Abstract: A field experiments were conducted at NARC, Islamabad during 1988-89 and 1989-90, with Waster rapeseed (Brassica napus L.) to determine the effect of topping at different growth stages on seed quality. Topping treatments included an untopped check, the removal of three-quarters of the top growth at each of pre-bud, bud and first flower, as well as removal of all secondary branches at first flower. Although topping showed positive response on oil content (%) in seed in all trials which ranged 43.9 to 48.0% which appeared non significant on erucic acid and glucosinolates in the seed.
Introduction
Rapeseed has become an important crop in many countries, following the introduction of rapeseed (canola) low in erucic acid and glucosinolates (Larsen and Soresen, 1985). The Brassica oilseeds are now the third most important source of edible vegetable oil in the world (Downey, 1990).
Many nutritionists view canola as a excellent vegetable oil, one of the best on the market. Like soybean, sunflower and corn, canola oil is low in saturated fats and like all vegetable oils, it contains no cholesterol (Younts, 1990). Kramer et al. (1977) conclude that low erucic acid rapeseed oils (<5% C22.1) present no problem regarding growth rate and longevity. They induce no myocardial lipidosis and have no effect on heart metabolism.
The high oil content in the seeds and high protein content in the oil cake has attracted special attention to canola meal as a source to feed for poultry and monogastric animals (Jensen et al., 1963).
Various rapeseed constituents present in too high concentration reduce, however, the rapeseed quality (Eggum et al., 1985). This is especially true for glucosinolate and degradation products thereof as reported by Sorensen (1988, 1990) and Jensen et al. (1963).
Glucosinolates, phenolic acid (free and bound e.g in dietary fibres Bjergegaard et al. (1991) and their derivatives are of the greatest importance for the quality of food and feed based on glucosinolate containing plant material including oilseed rape, vegetative parts of the plants, cabbage and kale (Nielsen et al., 1984; Eggum et al., 1985; Kozlowska et al., 1990; Sorensen, 1990). Older rapeseed varieties had 2 major defects. Firstly, they produced oil which contained about 45% erucic acid. a nutritionally undesirable fatty acid (Abdellatif and Vles, 1970). Secondly, the hydrolysis product of the glucosinolates in their meal had detrimental effects on animal growth thereby limiting use of the meal (Tookey et al., 1980).
The protein content of canola meal varies depending on the cultivars from which the meal is produced. On the average canola meal made from a mixture of low glucosinolate cultivate can be expected to contain 38 to 38% protein (Clandinin and Robblee, 1983). Generally speaking, canola meal is a richer source of minerals than soybean meal.
The purposes of this study were to determine the effect of topping B. napus L. rapeseed at different growth stages, on seed quality.
Materials and Methods
Field trials were established under rainfed conditions at the National Agricultural Research Centre (NARC), Islamabad during 1988-89 and 1989-90.
The B. napus rapeseed variety cv. Waster was used in all trials. Prior to seeding, soil samples were taken from all sites. Results of the physical and chemical analyses of the soils are presented in Table 1. The trial fields at NARC were cultivated with a sweep-typed cultivator to a depth of 10-15 cm. Fertilizer rate 90 kg/ha–1 of N and 30 kg/ha–1 of P were applied, using a modified Naeem rabi drill (Naeem and Co., Faisalabad, Pakistan) with fertilizer attachment, in bands 12-15 cm deep in rows 22 cm apart perpendicular to the direction of seeding. The fertilizer was a blend of urea and diammonium phosphate. Final preparation of the seedbed was accomplished by harrowing twice with a diamond harrow, followed by a wooden plank.
Trials were laid out in a randomized complete design with four replication during both the years respectively.
For all seeding, an Oyord plot drill (manufactured by Wintersteiger, Division Seedmech, Dimmelstrasse 9, A-4910 Ried, Austria) equipped with six openers spaced 30 cm apart, at a depth of 2-3 cm. was used. Seeding rate 5 kg/ha–1 was used.
Seeding was carried out in mid October in each of the trials conducted. Details of planting dates are presented in (Table 2). The sites received 33 and 17 mm prior to planting in October in 1988 and 1989 respectively. Meteorological data presented in Table 3 and 4.
Following treatments were used during the course of this study:
| T.1: | Check (no topping) |
| T.2: | At pre-bud stage, topping of 3/4 top growth as the plant was elongating, having just completed the rosette stage |
| T.3: | At bud stage, topping as for T.2 above |
| T.4: | At first flower, topping as for T.2 |
| T.5: | Secondary branches were removed at first flower appearance |
Seed samples after harvest from all ratooned crop treatments topped at different stages were subjected to quality determinations.
After laboratory analysis, the data were analyzed using the analysis of variance procedure and LSD (p<0.05) values were calculated for comparison among means (Steel and Torrie, 1980).
| Table 1: | Soil chemical and physical status of trial sites used in study |
| O.M = Organic matter, Text. = Texture, (c),= Clay; Cllm = Clay loam; Sacl = Sandy clay loam; Sicl = Silty clay loam | |
| Table 2: | Moisture regime and planting dates of trial sites used in the study |
| Table 3: | Meteorological data for rabi seasons at the National Agricultural Research Centre, Islamabad. a. Monthly precipitation (mm) |
| **1960-1983 from Robertson (1986) | |
| Table 4: | Monthly mean temperatures (°C) |
Results and Discussion
Effect on oil contents on seed (%): During 1988-89, the oil contents of seed (%) ranged from 44.4 to 45.5% whereas it ranged 43.9 to 48.0% during 1989-90. Although both the year it did not differ significantly but it was positively affected by ratooning as compared to check plots (Table 5). Results from this study showed that the oil content in seed were positively changed by topping.
Inanaga et al. (1986) reported that percentage of oil in seed was changed by cutting of all leaves and leaf shading at various stages during the period from the beginning of flowering to near maturity and increased with increase in dry matter of plant per seed.
| Table 5: | Effect of ratooning on oil content of seed (%), erucic acid (%) content of seed and total glucosinolates (micromoles/g) under barani (rainfed) conditions at NARC, Islamabad, 1988-89 and 1989-90 |
Evans and Abdel Wahab (1983) found in winter oil seed rape in the U.K that leaf removal up to the stem elongation phase decreased the seed oil percentage.
Effect on erucic acid and glucosinolate in seed: The level of erucic acid ranged from 3.85 to 4.73% and 1.41 to 2.22% during 1988-89 and 1989-90. It reduced during first year whereas minimum increase occured in the second year but the differences appeared non significant which may have been the result of different locations each year. Parnell et al. (1983) showed that glucosinolates in certain new double-low varieties varied with location, but the effect of time of sowing was not examined. Concerned to the glucosinolate which ranged 26.7 to 30.7 (micromoles/g) and 26.5 to 32.9 (micromoles/g) during both the year respectively. But the ratooning did not influence on erucic or glucosinolate levels in the seed. Declining glucosinolate contents in vegetative plant parts in the course of growth were reported by McGregor and Love, 1988).