A comparison of a hybrid stubble turnip (cv. Appin) with other cruciferous catch crops for lamb fattening. 2. Animal performance and toxicological evaluation

Abstract

AbstractForage brassica catch crops can provide a valuable source of additional feed in the autumn when supplies of other feed are limited. Weaned lambs grazed either (A) a novel hybrid crop, stubble turnip cv. Appin, or (B) a sequence of crops consisting of fodder radish cv. Nerys, followed by forage rape cv. Canard. Forty, 60 or 80 g of crop DM per kg current live weight (LW) were allowed daily and the performance, intake and carcass yield of the lambs were monitored. Lamb LW gains were lower on treatment A (45, 65 and 72 g d‐1) than on B (78, 111 and 117 g d‐1) at the 40‐, 60‐ and 80‐g DM d‐1 allowances respectively. Intakes of forage crops, however, did not differ significantly, probably because the estimates of intake were made in the middle of each 14‐d allocation period of the crops. The intake values are thus an indication of the potential intake when a relatively plentiful supply of food is available; they were 19·2, 17·2 and 18·2 g OM kg‐1 LW for treatment A and 16·2, 19·2 and 19·2 g OM kg‐1 LW treatment B at the 40‐, 60‐ and 80‐g DM d‐1 allowances respectively.Mild anaemia occurred in lambs feeding on both crops. Anaemia was more marked on treatment B, which was consistent with the higher concentrations of S‐methyl cysteine sulphoxide (SMCO) but as the performance of animals was superior on treatment B, SMCO intake and anaemia are not thought to be the first or the most important factors in limiting animal performance. Goitrogenic changes in thyroid glands occurred on both crops. Mean weights of paired thyroid glands were 3·22, 4·28 and 4·21 g on treatment A and 3·29, 5·27 and 5·21 g on B at the 40‐, 60‐ and 80‐g DM d‐1 allowances respectively; the effects of both treatment and allowance were significant (P <0·21). Thiocyanate concentrations were higher on treatment A than B, indicating differing glucosinolate concentrations. While this was consistent with animal performance, it was not consistent with the weight of thyroid glands. However, the goitrogenic responses may have been complicated by the ingestion of soil iodine because very high faecal ash values were recorded. Thus, glucosinolates appear not to have affected lamb growth rate by their goitrogenic effect but the possibility remains that they may have influenced performance by other means.