Canola growth and development
Canola stand establishment and seedling survival in the inland Pacific Northwest tends to be more variable than cereals. This is largely attributable to its “epigeal emergence” whereby the cotyledons and the shoot growing point emerge above the soil surface, increasing the plant’s exposure to environmental stress. In contrast, cereals exhibit “hypogeal emergence,” resulting in the shoot growing point remaining below ground and therefore more protected from extreme aerial climatic conditions. Seeding during favorable temperature and moisture conditions is therefore more critical for canola than for wheat stand establishment.
Canola also has an indeterminate growth habit. This means that individual plants are capable of expanding to utilize available space, water, and nutrient resources by increasing the length and number of lateral branches and pods per branch, and therefore seed yield. This largely explains why uneven stands of canola are still capable of producing high yields. In fact, canola is reported to be capable of producing near maximum yields with stand reductions of 50% or more (OMAFRA 2011).
Canola will continue to flower and develop seed until stress terminates these processes. Canola is more sensitive to heat stress at flowering than wheat since flowering and seed initiation occur over a long period of time, and a long duration of flowering is directly related to high seed yields. Collectively, the greater sensitivity of canola to environmental stresses at seedling establishment and at flowering accounts for higher yield variability with canola than with wheat.
Canola root systems exhibit typical taproot architecture shaped like an inverted cone where soil volume in contact with roots decreases with depth. Canola has an extensive root system (Weiss 1983) and root hairs (Hammac et al. 2011), which give it high root surface area and potential to remove nutrients from soil. The rooting depth for winter and spring canola has been reported as 65 and 46 inches, respectively (Johnston et al. 2002). However, Johnston et al. (2002) reported a deeper penetrating root system is often a response to limited water. In addition to nutrient uptake, canola’s root system provides stability against lodging (Goodman et al. 2001).
The proportion (%) of total aboveground plant dry matter that is seed (harvest index, HI) ranges from 20 to 35% for canola compared to a relatively stable average of 40% for wheat (Hocking and Stapper 2001, Hocking et al. 1997). Reported HI values vary widely in part because the date of planting and the timing of stress markedly affect seed yield, and many leaves
Canola follows dry matter and nitrogen (N) uptake patterns similar to wheat (Figure 1). Maximum dry matter and N accumulation occur between the beginning of stem elongation/branching and the end of flowering. Stress during this time will limit dry matter accumulation, N uptake, and seed yield by limiting lateral branching and flowering. Dry matter and N peak during seed fill and then decline as seed matures due primarily to leaf senescence and pod shattering.
Table 1 summarizes nutrient uptake, partitioning, and removal estimates for canola and wheat. On an equivalent yield basis, canola accumulates more nitrogen (N), phosphorus (P), potassium (K) and sulfur (S) than wheat. Due in part to a low HI and high nutrient concentration in the residue, canola also leaves more nutrients in the field after harvest than comparable yields of wheat. For example, Jackson (2000) reported 40% of N, 30% of P, and 85% of K accumulated by spring canola remained in the residue left after harvest. Cycling of nutrients in this residue to subsequent crops is likely one important rotational benefit of canola (Kirkegaard et al. 1994, 1997).
The following sections discuss major nutrient responses, recommendations, and management for canola.
Canola seed yield responds well to applied N when residual soil levels are low (Grant and Bailey 1993, Hocking and Stapper 2001, Jackson 2000). In Montana, Jackson (2000) measured canola seed yield responses of 2,000 to 3,000 lb/acre when up to 225 lb N/acre was applied to soils with available N below 50 lb/acre. Similar yield responses were obtained when 90 lb N/acre was applied at a site with similar residual N levels in Australia (Hocking and Stapper 2001). A few references state that canola N requirements are similar to those of wheat, though most acknowledge that canola requires more N than an equivalent yield unit of wheat (Grant and Bailey 1993).
Gan et al. (2007) and Hammac et al. (2010) showed that canola had little or no yield response to N application when residual soil N was moderate to high. Similarly, grain N increases very little with an increasing N rate as maximum yield is approached. Nitrogen fertilization resulted in linear decreases in canola seed oil concentration (Jackson 2000, Ramsey and