Variation between forage rape (Brassica napus) cultivars for live-weight gain is due to anti-nutritional compounds

Two field experiments were carried out at Kimihia Research Centre, Canterbury, to evaluate the importance of the secondary compounds, S-methyl cysteine sulphoxide (SMCO) and glucosinolates for the liveweight gain of lambs grazing different forage rape (Brassica napus) cultivars of similar nutritional value. Previous research on the secondary compounds has tended to focus on the independent effects of SMCO and glucosinolates on animal performance and health. However, the current study suggests that for ruminants fed a predominately brassica based diet it is important to look at their interactive effects rather than at each in isolation. The first experiment was a split-plot agronomy trial with six cultivars of forage rape (95A3, AMX2f, AR5, H103d, Bonar, Rangi) sown on two sowing dates (19th October and 21st November 2002) with measurements made at five stages of growth (approximately 40,60,80, 100, 120 days post-sowing). At each stage of growth, cultivars were sampled for dry matter yield and leaf and stem analysis of major nutrient concentrations (dry matter, (DM); organic matter digestibility, (OMD); crude protein, (CP); non-structural carbohydrate, (NSC); neutral detergent fibre, (NDF) and acid detergent fibre (ADF)) and secondary compound concentration (SMCO and total and individual glucosinolates). In terms of nutritive value, forage rape leaf was of higher value (OMD > 0.85, CP > 160 g/kg DM, NDF 250 g/kg DM) with only small differences in concentration between cultivars. The major difference between forage rape cultivars was in the concentration of glucosinolates, of which three aliphatic glucosinolates, 2-hydroxy-3-butenyl (progoitrin), 3-butenyl (gluconapin) and 4-pentenyl glucobrassicanapin) predominated. The concentration of each respective glucosinolate was not only dependent on cultivar, but was also influenced by stage of maturity, date of sowing and plant part (leaf or stem). The concentration of SMCO was less dependent on genotype and more affected by stage of maturity and time of sowing. Concentrations of SMCO varied from 1.5 to 6 g/kg DM, which could cause a mild haemolytic anaemia in ruminants. When offered all of the forage rape cultivars ad libitum, Romney ram lambs showed clear preferences, irrespective of stage of growth, between cultivars as determined by the proportion of leaf dry matter eaten relative to the amount of leaf dry matter on offer during a period of no less than 48 hours (H103d, 83% > Rangi, 66% > AMX2F, 63% > 95A3, 59% > Bonar, 53% > AR5, 47%). The current study provides evidence that lambs select against cultivars of forage rape ,that have a high leaf concentration of glucosinolates, particularly those with a high concentration of gluconapin. Regression analysis showed a negative relationship between proportions of leaf dry matter eaten with the concentration of gluconapin (R² = 0.76, P < 0.001). Irrespective of sowing date, cultivars all matured at the same date as indicated by maximum dry matter yield and a change in leaf colour from lush green to blue/green with a reddish tinge on the outer leaf. Coinciding with maximum dry matter yield was a decline in CP concentration, carbohydrate concentration particularly the fibre component and a decrease in concentrations of secondary compounds especially SMCO. It was suggested that the optimum time to commence feeding a forage rape crop is just before maximum DM yield. By commencing grazing at this point of growth more animals can be carried per hectare or the allowance of those already on the crop can be increased. Additionally, health risks such as acidosis caused by eating a low fibre, high-energy forage and from anti-nutritional compounds are reduced due to an increasing fibre and decreasing secondary compound concentration respectively. Experiment 2, compared the live-weight gain (LWG) of weaned lambs grazing different forage rape cultivars (sown 14th November 2000) offered ad-libitum for a 29 day period starting on the 27th February 2001. Mean LWG across cultivars was only 109 g/d, which was low because of a loss of liveweight (149 g/d) during the first 7 days of feeding. The only significant difference (P 0.05) difference between cultivars during grazing days 0- 7 (period 2) and 8-15 (period 2) may reflect the large variability in individual LWG's. Coinciding with negative weight gain in period 1 was the appearance of Heinz bodies in the erythrocytes and a decreasing red blood cell and haemoglobin concentration, suggesting a sub-acute haemolytic anaemia caused by dimethyl disulphide (DMDS) the rumen metabolite of SMCO. Between cultivars, there was a significant difference in the degree of anaemia observed in periods 1 and 2. Differences in the degree of anaemia were attributed to small differences in SMCO concentration between cultivars and more importantly to significant (P< 0.05) differences in dry matter intake. There was a trend for cultivars that had a lower dry matter intake to also have lower LWG. Potentially, causes of differences in dry matter intake include glucosinolate concentration, specifically gluconapin and morphological differences between cultivars such as the proportion of leaf dry matter on offer. A positive relationship was found between leaf gluconapin concentration and liveweight gain during period 1 (R² = 0.48; P < 0.004 respectively) and Heinz body numbers during period 3 (R² = 0.28; P < 0.05). It was suggested that differences in dry matter intake between cultivars caused by factors such as gluconapin concentration could affect the intake of both major nutrients and other secondary compounds including SMCO. Thus, glucosinolates may lower the risk from anti-nutritional factors such as SMCO and nitrate by reducing the amount of brassica crop consumed during the early stages of feeding until the animal adapts to their presence. This work demonstrated that the nutritive value of forage rape is high, particularly the leaf, but because of secondary compounds animal performance on these crops may be reduced. Further, the presence of secondary compounds may explain why differences in animal performance may occur between cultivars of the same brassica species