Milk production is largely dependent on the shape of the lactation curve. Relevant elements of the
lactation pattern are the peak yield, which represents the maximum milk yield during the lactation,
and the lactation persistency, which expresses the ability of animals to maintain a reasonably constant
milk yield after the lactation peak. Thus, persistent animals are those that show flatter
lactation curves. Several measurements of persistency have been proposed (Broster and Broster, 1984;
Gengler, 1996): the rate of fall of milk yield per week or per month; combinations of parameters of
mathematical functions used to model the lactation curve; the variation of test day yields throughout
the whole lactation or part of it; the proportion of total milk yield achieved in a certain period (e.g.
second half of lactation). However, none of the above mentioned measurements seems to be able to
become the reference method (Grossman et al., 1999). For example, the definition of persistency as
the rate of fall of milk yield per unit of time can be misleading if the absolute level of production is
not considered. Usually curves with high peak yield show low persistency because the rate of milk
yield declines faster in animals that have a fast milk yield increase after calving. Thus in
this review, we will consider persistency in a broad sense, and we will analyze the nutritional and nonnutritional
factors that affect and limit milk production in mid-late lactation in sheep
