A gronomy J our n al • Volume 110 , I ssue 1 • 2 018 1 T he goal of an N recommendation system is to accurately estimate the gap between the N provided by the soil and the N required by the plant. Accurately estimating this gap depends on the ability of the recommendation system to accurately estimate fi eld or subfi eld specifi c economically optimal nitrogen rates (EONR). Current recommendation systems are not as accurate as needed to provide consistently reliable estimates of N needs across years at the fi eld or subfi eld scale. Uncontrollable factors like temperature, rainfall timing, intensity and amount, and interactions of temperature and rainfall with factors such as N source, timing and placement, plant genetics, and soil characteristics combine to make N rate recommendations for an individual fi eld or rates for subfi elds a process guided as much by science as by the best professional judgement of farmers and farm advisors.Substantial evidence has accumulated that EONRs can vary widely across fi elds, within fi elds and over years in the same fi eld for a wide range of crops and geographies. Examples ABSTRACTNitrogen fi xation by the Haber-Bosch process has more than doubled the amount of fi xed N on Earth, signifi cantly infl uencing the global N cycle. Much of this fi xed N is made into N fertilizer that is used to produce nearly half of the world's food. Too much of the N fertilizer pollutes air and water when it is lost from agroecosystems through volatilization, denitrifi cation, leaching, and runoff . Most of the N fertilizer used in the United States is applied to corn (Zea mays L.), and the profi tability and environmental footprint of corn production is directly tied to N fertilizer applications. Accurately predicting the amount of N needed by corn, however, has proven to be challenging because of the eff ects of rainfall, temperature, and interactions with soil properties on the N cycle. For this reason, improving N recommendations is critical for profi table corn production and for reducing N losses to the environment. Th e objectives of this paper were to review current methods for estimating N needs of corn by: (i) reviewing fundamental background information about how N recommendations are created; (ii) evaluating the performance, strengths, and limitations of systems and tools used for making N fertilizer recommendations; (iii) discussing how adaptive management principles and methods can improve recommendations; and (iv) providing a framework for improving N fertilizer rate recommendations.

In late September and October 1987–1990 and early July 1988–1991, 66 radio-collared female caribou (Rangifer tarandus granti) of the Central Arctic Herd were captured and weighed 117 times. Caribou were relocated repeatedly during early June 1988–1991; parturition status, calving date, and perinatal calf survival were determined. Mean autumn body weights of subsequently parturient (90.0 kg) and nonparturient (82.5 kg) females differed significantly (P < 0.01). Mean summer weights 4 – 5 weeks after parturition were significantly higher for females that had calved on or before 7 June (82.2 kg) than for those that had calved after 7 June (72.1 kg; P < 0.01), and for females whose calves survived at least 2 days post partum (80.2 kg) than for those whose calves died within 2 days (70.3 kg; P < 0.01). Significant logistic models were generated for relationships between parturition rate and autumn weight (P < 0.01), between the occurrence of early calving and summer weight (P < 0.05), and between calf survival rate and summer weight (P < 0.02). Body weight appears to be a reasonable index of body condition, which in turn is related to reproductive performance. The probability of a successful pregnancy is largely predetermined at breeding, based on autumn condition, whereas calving date and early calf survival appear to be influenced primarily by maternal condition during late pregnancy.

We synthesize findings from cooperative research on effects of petroleum development on caribou (Rangifer tarandus granti) of the Central Arctic Herd (CAH). The CAH increased from about 6000 animals in 1978 to 23 000 in 1992, declined to 18 000 by 1995, and again increased to 27 000 by 2000. Net calf production was consistent with changes in herd size. In the Kuparuk Development Area (KDA), west of Prudhoe Bay, abundance of calving caribou was less than expected within 4 km of roads and declined exponentially with road density. With increasing infrastructure, high-density calving shifted from the KDA to inland areas with lower forage biomass. During July and early August, caribou were relatively unsuccessful in crossing road/pipeline corridors in the KDA, particularly when in large, insect-harassed aggregations; and both abundance and movements of females were lower in the oil field complex at Prudhoe Bay than in other areas along the Arctic coast. Female caribou exposed to petroleum development west of the Sagavanirktok River may have consumed less forage during the calving period and experienced lower energy balance during the midsummer insect season than those under disturbance-free conditions east of the river. The probable consequences were poorer body condition at breeding and lower parturition rates for western females than for eastern females (e.g., 1988-94: 64% vs. 83% parturient, respectively; p = 0.003), which depressed the productivity of the herd. Assessments of cumulative effects of petroleum development on caribou must incorporate the complex interactions with a variable natural environment.

Aerial surveys were conducted annually in June 1978-87 near Prudhoe Bay, Alaska, to determine changes in the distribution of calving caribou (Rangifer tarandus granfi) that accompanied petroleum-related development. With construction of an oil field access road through a calving concentration area, mean caribou density (no./km2) decreased from 1.41 to 0.31 (P = 0.05) within 1 km and increased from 1.41 to 4.53 (P = 0.04) 5-6 km from the road. Concurrently, relative caribou use of the adjacent area declined (P < 0.02), apparently in response to increasing surface development. We suggest that perturbed distribution associated with roads reduced the capacity of the nearby area to sustain parturient females and that insufficient spacing of roads may have depressed overall calving activity. Use of traditional calving grounds and of certain areas therein appears to favor calf survival, principally through lower predation risk and improved foraging conditions. Given the possible loss of those habitats through displacement and the crucial importance of the reproductive process, a cautious approach to petroleum development on the Arctic Slope is warranted.

Summary Three automated listening post‐telemetry studies were undertaken in the Suwannee and Apalachicola estuaries to gain knowledge of habitats use by juvenile Gulf Sturgeons (Acipenser oxyrinchus desotoi) on winter feeding grounds. A simple and reliable method for external attachment of small acoustic tags to the dorsal fin base was developed using shrink‐tubing. Suspending receivers on masts below anchored buoys improved reception and facilitated downloading; a detection range of 500–2500 m was realized. In the Apalachicola estuary, juvenile GS stayed in shallow water (< 2 m) within the estuarine transition zone all winter in the vicinity of the Apalachicola River mouth. Juvenile GS high‐use areas did not coincide with high density benthic macrofauna areas from the most recent (1999) benthos survey. In the Suwannee estuary, juveniles ranged widely and individually throughout oligohaline to mesohaline subareas of the estuary, preferentially using mesohaline subareas seaward of Suwannee Reef (52% of acoustic detections). The river mouth subarea was important only in early and late winter, during the times of adult Gulf Sturgeon migrations (41% of detections). Preferred winter feeding subareas coincided spatially with known areas of dense macrofaunal benthos concentrations. Following a dramatic drop in air and water temperatures, juvenile GS left the river mouth and estuary, subsequently being detected 8 km offshore in polyhaline open Gulf of Mexico waters, before returning to the estuary. Cold‐event offshore excursions demonstrate that they can tolerate full‐salinity polyhaline waters in the open Gulf of Mexico, for at least several days at a time. For juvenile sturgeons, the stress and metabolic cost of enduring high salinity (Jarvis et al., 2001; McKenzie et al., 2001; Singer and Ballantyne, 2002) for short periods in deep offshore waters seems adaptively advantageous relative to the risk of cold‐event mortality in shallow inshore waters of lower salinity. Thus, while juveniles can tolerate high salinities for days to weeks to escape cold events, they appear to make only infrequent use of open polyhaline waters. Throughout the winter foraging period, juvenile GS stayed primarily within the core area of Suwannee River mouth influence, extending about 12 km north and south of the river mouth, and somewhat seaward of Suwannee Reef (< 5 km offshore). None were detected departing the core area past either of the northern or southern acoustic gates, located 66 and 52 km distant from the river mouth, respectively.