**2. Agronomic performance**

Average visual chlorosis scores (VCS) in chlorosis screening nurseries and in management trials involving various treatments are commonly accepted as reasonable estimates of the severity of Fe deficiency. Minor, although statistically significant, differences in VCS observed in the near absence of chlorosis, or in another trial, the near death of many cultivars, have little meaning. In the research discussed here, the severity of Fe deficiency ranged from almost no chlorosis (VCS= 1.2) to mild chlorosis (VCS=2.3) to moderate chlorosis (VCS=3.0) to severe chlorosis (VCS=4.2), nonetheless, consistent genotypic differences usually were observed when genotypes were first grouped into classes based on published VCS, field VCS observed at V3, or planting seed Fe concentration or content.

 1[Fe] is iron concentration.

developed (Cianzio et al. 1979) may be more a reflection of planting seed [Fe]1 than resistance to Fe deficiency (Ambler and Brown, 1974; Tiffin and Chaney, 1973; Chaney et al., 1992). Furthermore, Naeve and Rehm (2006) concluded that varietal screening based on VCS likely requires that evaluation is conducted at multiple locations to be predictive. This suggests that using VCS to identify more resistant cultivars may not be the most efficient or least expensive procedure. It has been suggested that the plant character (plant height, seed number, grain yield, seed [Fe], VCS, relative chlorophyll [SPAD] reading) used to measure Fe deficiency is of primary importance in the classification of genotypes for resistance to Fe deficiency (Wiersma, 2007). Many of the characters mentioned are known to vary markedly in screening nurseries as well as in management studies (Helms et al., 2010; Naeve and

In measuring the indirect effects of recurrent selection for Fe efficiency in soybean, Beeghly and Fehr (1989) reported that Fe efficiency was not associated closely with grain yield, time of maturity, plant height, seed protein or oil, leaflet traits, and most micronutrients, except seed [Fe]. Seed weight declined 12%; seed [Fe] increased 13%; whereas, seed Fe content did not change over seven cycles of selection (Beeghly and Fehr, 1989). For soils known to have yield-limiting availabilities of specific micronutrients, increasing the concentration of that micronutrient in seed used for planting has reduced Mo deficiency in corn (*Zea mays* L.) (Weir and Hudson, 1966), Zn deficiency in several species (Rashid and Fox, 1992), Fe and Zn deficiency in rice (*Oryza sativa* L.) (Gregorio et al., 2000), B deficiency in soybean (Rerkasem, et al., 1997), and Fe deficiency in dry bean (*Phaseolus vulgaris* L.) (Beebe et al., 2000) and wheat (*Triticum aestivum* L.) (Shen et al., 2002). Since seed [Fe] can be regarded as an integrated measure of resistance to Fe deficiency that is manifest at maturity, perhaps seed [Fe] should be considered the "measure of choice" in determining susceptibility or

This chapter presents evidence that supports the use of seed [Fe] as an accurate and consistent measure of genotypic differences in Fe efficiency and agronomic performance. This 'evidence' has been garnered from recent soybean Fe deficiency trials conducted on high pH, highly calcareous soils in the North Central region of the USA (Wiersma, 2005, 2007, and 2010), from variety evaluation trials of the Univ. of Minn. Soybean Plant Breeding and Genetics Project, from IDC nurseries managed by R.J. Goos (http://www.soilsci.ndsu.nodak.edu/yellowsoybeans/) and from varietal trials conducted

Average visual chlorosis scores (VCS) in chlorosis screening nurseries and in management trials involving various treatments are commonly accepted as reasonable estimates of the severity of Fe deficiency. Minor, although statistically significant, differences in VCS observed in the near absence of chlorosis, or in another trial, the near death of many cultivars, have little meaning. In the research discussed here, the severity of Fe deficiency ranged from almost no chlorosis (VCS= 1.2) to mild chlorosis (VCS=2.3) to moderate chlorosis (VCS=3.0) to severe chlorosis (VCS=4.2), nonetheless, consistent genotypic differences usually were observed when genotypes were first grouped into classes based on published VCS, field VCS observed at V3, or planting seed Fe concentration or content.

Rehm, 2006; Wiersma, 2005, 2007, and 2010).

resistance to IDC (Bouis et al., 2003; Nestle et al., 2006).

**2. Agronomic performance** 

1[Fe] is iron concentration.

on partially limed and fully limed, acid soils in Brazil (Spehar, 1994).

Class variances were calculated and tested for homogeneity (Snedecor and Cochran, 1980; Gomez and Gomez, 1984) and when class variances were homogeneous, regression equations were developed using class means consisting of both independent and dependent variables. Management studies involving increasing rates of seeding, increasing rates of Fe-EDDHA application, and increasing rates of N application were conducted using resistant, moderately resistant, and susceptible cultivars, without first categorizing the smaller number of genotypes into classes.
