In essence, each class of cattle has defined nutritional needs for the task and level of production you expect it to achieve and uses the nutrients it takes in each day in a certain order of priority.
This hierarchy is illustrated below. An animal will use its daily intake of nutrients in this order beginning at the bottom and moving up the hierarchy. Performance stops when the nutrients run out.
Notice that parasites take their share of nutrients first. This is likely happening right now in herds in our area with lice infestations.
Although lice spend their entire life cycle on the host and are present year-round, their numbers begin increasing in late fall and peak from winter until late spring when cattle begin to shed their long winter coats.
Determining the economic loss from an infestation is difficult. A relatively low population can cause irritation and itching, which causes the animal to expend energy scratching and licking.
This extra activity can lower gain in calves and stockers and reduce milk production in cows. Even moderate persistent scratching can also cause reduced value due to hide damage.
A severe infestation can lead to lower immunity, anemia and even death. Problems with lice add to other common challenges during the winter, such as increased nutrient requirements and weather-related stress.
Some researchers say that as few as three lice per square inch justifies treatment, but treatment can be tricky. For instance, some products can’t be used on calves or lactating cows.
Others may also kill cattle grubs, which can be disastrous during winter. Many products require subsequent treatments to control the newly hatched nymphs.
As with other parasites, inappropriate treatment and diagnosis can lead to buildup of a resistant population.
Often, producers will let lice populations decline naturally as cattle shed their winter hair, but the major damage has already been done by then.
If cattle exhibit the classic symptoms of excessive scratching, a rough appearance and/or reduced performance, it is worth the time and effort to run a few through the chute to look for lice and treat accordingly. As always, read and follow label directions. 
PHOTO
TOP: Notice that parasites take their share of nutrients first. Photo by Colorado State University Extension.
