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318 dairy cows out of 6 dairy farms in the Allgäu area were included in a field study to test the efficacy of a single oral dose of 50 or 100 g L- tyrosine at day 8, 12 or 21 post partum through the method of a double blind trial in the cases of health traits (frequencies of anoestrus, cystic ovaries and endometritis), cyclus traits (observed heats, palpated oestrus follicles, expression of C. l.) and reproductive parameters (intervals application ? first heat, partus ? first heat, partus ? first service, partus ? last service, first ? last service, first service index, conception rate after first service and total conception rate). In detail the following results were obtained: 1. The use of tyrosine during the postpartum period makes cows coming in heat more sure: The frequency of anoestrus is decreased in the verum levels compared to the placebo levels in 5 of 6 treatment couples, the differences between the verum levels 50 and 100 g are only low. All differences are statistically not significant. 2. As the endometritis frequencies for the verum groups are higher in 5 of 6 treatment couples than for the placebo groups ? statistically not significant- it can be supposed that due to the early onset of luteal activity following tyrosine application, oestrogen induced self cleaning processes of the uterus are reduced or prevented. 3. A higher incidence for cystic ovaries was ascertained for the verum groups compared to the placebos, statistically not significant. A low frequency of cysts in the medium classes for the parameters tyrosine/ kg BW and tyrosine/ kg protein yield, in addition decreasing frequence from d 8 to d 21 in the 50 g level are showing, that a moderate aimed tyrosine input or a adequate tyrosine supply at the end of the puerperal period could be an efficient preventive step against cystic ovaries. 4. Following the tyrosine input there was found a higher frequency of oestrus follicles in all 6 verum in contrast to the placebo groups with advantages for the 100 g doses. The differences are not significant. In 5 of 6 verum groups there is an increasing frequency of palpated follicles with increasing distance from parturition. 5. The frequency of observed heats amounts 8,5 %, all direct comparisons brought higher numbers for the verum groups. The 50 g verum level dominated the 100 g level in every application point. Significance was not obtained. 6. 38,4 % of the cows were palpated with C. l. 15 d post applicationem. The frequencies of palpated corpora lutea were higher in all verum groups than in the placebo groups. The high tyrosine dosages were superior to the 50 g doses. The differences are not statistically significant. 7. The interval application- first heat was shortened in 5 of 6 compared couples for the verum groups, with severe advantages for the 100 g tyrosine groups, but lacking statistical significance. The shortening is most expressed in the verum level 2 with increasing distance from parturition. 8. The duration of postpartal anoestrus averages 41,52 d and is shortened in 5 of 6 compared couples for the verum levels. Statistical significance due to Kruskal Wallis was obtained. High tyrosine doses are superior to the low doses and the placebos. 9. The interval partus ? first insemination amounts to an average of 61,47 d and is in 4 of 6, the median in 5 of 6 direct comparisons shorter for the verum levels than the placebos, but without statistical significance. The higher tyrosine quantities have advantages over the 50 g doses and the placebos. 10. The average interval partus ? pregnancy is 89,25 d , whereby the placebos show the desirable lower values in 4 of 6 comparisons, with no statistical significance. The longest intervals are caused by the 50 g verum group. 11. The mean for the service period was calculated with 27,82 d, whereby in 4 of 6 comparisons the placebos achieve the more favourable values, but without statistical significance. 12. The first service index lies in 4 of 6 direct comparisons in the placebo groups better than in the verum groups, but there is no significance. The placebo groups show the lowest index with 1,67 and the verum 50 g groups with 1,86 the highest index. 13. The conception rate after the first service reaches in 5/6 of the comparisons in the placebo groups higher values than the corresponding verum groups. Parallel are the lowest conception rates after first service found for the parameters tyrosine/ kg BW and tyrosine/ kg protein yield caused by class 2, representing the 50 g tyrosine doses, while the placebo class 1 and the 100 g tyrosine class 3 are lying on nearly the same range. 14. In this study 93,8 % of the involved cows became pregnant. The highest total pregnancy rates were achieved by the 50 g tyrosine with 97,1 % in opposite to 100 g tyrosine level with 91,7 %. The placebos occupy a mid position. Statistical significance was not possible.