Optimal Level of Rapeseed Meal in Diets of Lambs

Abstract

Two experiments were carried out to improve the strategy of rapeseed meal (RPM) incorporation in the diets of lambs. In the first experiment, the effects of RPM obtained from either a high-glucosinolate (HG) cultivar (Honk RPM) or a double-low strain (Samourai RPM) were studied. Two types of concentrates containing 25% of RPM were compared to a control. In the second experiment, the effects of various proportions (0–40%) of an industrial low-glucosinolate (LG)-RPM were studied to determine the disorder-threshold in diets for young ruminants. One hundred forty-six Texel, Suffolk, or crossbred lambs aged 1 to 2 months were used. Neither the Samourai nor the Honk RPM negatively affected animal performance, regardless of the parameter considered (growth, food intake and conversion, slaughter performances). Low (P < 0.05) proportions of C10:0, C12:0, and C14:0 and high (P < 0.05) contents of C18:1trans and C22:2 were determined in the perirenal fat of lambs receiving the Honk RPM. The industrial LG-RPM had no negative effects on animal performance, with the best results obtained with 25% and 30% RPM. The Samourai RPM had no effect on thyroid weight, whereas the Honk RPM modified (P < 0.05) the histology of this organ; the percentages of large thyroid follicles were higher (P < 0.05) in the Honk group than in the control and Samourai groups. High levels of industrial LG-RPM (20–40%) also induced hyperthyroidism. The Samourai and Honk RPM decreased (P < 0.05) the secretions of thyroid hormones, while the industrial LG-RPM did not affect these hormones. LG-RPM had no significant effects on other hormones. It was concluded that disturbances in thyroid histology and activity induced by RPM ingestion did not affect the physiology and performance of lambs and that levels between 25% and 30% of LG-RPM were optimal for growing and fattening lambs.

Keywords: Rapeseed meal, disorder-threshold, lambs, performance, physiology

Introduction

Data concerning the maximal proportions of rapeseed meal (RPM) in ruminant diets are not well defined, although it is known that the effects of antinutritional factors vary among species and depend on the age of the animal (Bell, 1993; Mawson et al., 1993, 1994). Contents of erucic acid and some types of glucosinolates have been reduced in rapeseed through genetic improvement, but concentrations of other antinutritional factors, such as progoitrin, remain high in some cultivars, and their effects depend on the amount of RPM in the diet. The general objective of our experiments was to determine the optimal amount of RPM that may be included in the diet of young lambs to optimize growth and fattening.

Materials and Methods

In the first experiment, the effects of RPM from either a high-glucosinolate (HG) cultivar (Honk RPM, 33 µmoles of glucosinolates/g DM of seed) or a double-low strain (Samourai RPM, 15 µmoles/g DM of seed) were studied. Two types of concentrates containing 25% RPM were compared to a control; they contained 1.95 and 4.22 µmoles of glucosinolates/g DM, respectively. Sixty-six Texel, Suffolk, or crossbred lambs (approximately 1 month of age) were used. From weaning (88 ± 8 days) to slaughter weight, concentrates were the sole component of the diet apart from a small quantity of hay (0.4 kg/day/animal). In the second experiment, the effects of various proportions (0–40%) of an industrial low-glucosinolate (LG)-RPM were tested to determine the disorder-threshold in lamb diets. Eighty lambs (approximately 2 months of age) receiving concentrates with 0%, 5%, 10%, 15%, 20%, 25%, 30%, and 40% RPM as the major component of their diet were used. The experimental concentrates contained 0.70, 1.73, 2.66, 3.76, 4.91, 6.31, and 8.37 µmoles of glucosinolates/g DM, respectively.

For both experiments, growth performance, feed intake, slaughter parameters, thyroid weight and histology, and circulating hormones were investigated. Thyroid tissue slices (7 µm) were stained and photographed for follicle size analysis. Plasma hormone concentrations were quantified by radioimmunoassay. In Experiment 1, the fatty acids of perirenal fat were determined by gas-liquid chromatography.

Results

Experiment 1

For animal performance, no significant effects of RPM were observed, regardless of the parameter considered. Only the quality of perirenal fat showed some modifications. The concentrates with RPM induced a significant (P < 0.05) decrease in the percentages of some saturated acids (C10:0, C12:0, and C14:0). Moreover, an increase (P < 0.01) in C22:2 was noted for the two experimental concentrates, and this was also apparent for trans C18:1 in the Honk group. Thyroid weights were comparable for the control and Samourai groups but were 15–18% higher (P < 0.05) in the Honk group. The proportion of small follicles was significantly lower (P < 0.05) in the Honk group compared to the control group. Plasma T3, T4, and insulin concentrations were higher (P < 0.05) in the control than in the Samourai and Honk groups.

Experiment 2

No detrimental effect of LG-RPM was detected, apart from low concentrate intake, which was compensated by high hay ingestion. For groups receiving 25% and 30% RPM, the growth rate was comparable to the control group, associated with low concentrate intake and improved feed conversion. Thyroid weights were higher (P < 0.05) for proportions between 20% and 40% than for 0–15%. Plasma T3 concentration was comparable among groups, while plasma T4 was lower (P < 0.05) in lambs receiving 15%, 20%, and 40% LG-RPM. Plasma cortisol was lower (P < 0.05) in groups receiving LG-RPM.

Discussion

The results show that incorporating LG-RPM (< 10 µmoles of glucosinolates/g diet) in lamb diets has no detrimental effects on animal performance. Best growth rates were observed with 25% and 30% LG-RPM. The use of LG-RPM improved the quality of perirenal fat, as the ratio of unsaturated to saturated fatty acids increased. The Samourai RPM had no effect on the thyroid, whereas the Honk RPM and high levels of industrial LG-RPM modified thyroid weight and histology. However, these changes did not affect animal physiology or performance.

Conclusion

Disturbances in thyroid histology and activity induced by RPM ingestion did not affect lamb physiology or performance. Levels between 25% and 30% of LG-RPM were optimal for growing and fattening lambs.

Acknowledgements

This work was supported by the General Office of Research and Development of the Belgian Agricultural Ministry and the General Direction of Technologies, Research, and Energy of the Ministry of "Region Wallonne" in Belgium. The authors are grateful to M.A. Bouckoms-Vandermeir of the Laboratory of Animal Physiology of Namur, the staff of the Ovine Research Centre of Faulx-les-Tombes, and M. Hardenne of the Department of General and Organic Chemistry of Gembloux for their technical assistance.

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