Tetracycline has long been added to livestock and poultry feed in the united states in order to produce rapid growth as well as protect herds and flocks from infectious diseases. why is this problematic?

It is problematic due to antibiotic-resistance that would occur. This extensive use and abuse of antibiotics result in the emergence  of resistant strains of bacteria.  Some types of bacteria that cause serious infections in humans have already developed resistance to most or all of the available treatments, and there are very few promising options in the market.

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the nitrogen cycle provides nitrogen to the ecosystem from the atmosphere, ground and oceans. nitrogen is an important component of complex molecules such as amino acids and nucleotides, which lead to the creation of proteins and dna, the building blocks of all life. plants absorb nitrogen in the form of ammonium, nitrate ion and, on rare occasions, as amino acids. animals receive nitrogen necessary for biological processes from feeding on living or dead organic matter.