
2016年 | 37篇 |
2015年 | 34篇 |
2014年 | 30篇 |
2013年 | 63篇 |
2012年 | 43篇 |
2011年 | 58篇 |
2010年 | 38篇 |
2009年 | 47篇 |
2008年 | 57篇 |
2007年 | 57篇 |
2006年 | 60篇 |
2005年 | 54篇 |
2004年 | 34篇 |
2003年 | 34篇 |
2002年 | 44篇 |
2001年 | 28篇 |
2000年 | 38篇 |
1999年 | 27篇 |
1994年 | 11篇 |
1992年 | 25篇 |
1991年 | 24篇 |
1990年 | 20篇 |
1989年 | 18篇 |
1988年 | 13篇 |
1987年 | 40篇 |
1986年 | 57篇 |
1985年 | 51篇 |
1984年 | 38篇 |
1983年 | 41篇 |
1982年 | 35篇 |
1981年 | 33篇 |
1980年 | 35篇 |
1979年 | 35篇 |
1978年 | 33篇 |
1977年 | 25篇 |
1976年 | 24篇 |
1975年 | 26篇 |
1974年 | 31篇 |
1973年 | 17篇 |
1972年 | 16篇 |
1971年 | 16篇 |
1970年 | 15篇 |
1969年 | 16篇 |
1966年 | 13篇 |
Using a cross-sectional survey, we determined the prevalence and risk factors associated with bovine brucellosis in herds under extensive production system in southwestern Nigeria. Antibodies to Brucella species in serum samples were tested using the Rose Bengal test (RBT) and competitive enzyme-linked immunosorbent assay (cELISA); for milk, the milk ring test (MRT) and indirect-ELISA (i-ELISA) were used. Questionnaire was administered to cattle herdsmen to determine factors predisposing the animals to bovine brucellosis. Data were analyzed using STATA 12. From 513 serum and 635 milk samples tested among 120 herds, overall animal-level prevalence of 10.1% (95% CI 7.5–12.7%) and 20.2% (95% CI 17.1–23.3%) were recorded by RBT and MRT, respectively; while 9.4% (95% CI 6.9–11.9%) and 17.8% (95% CI 14.8–20.8%) were obtained using cELISA and i-ELISA, respectively. In all, from the 120 herds tested, 29.2% and 43.3% were positive by RBT and MRT, respectively. Multivariable logistic regression revealed that herd location (OR?=?8.12, 95% CI 1.68–38.90) and improper disposal of placenta/fetus (OR?=?17.33, 95% CI 4.81–62.33) were predictors for a seropositive herd using RBT; while herd location (OR?=?5.13, 95% CI 1.27–20.28), large herd size (OR?=?2.62, 95% CI 1.15–5.85), and occurrence of abortion for a year or more (OR?=?4.62, 95% CI 1.53–13.71) were predictors of seropositivity to antibodies to Brucella spp. using MRT. We found high prevalence of brucellosis in cattle herds under extensive management system in southwestern Nigeria. Urgent and coordinated control strategies are required to mitigate this problem.
相似文献The dry-matter digestibility of the diets was slightly higher when the grass was supplemented with barley than when grass alone was given. Although barley depressed the digestibility of protein, the absolute and the proportionate retention of nitrogen was increased. This was particularly marked on Treatment 3, presumably because of the higher net energy value of the diet containing the greater amount of barley. The N-retention data corresponded to the rates of liveweight gain obtained. 相似文献
The study documented gross anatomical and histological differences in the reproductive organs of 28 breeding and non-breeding female guinea fowls. Peripheral progesterone and 17β-oestradiol concentrations were also compared in breeding and non-breeding hens.
In non-breeding females, all ovarian and oviducal gross anatomical features had significantly regressed. Histologically, some of the changes in a regressing oviduct include systematic changes in height and size of all epithelial cells in all regions of the duct, absence/sparse ciliation of portions of surface epithelium in the magnum, isthmian and uterine regions, general loss of cytoplasmic mass, reduction in size and degeneration of tubular glands. Mucosal folds in all regions of the oviduct except the infundibular lip were higher in breeding females.
No difference was found between the two groups in plasma progesterone concentrations. Breeding females, however, had higher peripheral oestradiol concentrations than non-breeding females. About 2 h prior to oviposition, plasma oestradiol concentrations peaked at 2.4-fold (230 pg/ml) compared with baseline concentration and plasma progesterone concentrations by nearly 9-fold (5.29 ng/ml) of baseline.
Significant regression and changes in the histological structure of the ovary and oviduct had occurred in non-breeding females, and lower peripheral oestrogen concentrations may be responsible for this phenomenon.