Investigating the Relationship Between Site-speciﬁc
Yield and Protein of Cereal Crops
S. NORNG AND A. N. PETTITT Sorn.Norng@dpi.vic.gov.au
School of Mathematical Sciences, Queensland University of Technology, 2 George Street, Brisbane 4000,
R. M. KELLY, D. G. BUTLER AND W. M. STRONG
Department of Primary Industries (DPI), PO Box 102, Toowoomba 4350, Queensland Australia
Abstract. Agronomists use overlaying protein and yield maps to identify factors limiting cereal crop
growth and development. Management decisions can be derived from knowing what and where these
limiting factors are. In using protein and yield in this manner, there is an assumption that a physiologically
or biologically signiﬁcant relationship exists between grain protein and grain yield at the local level. In this
paper, we investigate whether within-ﬁeld yield and protein data support this relationship. The protein-
yield relationship was modelled using weighted regression with global and local neighbourhoods in both
1-D and 2-D spatial location frameworks. The results from both the 1-D and 2-D analyses showed that the
relationships between protein and yield are signiﬁcant at both the macro (ﬁeld level) (r
¼ 0.25) and the
micro-scale (local within ﬁeld level) (r
¼ 0.69). The assumption of a signiﬁcant local relationship between
protein and yield is supported by these data, suggesting that management decisions may be determined
using such a relationship.
Keywords: yield maps, grain protein, protein-yield relationship, local neighbourhood, weighted regression
Within-ﬁeld variation in grain protein has been observed in North America (Long
et al., 1998, 2000, 2001; Engel et al., 1999) and Europe (Reyns et al., 1999, 2000;
Staﬀord, 1999) in grain crops such as wheat and barley. Similar observations have
been made in southern (Low et al. 1997) and northern grain-growing regions of
Australia (Kelly et al., 2001a; Stewart et al., 2002), with the range in protein content
varying from 2% to as much as 5%.
For Australian grain farmers, these ﬁndings are signiﬁcant for two reasons.
Firstly, payment for wheat and barley commodities is based on the protein content
found within the grain at harvest. In 2001 for example, malting-grade barley, with a
protein content from 9% to 11.5%, fetched a premium of nearly 20% above the price
paid for feed barley (i.e. A$200 versus $235; Grainco, 2001). A similar quality
payment system, although with a greater degree of segregation, exists for hard-red
wheat thus providing incentives to maximise grain protein content. An under-
standing of the within-ﬁeld variation in protein content for these crops may
advantage farmers who can segregate high quality grain from low quality grain
(Kelly et al., 2001b; Stewart et al., 2002).
Precision Agriculture, 6, 41–51, 2005
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