the above objects are achieved according to the invention by providing the feed additive containing zinc bacitracin with a polymer coating . for embedding the feed additive in this way it is theoretically possible to use all those substances which are capable of forming a coating over the substance which is to be protected and thus providing mechanical protection ; for example : it should be borne in mind that the substances which form the coating should dissolve or swell under physiological conditions ( in the stomach , intestine or first stomach in the case of ruminants ) to enable the active substance to be released . coatings of polyacrylates and polysaccharides have proved satisfactory , but coatings of hydroxypropylmethyl cellulose , hydroxyethyl cellulose and methyl cellulose are particularly preferred . the proportion of polymer is between 1 and 30 % by weight , preferably from 5 to 20 % by weight , based on the weight of the starting material which is a mixture of 5 - 40 % by weight , preferably 15 - 25 % by weight , of zinc bacitracin and 60 - 95 % by weight , preferably 75 - 85 % by weight , of a carrier . it goes without saying that stabilization of the feed additive according to the invention is achieved even with higher polymer concentrations , but it is within the purview of the present invention to keep the quantity of polymer as small as possible . the carrier may consist of 20 - 80 % by weight of fermentation residue ( silage ) and 20 - 80 % by weight of calcium carbonate ; preferably , it consists of 75 % by weight of fermentation residue and 25 % by weight of calcium carbonate . other carriers ; particularly those specified in the feed regulations of may 1983 published in the federal german law bulletin , may be used . a variety of different methods are available for preparing the feed additive containing zinc bacitracin according to the invention . the simplest method of coating with polymers is the moist mixture method . the substance which is to be protected is intensively mixed with a solution of the coating material and then granulated . after the granules have been dried , the agglomerates produced are ground again . another method is spray - hardening , for example , in which the substance to be protected is incorporated in molten materials . during spraying , small spherical particles are produced which harden upon cooling . in micro - encapsulation , the substance to be protected is coated with polymers , using the methods described in the literature . whereas dissolved or liquid substances are relatively easy to encapsulate , problems may occur with solids , depending on the particle form , as a result of incomplete application of the coating . in a preferred process according to the invention , the feed additive containing zinc bacitracin is coated with a polymer in a fluidized bed granulator . the feed additive consisting of a mixture of 5 - 40 % by weight , preferably 15 - 25 % by weight , of zinc bacitracin and 60 - 95 % by weight , preferably 75 - 85 % by weight of a carrier is fluidized by means of an air current , and then the polymer , preferably hydroxypropylmethyl cellulose , hydroxyethyl cellulose or methyl cellulose , is sprayed into the fluidized bed in dissolved form through a nozzle . any organic and aqueous solvents or mixtures of solvents wherein the polymer is soluble may be used to prepare the polymer solution . if necessary , a suspension of the polymer in a suitable solvent may also be used . the preferred solvents are water , methanol or dichloromethane or mixtures of solvents such as methanol / dichloromethane in the ratio 1 : 1 . when selecting the solvents it is important to choose those having a relatively low boiling point , so that no residues of solvent remain in the feed mix after processing . the smallest possible particle size for the feed additive containing zinc bacitracin is desirable in order to ensure uniform distribution of the feed additive in the feed mixture . the formation of agglomerate , and hence the particle size , can be controlled by means of the rate of spraying ; the formation of agglomerate increases as the spraying rate increases . the feed additive containing zinc bacitracin coated with polymer by the process of the invention is distinguished over the untreated commercial product in admixture with feeds by a substantially lower decomposition of the zinc bacitracin even under unfavorable storage conditions at elevated temperatures , so that , contrary to present practice , the content of zinc bacitracin in finished feed mixtures can be determined even after a fairly long storage time and the tolerances prescribed by the statutes can be monitored . additional advantages of the feed additive having a polymer coating are its noticeably reduced odor compared with the untreated product and a substantially reduced production of dust during processing . the feed additive according to the invention is suitable for improving utilization of fodder in the fattening of animals . the following examples illustrate the present invention and will enable others skilled in the art to understand it more completely . it should be understood , however , that the invention is not limited solely to the particular examples given below . hydroxypropyl methyl cellulose in the form of an aqueous solution ( 5 % or 7 %) 5 % or 20 % by weight based on the weight of starting material the starting material was fluidized in a fluidized bed granulator by means of an air current , and then a solution of the hydroxypropyl methyl cellulose was sprayed into the fluidized bed through a nozzle . the feed additive prepared according to example a had the following particle size distributions : ______________________________________screen size ( μm ) proportion (%) ______________________________________quantity of polymer 5 %: & lt ; 250 8250 - 500 32 500 - 1000 60quantity of polymer 20 %: 250 - 500 6 500 - 1000 60 & gt ; 1000 34______________________________________ the following example shows that the particle distribution can be shifted towards smaller particles at the same spraying rate by using an organic solvent ( methanol / methylene chloride = 1 : 1 ): ______________________________________quantity of polymer 10 %: screen size ( μm ) proportion (%) ______________________________________ & lt ; 125 30125 - 250 20250 - 500 35 & gt ; 500 15______________________________________ the composition of the feed mixtures tested is shown in table i below . the coating in recipes r 2 and r 3 was prepared according to the preferred embodiment of the process according to the invention , but in recipe r 1 the starting material was not treated . the feed mixtures were prepared by making 2 preliminary mixtures in a mortar and carrying out the final mixing in a cube mixer . piglet fodder ii ( composition according to the fodder regulations of may 1983 ) was used as the fodder since it had a particularly high content of iron , copper , manganese and zinc . table i______________________________________composition of the feed additive mixtures tested ( values in g ): r . sup . 1 r . sup . 2 r . sup . 3______________________________________zinc bacitracin 20 % calcium carbonate 20 % 0 . 25 0 . 25 0 . 25fermentation residue 60 % hydroxypropylmethyl - -- 0 . 0125 0 . 05cellulosepiglet fodder ii 999 . 75 999 . 7375 999 . 70total 1000 . 00 1000 . 0000 1000 . 00______________________________________ the samples were stored for 4 weeks at the following temperatures : rt ( room temperature , about 21 ° c . ), 31 ° c ., 41 ° c . and 51 ° c . they were then tested microbiologically ( official journal of the european communities , jan . 18 , 1984 , no . l 15 / 28 ). the results of the microbiological investigations are shown in table ii below : table ii______________________________________residual content of zinc bacitracin as a function of thecomposition and storage conditions ( values in %) temperature (° c .) r . sup . 1 r . sup . 2 r . sup . 3______________________________________rt 90 . 4 96 . 7 96 . 231 52 . 9 70 . 0 81 . 441 21 . 4 60 . 0 59 . 551 7 . 6 46 . 2 59 . 5______________________________________ these results indicate that zinc bacitracin in fodder is stabilized by hydroxypropyl methyl cellulose . compared with the composition r 2 sprayed with 5 % polymer , r 3 ( 20 % polymer ) is marginally better . in addition to these tests at unfavorable elevated storage temperatures , the long - term characteristics of fodder according to the invention and standard commercial fodder containing zinc bacitracin were tested . unless otherwise stated , the polymer concentration was 20 % based on a feed additive consisting of 20 % zinc bacitracin , 20 % calcium carbonate and 60 % fermentation residues . table iii shows the residual content of zinc bacitracin in a sole fodder for fattening chickens ( type 7 . 7 ) with an initial concentration of zinc bacitracin of 50 ppm after storage for 1 , 3 and 6 months at various temperatures . in order to investigate the effect of pelleting , ( steam conditioning ) is which the feed mixture is briefly exposed to high temperatures ( up to 110 ° c .) and high humidity , the batches were subdivided ; half were then pelleted and subsequently subjected to the same storage conditions as the non - pelleted samples . as is shown by tests r 4 to r 7 , the samples coated with hydroxypropyl methyl cellulose ( r 5 and r 7 ) have a higher content of bacitracin than the untreated samples r 4 and r 6 in every case , even after prolonged storage . table iii______________________________________residual content of zinc bacitracin in a fodder for fatteningchickens ( type 7 . 7 ) with an initial content of 50 ppm ( values in %). r . sup . 6 r . sup . 7stored r . sup . 4 r . sup . 5 ( untreated , ( hpmc , at ( untreated ) ( hpmc ) pelleted ) pelleted ) ______________________________________1 month rt -- -- 36 65 26 ° c . -- -- 24 602 months rt 54 93 23 47 26 ° c . 32 77 8 336 months rt 47 95 17 47 26 ° c . -- 73 -- 20______________________________________ hpmc = hydroxypropylmethyl cellulose rt = room temperature 21 ° c . table iv______________________________________residual content of zinc bacitracin in a fodder for fatteningpigs ( type 2 . 6 ) with an initial content of 20 ppm ( values in %). r . sup . 9 r . sup . 11 stored r . sup . 8 ( untreated , r . sup . 10 ( hpmc , at ( untreated ) pelleted ) ( hpmc ) pelleted ) ______________________________________initial 100 66 100 100value6 months rt 79 52 100 73______________________________________ as is shown by a comparison of the initial values of tests r 8 to r 11 in table iv , the loss of activity of zinc bacitracin in an untreated sample ( r 8 and r 9 ) is already 30 % at the pelleting stage , whereas the sample r 10 coated with hydroxypropyl methyl cellulose shows no measurable loss of activity caused by pelleting ( r 11 ). even after 6 months &# 39 ; storage , the feed additive according to the invention has a higher zinc bacitracin activity than the untreated feed additive . table v______________________________________residual content of zinc bacitracin in a fodder for laying hens ( type 7 . 4 ) with an initial concentration of 100 ppm of zincbacitracin after 6 months &# 39 ; storage ( values in %). r . sup . 13 r . sup . 15 stored r . sup . 12 ( untreated , r . sup . 14 ( hpmc , at ( untreated ) pelleted ) ( hpmc ) pelleted ) ______________________________________6 months rt 77 28 100 70 26 ° c . -- -- 96 58 31 ° c . -- -- 88 42______________________________________ in another test arrangement , both untreated feed additive and also feed additive containing zinc bacitracin and coated with hydroxypropyl methyl cellulose ( hpmc ), methyl cellulose ( mc ) and hydroxypropyl cellulose ( hpc ) was stored for 4 weeks in glass and polyethylene containers ( pe ) at 21 ° c . and at a relative humidity of 60 %. in this series of tests , the polymer concentration was varied between 10 and 20 %. the results are shown in table vi . table vi______________________________________residual content of zinc bacitracin in a fodder for fatteningchickens ( type 7 . 7 ) with an initial content of 50 ppm ( values in %): initial value initial ( non - ( non - value pelleted ) ( pelleted ) test polymer pelleted ) ( pelleted ) glass pe glass pe______________________________________r . sup . 16untreated 77 68 48 41 19 24r . sup . 17hpmc 10 % 100 94 80 94 53 60r . sup . 18hpmc 15 % 97 85 79 96 61 69r . sup . 19hpmc 20 % 96 82 77 97 70 73r . sup . 20mc 10 % 96 80 93 92 63 68r . sup . 21mc 20 % 100 82 73 93 84 67r . sup . 22hpc 10 % 95 85 92 74 54 51r . sup . 23hpc 15 % 99 86 88 90 64 55r . sup . 24hpc 20 % 99 81 93 84 65 54______________________________________ table vii shows the 4 weekly values of a feed additive coated with polyacrylate . the polymer content was 30 %. table vii______________________________________residual content of zinc bacitracin in a piglet fodder ii with aninitial content of 50 ppm ( values in %): r . sup . 25 ( 30 % eudragit . sup . r e 100 ) temperature (° c .) 4 weeks______________________________________21 ° c . 7131 ° c . 6341 ° c . 47______________________________________ the above examples demonstrate the stabilization of zinc bacitracin in feed mixtures by treatment with - the coatings according to the invention compared with untreated feed additives . preparation of a feed additive according to the invention , using the preferred process . batch : 240 kg 200 kg of commercially available feed additive in the form of a powder consisting of 20 % zinc bacitracin , 20 % caco 3 and 60 % fermentation residues were sprayed with a solution of 40 . 0 kg of hydroxypropyl methyl cellulose in 531 . 42 liters of purified water in a fluidized bed granulator ( type wsg 200 made by the glatt co ., fitted with a lifting piston pump , type hl 1l made by the lewa co .). then , 3 liters of purified water were sprayed into the fluidized bed . after spraying had ended , the air heating device was turned off and the product was cooled to a temperature of 45 ° c . at the start of spraying , the feed additive was present as a fine compact powder which formed a light , loose granulate in the course of the spraying phase . the formation of granules with a resultant substantial increase in volume must be taken into account in the batch size . therefore , spraying must also be started with a relatively small quantity of air and increased during the spraying phase as more and more granulate is formed . after spraying had ended , the granules were screened in a screen granulator made by the frewitt co . with a screen insert having a mesh size of 1 . 0 mm ( rotating , moderate speed ), and then mixed for 100 minutes in a rotating spit mixer made by the lermer co . while the present invention has been illustrated with the aid of certain specific embodiments thereof , it will be readily apparent to others skilled in the art that the invention is not limited to these particular embodiments , and that various changes and modifications may be made without departing from the spirit of the invention or the scope of the appended claim .
a novel zinc bacitracin -- containing animal feed additive , in particulate form , has been discovered having an externally coating with a polymer such as an alkyl cellulose polymer . the novel composition shows significant improvement in stability with respect to long term storage over a wide range of temperatures and in admixture with a wide range of animal feeds , and a substantial reduction in odor . in a preferred embodiment , the zinc bacitracin -- containing feed additive comprises zinc bacitracin , a fermentation residue and calcium carbonate . a novel process for producing the zinc bacitracin containing feed additive minimizes the formation of dust sized particles .