Pablo L. Cicore

Phosphorus (P) deficiency limits plant growth. Suboptimal or excessive P fertilization of crops and pastures implies unwanted economic and environmental impacts. Critical P concentration (Pc) dilution curves can be used to diagnose P status. However, P and nitrogen (N) metabolisms are closely interrelated, and it is unknown whether Pc is affected by N deficiency. This study evaluated the effect of N deficiency on Pc dilution curves and then assessed the usefulness of the N:P ratio to distinguish between N vs. P deficiency vs. N and P co-limitation. For this purpose, a factorial combination of several N and P fertilization rates was applied in nine field experiments conducted during winter and spring, on annual ryegrass (Lolium multiflorum) and tall fescue (L. arundinaceum (Schreb.) Darbysh) pastures, at different sites in Uruguay and Argentina. Across all sites, N deficiency did not alter Pc. However, Pc values did differ among sites. This was due to variations in the scale parameter, not the dilution parameter. Combining Pc and Nc curves revealed that the critical N:P ratio decreased as shoot biomass increased, given that N was diluted more rapidly than P. In general, the N:P ratio had low accuracy to discriminate between N and P deficiency. Instead, at low shoot biomass, using individual [N] and [P] values predicted better plant nutritional status. These results support the use of reference Pc dilution curves under contrasting N nutritional conditions and indicate that [N] and [P] (not N:P ratios) could yield useful inferences whenever shoot biomass data is unavailable.

Mapping of the apparent soil electrical conductivity (ECa) can be used to estimate the variability of forage yield within a plot. However, forage production can vary according to the growing season and to soil properties that do not affect the ECa (e.g. nitrogen (N) content). The aim of this study was to assess the relationship between ECa and forage yield of tall fescue (Lolium arundinaceum (Schreb.) Darbysh.) during different regrowth periods and contrasting levels of N availability and then use this information to determine potential management zones. The ECa was measured and geo-referenced in a 5.75-ha paddock that sustained a permanent pasture dominated by tall fescue. In addition, a 30 m by 30 m grid cell size was chosen and 43 sampling areas, each 4 m2 in size, were geo-referenced and divided into two experimental units of 1 m by 2 m, one of which was fertilised with 250 kg N ha–1 (N250) at the beginning of four regrowth periods (spring 2015, spring 2016, autumn 2016 and autumn 2017) and the other was not fertilised with N (N0). At the end of each regrowth period, we estimated the accumulated biomass. During the spring growing season, accumulated biomass was positively associated with ECa in both N0 and N250 treatments (R2 = 47% and 54%, respectively). By contrast, in autumn, accumulated biomass and ECa were poorly associated (R2 = 10% and 27% for N0 and N250). This may be due to seasonal interactions that alter soil–yield relationships. To assess whether ECa can be used to determine management zones, the differences in accumulated biomass were compared through analysis of variance. Results showed that ECa is associated with the spatial distribution of tall fescue forage yield variability in spring at different N availabilities. Thus, ECa can be reliably used for defining management zones in marginal soils under permanent pastures.

Little information is available on the degree of within-field variability of potential production of Tall wheatgrass (Thinopyrum ponticum) forage under unirrigated conditions. The aim of this study was to characterize the spatial variability of the accumulated biomass (AB) without nutritional limitations through vegetation indexes, and then use this information to determine potential management zones. A 27-×-27-m grid cell size was chosen and 84 biomass sampling areas (BSA), each 2 m² in size, were georeferenced. Nitrogen and phosphorus fertilizers were applied after an initial cut at 3 cm height. At 500 °C day, the AB from each sampling area, was collected and evaluated. The spatial variability of AB was estimated more accurately using the Normalized Difference Vegetation Index (NDVI), calculated from LANDSAT 8 images obtained on 24 November 2014 (NDVI_nov) and 10 December 2014 (NDVI_dec) because the potential AB was highly associated with NDVI_nov and NDVI_dec (r² = 0.85 and 0.83, respectively). These models between the potential AB data and NDVI were evaluated by root mean squared error (RMSE) and relative root mean squared error (RRMSE). This last coefficient was 12 and 15 % for NDVI_nov and NDVI_dec, respectively. Potential AB and NDVI spatial correlation were quantified with semivariograms. The spatial dependence of AB was low. Six classes of NDVI were analyzed for comparison, and two management zones (MZ) were established with them. In order to evaluate if the NDVI method allows us to delimit MZ with different attainable yields, the AB estimated for these MZ were compared through an ANOVA test. The potential AB had significant differences among MZ. Based on these findings, it can be concluded that NDVI obtained from LANDSAT 8 images can be reliably used for creating MZ in soils under permanent pastures dominated by Tall wheatgrass.

Use of geophysical survey as a predictor of the edaphic properties variability in soils used for livestock production