On-farm assessment of constraints to wheat yield in the south-eastern Pampas

The aim of this study was to identify environmental constraints to wheat yield in the south-eastern wheat belt of the Pampas. We were concerned with the relevance of known physiological mechanisms in commercial crops. Our approach involved three steps. First, we used data from 103 large (>20 ha) grower managed fields in two locations (annual rain: 990 mm) to develop a parsimonious model based on knowledge of the system, i.e. a model that uses the minimum number of variables and parameters without losing predictive power. Second, the model was tested with independent data from 49 crops in two drier locations (annual rain: 790 mm). Third, yield of crops grown under a range of soil depths and management conditions (e.g. varying fertiliser rate) were compared with predicted yield. Actual, attainable and potential yields were also estimated and analysed in the light of environmental and management constraints. For a period including six growing seasons, potential yield ranged from 7.5 to 8.1 t ha(-1), attainable yield from 4.6 to 6.6 t ha(-1) and average yield from 3,5 to 5.2 t ha(-1). Water deficit in the period from 30 days before to 10 days after flowering accounted for 65% of the variation in yield, Extending the critical period to account for water stress during earlier growth stages increased r(2) to 0.71. Actual yield was also negatively associated with air temperature during grain filling. An empirical model using rainfall and grain filling temperature as independent variables accounted for 55-69% of the variation in yield in an independent data set. This model together with long-term rainfall records indicated that closing the gap between attainable and potential yield would require improved management to attenuate water deficit, particularly late in the season. Efforts to alleviate the effect of early drought, which had a dramatic effect on yield not reversed by good water supply later in the season, would be less profitable given the low frequency (10%) of this type of yield restriction. Strategies to cope with late season drought are particularly important in farms where the detrimental effect of low rain is compounded by shallow soil. (C) 2002 Elsevier Science B.V All rights reserved.