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.

the corpus luteum secretes estrogens and progesterone. the latter hormone causes changes in the uterus that make it more suitable for implantation of the fertilized ovum and the nourishment of the embryo. if the egg is not fertilized, the corpus luteum becomes inactive after 10–14 days, and menstruation occurs.

i hope this : )

a bird that nests only in a particular tree species.