Advances in technology are making global positioning systems, on-the-go yield
monitors, geographic information systems, remote sensing, communication
networks, and variable-rate application techniques available to crop
producers. There is, however, no established methodology for using these
technologies in agricultural research to develop basic knowledge needed to
improve crop production practices. This report describes how these new
technologies are being used to develop recommendations for N management during
non-irrigated corn production in the corn belt of the USA. The discussion
begins with relevant background information about the prevailing N-management
practices for this crop and some challenges posed by the advent of the new
technologies. This is followed by a description of the various methods being
used to collect data in field trials. An example of a field trial is then
presented and the potential importance of management-induced variability is
illustrated. This is followed by some general observations derived from the
results of many field studies. The discussion concludes with speculation about
the nature of N fertiliser prescriptions during crop production with precision
farming technologies
Data on yields of Atlantic salmon Salmo salar from 59 Icelandic rivers were analyzed with data on streamflows and sea and air temperatures in an attempt to identify why some stocks exhibited more variable yields than others. A group of 24 northern and northeastern rivers, which flowed into seas with wide annual variations in climatic and oceanic conditions (as indicated by coastal sea and air temperatures), exhibited significantly greater variation in Atlantic salmon yields of both grilse and two‐sea‐winter fish (two winters at sea before first spawning) than the 31 western rivers, which flowed into seas with more stable climatic and oceanic conditions. Yields were the most varied for stocks in the northeast, the region with the greatest annual variation in sea temperatures during spring and summer – the time when smolts migrate to the sea and begin to feed. Rivers with more variable seasonal streamflows also tended to have more variable two‐sea‐winter Atlantic salmon yields (P < 0.05) but not more variable grilse yields (P > 0.05). However, variations in stream flows were less useful than variations in sea and air temperatures for explaining variations in yield. We concluded that climatic and oceanic factors exert important influences on the variability in yield and abundance of Icelandic stocks of Atlantic salmon.
Investigations were conducted on the effects of oceanic variations (as measured by sea temperatures) and catches by the West Greenland salmon fishery on the sea age composition of Atlantic salmon (Salmo salar) stocks from 21 Icelandic west coast rivers. Annual ratios of grilse to two-sea-winter (2SW) salmon were strongly correlated among the 21 rivers. All eight rivers with time series extending back before the expansion of the West Greenland fishery showed lower ratios during the earlier period. Only 2 of the 21 rivers, however, had significantly declining ratios over their time series. In addition, for only one river was West Greenland catch significantly related to the ratios (P < 0.05), and for only one river did ratios increase when the expanded West Greenland fishery was active. Overall, the effects of the fishery on stock composition are evidently minimal. The mean April–May temperature when the smolts were to migrate out of rivers was significantly and positively related to subsequent ratios for five of the rivers, which, along with correlations among the ratios, indicated that more rapid growth of smolts in their first summer may have increased the ratios of grilse to 2SW salmon on several rivers.
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