rations of cattle (119), where it was used to bind as well as increase the protein content of the feed. When MBM was banned in 1988, chicken manure was one of the cheaper sources of protein used to replace it. Its use subsequently increased for a short while until it was banned in 1991 (119) - at the peak of the UK's BSE incidence rate (Fig. 8). Poultry were fortified with various Mn complexes (Mn sulphate, Mn oxide, etc.) for promoting egg and broiler production as well as rearing. Mn was generally fed at high rates between 100 and 120mg/kg of dry matter composition of diet (135) because of the inefficient 2-5% rate of dietary absorption of Mn by monogastric poultry (135). Consequently, 95-98% of the Mn content of poultry feed is excreted in the manure.
Cows were also increasingly exposed to foods high in Mn via various cost-cutting byproduct ingredients that were added into cattle concentrate feed during the 1970s/1980s (135); palm kemal meal (164p.p.m. Mn), wheat bran (122 p.p.m.), rice bran (260 p.p.m.), soya bean meal (35 p.p.m.) tea waste (275 p.p.m.) coffee waste (20.6 p.p.m.) red clover (158 p.p.m.), dried alfalfa (37 p.p.m.) (3,4,69). Nickel was used to extract certain types of oil (4) that are added to concentrated feeds could have entered the bovine food chain.
Concentrated Dairy Cow feed was also supplemented with high rates of Mn at 120 mg/kg (calves at 80 mg/kg) whilst beef cattle that received considerably smaller quantities of concentrated feeds than dairy cattle were
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only fed 70 mg/kg (135). (NB. Beef cattle experienced 80% lower rates of BSE relative to dairy cattle) Ruminants absorb Mn more efficiently than monogastrics, with 10-18% Mn traversing the gut wall (135).
Exposure to Mn residues resulting from the use of Mn fertilizers and 'maneb'/'mancozeb' fungicides on fodder and forage crops for cattle also occurred - particularly during the period of peak usage of these compounds in the late 1980s/early 1990s (131-133). Any Mn originating from these various sources would also survive the rendering process of meat and bone meal (MBM) manufacture, and subsequently bioaccumulate its way up the farm animal foodchain as a result of the practice of feeding farm animal back to farm animal via the MBM ingredient.
Alternatively, there are two other candidate cations which were used significantly in UK agriculture and along UK waterways. Diquat and Chloramquat cations (related to the auto oxidising paraquat molecule (17)) were used more intensively in the UK by weight per acre during the 1980s-1990s than in other countries. Diquat is applied as a crop desiccator in the UK (17,130) where it is used on peas, grass crops for seed, laid cereals, oilseeds, hops, lucerne, potatoes, beans, etc, immediately prior to harvest (and as a herbicide along waterways), and chloramquat is applied as a plant regulator on cereals shortly before harvest. Diquat use (Graph 3) increased by 800% from 20 tonnes of a. i. used per annum in the later half of the 1970s (131) to 163 tonnes used per annum in the
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