Towards a crop growth, development, and yield model for Lupinus angustifolius L. (narrow leafed lupin) in Tasmania

Experiments were conducted between 1988 and 1990 at Elliott, Cressy, and Ross in Tasmania using three cultivars (Yandee, Geebung, and 75A329) of narrow-leafed lupin (Lupinus angustifolius). The purpose of these experiments was to examine narrow-leafed lupin growth and development in Tasmania in relation to specific environmental factors. These factors were related to growth and development measurements. It was hoped to use these relationships in a simple crop model suitable for assessing sites for the commercial production of lupins. Preliminary experiments in 1988 examined the lupin cultivars for agronomic suitability in Tasmania. Increased grain yield was a function of more pods/m2 rather than increased pods/plant. This suggested that lower, yields of the indeterminate line 75A329 could be compensated for by a higher plant density than in the indeterminate cultivars. Lupins responded to higher rainfall and extended growing season at Elliott thus outyielding crops at Cressy and Ross. Detailed field experiments were conducted in 1989 and 1990 at Elliott and Cressy. Lupin crops developed very slowly in the first 8- 10 weeks, and then grew rapidly after flowering was initiated. It appeared floral initiation was a function of higher temperatures and longer days in Yandee and 75A329, with further responses to vernalisation in Geebung. Plant density significantly affected grain yield. 75A329 showed the largest yield responses to increased plant density. Although increased plant density resulted in increased leaf area, leaf senescence took place earlier in the highest density crops probably due to competitive effects. Optimum density for the indeterminate cultivars was 40 plants/m2. It may be higher for determinate cultivars. Low density crops were able to utilise their leaf area for light interception more efficiently than high density crops. In the latter, branches and leaves were pushed more towards vertical rather than horizontal thus less leaf area was presented to intercept light. The study established that early sowing of lupins in Tasmania allows more time to grow and develop and yield more grain. A direct relationship was established between increased total dry matter and increased grain yield. The model developed in this study used thermal time as its only external factor to determine L, intercepted radiation (%), and total dry matter (kg/ha) during crop growth. From the predicted figure for total dry matter accumulated by harvest time, an estimate of potential grain yield could be made for that crop. This study demonstrated the principle of collecting agronomic data and, guided by basic plant physiological principles and mathematical procedures, assembling simple sub-models that when linked can approximate a particular aspect of crop growth.