6903
Nonreturn rates can be used to derive more elementary biological measures for reproductive efficiency , such as conception rate and calving rate , which separately might be more reliable than nonreturn rate itself to evaluate the fertility of a bull or the performance of an AI technician .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: GENE 
Text: {0} 
Answer:
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