Livestock producers at one time or another have had a lice infestation in the herd.  When lice populations are heavy enough to cause cattle to be visibly scratching, it is apparent that cattle are not performing optimally.  Managing lice in purebred herds is important from a cosmetic sense - loss of hair can affect price at the purebred sales.

Livestock producers, whether they can actually see signs of lice or not, should consider a January or February treatment for lice, to knock populations down before the cattle get to the point of rubbing and itching.

Lice infest cattle all year, but numbers are usually low in summer because most of them are shed off in the spring with winter hair.  Also lice don't survive very well in heat. If the cow is standing in bright sunlight in summer, the temperature on the skin may go up to 115 to 120 degrees F.  Lethal limit for adults and eggs is about 104 degrees F.  Producers generally don't need to treat if a few show up in late March or early April, because the population won't grow at that late date. Even on untreated cattle, lice numbers are dropping at a rapid rate by that time.

Some animals, due to poor immune function or other factors, are more vulnerable to extensive lice populations, and transmit lice to the other cattle in the herd. This is the old 80-20 rule; about 80% of the cattle don't have lice and 20% do, for some reason. The carrier animals always have heavy loads, and general recommendation is to cull those.  Cattle with lice readily pass them to herd mates through direct contact, since cattle are social animals.