全文获取类型
收费全文 | 1022篇 |
免费 | 84篇 |
专业分类
林业 | 113篇 |
农学 | 33篇 |
基础科学 | 8篇 |
314篇 | |
综合类 | 38篇 |
农作物 | 58篇 |
水产渔业 | 109篇 |
畜牧兽医 | 331篇 |
园艺 | 23篇 |
植物保护 | 79篇 |
出版年
2023年 | 9篇 |
2022年 | 8篇 |
2021年 | 24篇 |
2020年 | 26篇 |
2019年 | 18篇 |
2018年 | 53篇 |
2017年 | 62篇 |
2016年 | 39篇 |
2015年 | 40篇 |
2014年 | 43篇 |
2013年 | 68篇 |
2012年 | 88篇 |
2011年 | 91篇 |
2010年 | 36篇 |
2009年 | 47篇 |
2008年 | 57篇 |
2007年 | 46篇 |
2006年 | 44篇 |
2005年 | 52篇 |
2004年 | 44篇 |
2003年 | 31篇 |
2002年 | 35篇 |
2001年 | 33篇 |
2000年 | 31篇 |
1999年 | 22篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 6篇 |
1995年 | 6篇 |
1994年 | 2篇 |
1993年 | 4篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1974年 | 2篇 |
1972年 | 1篇 |
1971年 | 2篇 |
1945年 | 1篇 |
排序方式: 共有1106条查询结果,搜索用时 62 毫秒
101.
102.
It is reasonable to assume that there is a relationship between the spatial distribution of forest fuels and fire hazards.
Therefore, if fire risk is to be included into numerical forest planning, the spatial distribution of risky and non-risky
forest stands should be taken into account. The present study combines a stand-level fire risk model and landscape level optimization
to solve forest planning problems in which the fire risk plays an important role. The key point of the method was to calculate
forest level fire resistance metrics from stand level indices and use these metrics as objective variables in numerical optimization.
This study shows that maximizing different landscape metrics produces very different landscape configurations with respect
to the spatial arrangement of resistant and risky stands. The landscapes obtained by maximizing different metrics were tested
with a fire spread simulator. These tests suggested that the mean fire resistance of the landscape, which is a non-spatial
metric, is the most important factor affecting the burned area. However, spatial landscape metrics that decrease the continuity
of fire resistance in the landscape can significantly improve the fire resistance of the landscape when used as additional
objective variables. 相似文献
103.
Perea-Ganchou F Soto Belloso E González Stagnaro C Soto Castillo G Hernández Fonseca H 《Tropical animal health and production》2005,37(7):559-572
To determine the relationship between first service conception rate (FSC) and the postpartum period (PP), 6160 calvings from
2250 crossbred dual-purpose cows were analysed. The effects of breed predominance (BT: Bos taurus and BI: Bos indicus), season of service (dry, intermediate and humid), parity (1 or 2 and more), agroecological region (Sub-humid and Dry Tropical
Forest) and milk production were studied. The postpartum period was grouped according to the calving–first service interval
as G1 ≤ 60 days (n = 1889); G2 > 60 ≤ 90 days (n = 1436); G3 > 90 ≤ 120 days (n = 1030); and G4 > 120 days (n = 1805). Data were analysed by chi-squared test. The overall mean for FSC was 58.2% and was lower in G1 (p < 0.01). All PP showed a breed effect on FSC, with mean values of 61.6% and 53.3% for BI and BT, respectively. Season of
service also affected FSC, which was significantly lower during the humid period: 51.2% versus 57.4% and 61.5% for the intermediate
and dry periods (p < 0.01), respectively. Parity and environmental effects were not observed on FSC. Levels of milk yield did influence the
FSC, which decreased from 66.7% and 61.0% for milk yields of < 1500 kg and between 1501 and 2000 kg to 46.0% in milk yields
> 2000 kg (p < 0.01). 相似文献
104.
Colunga GB Arriaga-Jordán CM Veláquez Beltran L González-Ronquillo M Smith DG Estrada-Flores J Rayas-Amor A Castelán-Ortega OA 《Tropical animal health and production》2005,37(Z1):143-157
The aim of this work was to describe the feeding strategies for donkeys used by peasant farmers in central Mexico. Feeding strategies for the dry and wet seasons are described, as well as the nutritional characteristics of the main forages and supplements used by the farmers. The extent to which the feeding practices used by farmers matched the digestible energy and crude protein requirements of donkeys for maintenance and work was determined. It was concluded that grazing on native grassland could provide enough energy and protein for donkeys at maintenance level and for those performing only a moderate amount of work. More research in required in order to have better estimates of intake and plant species preferred by donkeys under prevailing grazing conditions. 相似文献
105.
R N González D E Jasper T B Farver R B Bushnell C E Franti 《Journal of the American Veterinary Medical Association》1988,193(3):323-328
The California mastitis test (CMT) and bacteriologic culture were performed on samples of bulk-tank milk and cow-composite milk (n = 23,138 cows) from 50 California dairies, 19 of the 50 with known mastitis problems. Thirty-eight (76.0%) bulk-tank milk samples and 12,334 (53.3%) cows were positive by results of the CMT. Potential mastitis agents were isolated from 5,085 (22%) cows. Staphylococcus aureus was isolated from all 50 herds, Streptococcus agalactiae was isolated from 47 herds, and Mycoplasma sp was isolated from 24 herds. For cow-composite milk samples, the prevalences were 9.3% for Str agalactiae, 9.1% for S aureus, 0.9% for Mycoplasma sp, 1.2% for coliform bacteria, 0.9% for other streptococci, 0.8% for coagulase-negative staphylococci, and 1.3% for other organisms. The relative sensitivity and the relative specificity of the CMT performed on cow-composite milk samples were 83.4% and 55.2%, respectively, and the predictive value of positive CMT results was 34.2%. 相似文献
106.
González R Reguera E Mendoza L Figueroa JM Sánchez-Sinencio F 《Journal of agricultural and food chemistry》2004,52(12):3831-3837
The alkaline cooking of corn in a solution of Ca(OH)2 to produce corn-based foods is oriented to make corn proteins available, to incorporate Ca to the cooked grains, and also to remove the corn hull. This process (nixtamalization) is known in Mexico and Guatemala from prehispanic times; however, the effect of the alkaline cooking on the corn hull remains poorly documented. In this work, the physicochemical changes that take place in the corn hull during its cooking in a saturated solution of Ca(OH)2 were studied using infrared, X-ray diffraction, 13C cross-polarization/magic-angle spinning (CP/MAS) NMR, confocal imaging microscopy, differential scanning calorimetry, and thermogravimetry techniques. The main effect of this treatment on the hull is the removal of hemicelluloses and lignin, increasing the hull permeability and, as a consequence, facilitating the entry of the alkaline solution into the corn kernel. No significant changes were observed in the cellulose fiber network, which remains as native cellulose I, with a crystalline index, according to 13C CP/MAS NMR spectra, of 0.60. The alkaline treatment does not allow the cellulose fibers to swell and their regeneration in the form of cellulose(II). It seems any attempt to make use of the Ca binding capacity of the hull to increase the Ca availability in nixtamalized corn-based foods requires a separated treatment for the hull and kernel. On alkaline cooking, the hull hemicellulose fraction dissolves, losing its ability to bind Ca as a way to incorporate this element into foods elaborated from nixtamalized corn. 相似文献
107.
108.
Aparicio-Fernández X García-Gasca T Yousef GG Lila MA González de Mejia E Loarca-Pina G 《Journal of agricultural and food chemistry》2006,54(6):2116-2122
The antiproliferative effects of 100% methanol crude extract and of Toyopearl and silica gel fractions from the seed coats of black Jamapa beans (Phaseolus vulgaris L.) were evaluated using HeLa, human adenocarcinoma cells, and HaCaT, human premalignant keratinocytes. The 100% methanol crude extract [172.2 microM equiv of (+)-catechin] increased adhesion of HeLa cells; however, 3- and 5-fold higher concentrations decreased the number of cells attached as a function of the treatment time. The highest concentration tested diminished the cell adhesion until 40% (after 24 h) to almost 80% (after 72 h). The IC50 values showed that the 100% methanol crude extract was the most effective inhibitor of HeLa cell proliferation, even when it was dissolved in dimethylsulfoxide (DMSO) [34.5 microM equiv of (+)-catechin] or in medium [97.7 microM equiv of (+)-catechin]. The Toyopearl 5 (TP5) fraction and silica gel 2 (SG2) fraction inhibited 60% of the HeLa cell proliferation. The IC50 was 154 microM equiv of (+)-catechin of the 100% methanol crude extract on HaCaT cells. Toyopearl fractions TP4 and TP6 significantly inhibited HaCaT cell proliferation, but the silica gel fractions did not have a significant effect. The 100% methanol crude extract (35 microg of dry material/mL) decreased the number of HeLa cells in the G0/G1 phase from 68.9% (for control cells) to 51.4% (for treated cells) and increased apoptosis (2.9 and 21.2% for control and treated cells, respectively). The results indicated that black Jamapa beans could be a source of polyphenolic compounds, which have an inhibitory effect toward HeLa cancer cells but are less aggressive on HaCaT premalignant cells. 相似文献
109.
Perea F Soto E Hernández H González D Palomares R De Ondiz A González C 《Tropical animal health and production》2006,38(4):353-363
The aim of this study was to examine the monthly variation of the first service conception (FSC) and oestrus frequency (OF)
from 8308 artificial inseminations (AI) of 2960 crossbred dual-purpose cows on three commercial farms with improved management
located in the Maracaibo Lake Basin, Venezuela. The effects of month of the year, agroecological area (from dry tropical forest
to sub-humid tropical forest on the three farms), predominant breed (BT, Bos taurus; BI, Bos indicus), and season (December–April}; May–August; and September–November) were considered. Data were analysed using logistic regression
(FSC) and GLM (OF) from SAS. The mean FSC was 55.4% overall and 48.7%, 57.0% and 58.3% for farms A, B and C, respectively
(p < 0.01). Within the three agroecological areas (farms A, B and C), the FSC was highest during the cooler and drier months
of the year (season 1), while the lowest FSC was obtained during the months of highest rainfall and humidity (season 3) (53.3%
vs 37.4%; 58.5% vs 49.0% and 63.0% vs 52.3% on farms A, B, C, respectively; p < 0.01). In the three studied farms, OF was
higher during the first 4 months of the year; with a diminishing trend towards the end of the year (p % 0.05). In general,
BI cows had a higher FSC than BT cows, especially during the second half of the year, when environmental conditions were characterized
by higher rainfall and humidity with lower wind velocity (58.8%a 55.5%b and 46.4%c in season 1, 2 and 3 respectively; a, b, c p < 0.01). 相似文献
110.
Méndez D Giménez F Escalona A Da Mata O González A Takiff H de Waard JH 《Veterinary microbiology》2006,116(4):325-328
The ability of Mycobacterium paratuberculosis to survive the commercial pasteurization process of raw milk remains controversial. In a study undertaken in Venezuela to culture M. paratuberculosis from commercially pasteurized cows' milk, 83-200 ml containers of milk were processed and cultured on Herrold's egg yolk slants. No M. paratuberculosis was cultured but a total of six colonies of Mycobacterium bovis were isolated from one container each from two different milk providers. Because laboratory cross-contamination was suspected, the laboratory records were reviewed and spoligotyping was carried out on the isolated individual colonies. On the day before these milk specimens were processed, the biological safety cabinet had been used for the isolation of M. bovis from lymph nodes from infected cattle. Spoligotyping showed that that the colonies isolated from the milk all had the same pattern as the strains isolated from the lymph nodes that were processed the previous day. As far as we know, this is the first report of cross-contamination in a veterinary mycobacterial laboratory. False-positive cultures in the mycobacterial laboratory are not rare. In this setting M. bovis was isolated because it is the most common manipulated organism in this laboratory. We believe that reports on the isolation of M. paratuberculosis from commercially pasteurized milk should exclude cross-contamination before reporting, especially when this organism is routinely isolated from animal material in the same lab. 相似文献