The effect of different cultivars of rape in their ability to produce photosensitivity (rape scald) in lambs: A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Agricultural Science (Honours)

Brassica napus ( Rape ) is a high quality forage crop grown predominately for use in the summer and autumn, as it provides high quality feed when pasture quality and quantity can be limiting. Although rape has a high digestibility ( > 80% ), and protein content, liveweight gains of animals grazing these crops often doesn't reflect this, and certain disease conditions are often the cause. Between February and April, 1996, 48 mixed breed lambs were split into six groups and grazed on six different cultivars of rape, for two separate grazing periods of ten days each. Lambs were weighed on days -3, 0, 4, 10, 15, 21, 25, and 31 and blood samples taken. Pasture and leaf samples were also collected. The aim was to determine the effects of cultivars in producing photosensitivity in lambs, however no clinical signs of photosensitivity were observed during the trial period. There was a significant difference in lamb liveweight gain per day between cultivars for the first grazing period ( day 0 to day 10 ), with cultivar four being significantly different than cultivars three, five and six ( P<0.05 ). Weight gains recorded on cultivars one, two, three, five and six were not significantly different. In the second grazing period from day 21 to day 31, there was no significant difference between cultivars. Mean growth rates achieved were 232.50 and 311.25 grams per day for grazings one and two respectively. Haemoglobin changes over the two grazing periods were significantly different. In the first grazing period the mean haemoglobin concentration dropped by 3.35% on average, compared to a decrease of only 0.07% during grazing two. Cultivars had a highly significant effect (P<0.001) on glucosinolate concentration. The cultivars with the lowest glucosinolate concentrations generally promoted the highest liveweight gain per day in the lambs. Date of sampling was also highly significant (P<0.001 ), with the glucosinolate levels decreasing until day 25 and then increasing rapidly. The results show a large variation between cultivars for both lamb liveweight gain, and glucosinolate concentrations.