Accounting for residual effects of previously applied nitrogen fertilizer on intensively managed grasslands

Only 0·20–0·70 of the fertilizer-nitrogen (N) applied to grassland is taken up in herbage in the harvest directly following application. Residual effects at subsequent harvests can be large but are poorly quantified, and rarely taken into account in current management practices. An increased understanding of N-use efficiency per harvest can improve operational management. This study systematically assessed the residual effects of previously applied N fertilizer on N uptake, dry matter (DM) yield and soil mineral-N (SMN) during the whole of the growing season. It is based on field experiments conducted on peat and mineral soils in 1991–1994. Statistical models were derived for SMN, N uptake and DM yield as a function of previously and freshly applied N fertilizer. There were clear residual effects of previously applied N in later cuts. They were relatively greater at higher levels of N fertilizer. On peat soils, 0·15–0·25 of the N applied was recovered as SMN. On mineral soils the proportion was maximally 0·08. There was a clear relationship between SMN and N uptake in the subsequent cut on mineral soils but not on peat soils. The value of SMN as a tool to adjust fertilizer-N application rates was hence found to be limited. There were clear relationships between the amount of previously applied N and the N uptake in subsequent cuts, on both soil types and over the whole of the growing season. It was concluded that the total amount of previously applied N is a useful indicator for adjusting N-fertilizer application rates.     

Nitrogen Assimilation and Growth of Cotton Seedlings under NaCl Salinity and in Response to Urea Application with NBPT and DCD

Salinity stress and inefficient nitrogen fertilization adversely affect cotton growth and yield. The effect of salinity on the growth and stress response of cotton seedlings and the effect on N‐use efficiency from the use of the inhibitors of urease (NBPT) and nitrification (DCD) under salinity stress were studied in growth chambers. The study consisted of three levels of salinity – low (0.45 dS m^?1), moderate (8 dS m^?1) and high (16 dS m^?1) – and five N treatments – unfertilized control, 100 % N rate with urea, 80 % N rate with urea, 80 % N rate with urea +NBPT and 80 % N rate with urea +NBPT + DCD. The results indicated that salinity stress reduced plant growth (low leaf area and plant dry matter), decreased N assimilation (low NR, GS and protein), increased plant stress response (high GR and SOD), and decreased leaf chlorophyll, stomatal conductance and quantum yield. Addition of NBPT to urea improved N uptake by 22 % under low salinity; however, this effect was not observed with increasing salinity. No benefit of addition of DCD was observed in any of the parameters collected. In conclusion, salinity stress hindered the performance of the additive NBPT and negatively affected the growth and physiology of cotton.     

Development of a nitrocellulose-enzyme immunoassay for the detection of yellow-head virus from penaeid shrimp

A nitrocellulose-enzyme immunoassay (NC-EIA) employing horseradish peroxidase (HRP) was developed for the detection of yellow-head virus in the tissues of infected penaeid shrimp. Two substrates, DAB (3,3¢-diaminobenzidine retrahydrochloride) and TMB (3,3¢5,5¢-tetramethylbenzidine) for HRP were evaluated. The TMB was found to produce a more distinct and easily recognizable colour product and provided enhanced sensitivity (0.4 ng for TMB versus 0.8 ng viral protein for DAB). Under the present format, the NC-EIA was capable of detecting as little as 100 TCID₅₀ of virus in infected tissues. Uninfected tissue homogenates yielded no significant background colouration.     

Characterization of molecular variants of GnRH, induction of spermiation and sex reversal using salmon GnRH-A and domperidone in the protogynous diandric fish, Synbranchus marmoratus Bloch, (Teleostei, Synbranchidae)

This paper studies the molecular variants of gonadotropin-releasing hormone (GnRH) present in the brain of the protogynous swamp eel, Synbranchus marmoratus, and the effects of the administration of salmon GnRH analogue (sGnRH-A) and the dopamine receptor antagonist, domperidone (DOM) on final maturation and gamete release in this species. Evidence for the presence of two GnRH variants, sGnRH and cIIGnRH were obtained by reverse phase high-pressure liquid chromatography (RP-HPLC) and radioimmunoassay with different antisera. The effects of treatment with sGnRH-A+DOM were checked by three ways: oocyte and milt release by stripping, histological analysis of the gonadal tissue, and androgen serum levels at different times throughout the experiment. In males, spermiation was induced after three weeks of treatment. In the female group, sGnRH-A+DOM did not induce ovulation at the end of the experiment. Histological analysis of the gonads from the female group showed evidence of sex reversal. All the treated fish had elevated androgen serum levels from the third week, with respect to control fish. In all cases, serum estradiol levels were undetectable. These results suggest that treatment with sGnRH analog and DOM induce sex reversal in female and spermiation in males. This revised version was published online in July 2006 with corrections to the Cover Date.     

Differences in larval growth among families of Penaeus stylirostris Stimpson and P. vannamei Boone

Abstract. Full-sib families of Penaeus stylirostris Stimpson and Penaeus vannamei Boone were grown under controlled conditions in five experiments. Total length of shrimp larvae was measured at the following substages: protozoea one, mysis one and postlarva one. Analysis of variance showed that families differed significantly in size in all experiments at all substages tested. The average size possessed by a group of larvae early in development was significantly correlated with the size obtained in the later stages in P. siylirosiris , but not in P. vannamei . The component of variance in size due to family differences was determined and used to estimate heritabilities of size at the three larval substages in both species. Estimated heritabilities were higher for P. stylirostris than P. vannamei . In P. vannamei . the hcritability of size is greater for the protozoea and postlarval stages than the mysis stage.     

The role of the environment in partial migration: food availability affects the adoption of a migratory tactic in brown trout Salmo trutta

Abstract –  Alternative reproductive tactics are commonly reported for salmonids (Pisces) and typically involve large migratory and small resident individuals. Variation in migratory tendency should reflect the different benefits and costs that the two different phenotypes face with regard to fitness. Therefore, the effect of food availability on the adoption of a migratory tactic in brown trout was investigated. Fifty trout were placed in each of 12 tanks and fed at three different levels. Growth-related variables were measured regularly, and at the end of the experiment, the proportion of migrants and residents was recorded. Low food availability led to increased numbers of migratory fish. The expected sex-bias was also present, with a lower percentage of resident females than resident males. As all fish originated from the same gene pool, the changing proportions of the migratory tactics can be classified as phenotypic plasticity. The study provides evidence that the different phenotypes reflect alternative tactics within a conditional strategy. Some differences in growth-related variables were present between the sexes, and a very pronounced difference in condition factor was found between resident and migratory males, but not in females. Thus, the results provide evidence that different selective forces may be acting on the sexes.     

Transmission from Seed to Seedling and Secondary Spread of Xanthomonas campestris pv. Campestris in Brassica Transplants: Effects of Dose and Watering Regime

The effects of inoculum load and watering regime on the transmission of Xanthomonas campestris pv. campestris from seed to seedlings of cauliflower were investigated. Seed, inoculated with different concentrations of bacteria, was sown in commercial module trays and subjected to four different watering regimes: high frequency overhead spray, low frequency overhead spray, high frequency capillary and low frequency capillary. Visible symptoms were recorded and leaf washings were carried out to detect the pathogen on symptomless plants. The effects of treatments on symptoms and on the proportion of contaminated but symptomless plants was similar. Initially, they were influenced only by the dose of bacteria with little difference between the watering regimes, but later the proportion of plants with symptoms was greater for plants subjected to overhead watering, due to spread and secondary infection. Generalised linear models were fitted to the data relating the proportion of symptomless contaminated plants or the proportion of plants with symptoms, p, to the mean dose of bacteria per seed, d, and the number of overhead waterings, noh. The equations were: p=1–exp(–0.014·d ^0.32·noh ^0.045) for symptomless contaminated/infected plants and p=1–exp(–0.0056·d ^0.44·noh ^0.014) for plants with symptoms. These models indicated that the one-hit probability for transmission of the pathogen (i.e. with/without visible symptoms) was 0.014 and for infection (i.e. with visible symptoms) was 0.0056.