Milk composition and synthesis in dairy goats and sheep

Abstract Text:

Milk and dairy products from sheep and goats are of relevant economic importance worldwide and their demands are continuously increasing due to nutritional, functional, ethnical and gourmet reasons. Both species markedly differ in milk composition and mammary gland secretion process, which modifies the characteristics of their milk and the organoleptic traits of the produced dairy products. On average, ewe’s milk is richer in fat and protein than does’ milk, their secretion process being mainly merocrine and apocrine, respectively, which are responsible of the shedding of cytoplasmic particles and of the somatic cell counts in the milk, usually being greater in goats than in sheep. Moreover, the cisternal udder compartment is proportionally larger in goats than in sheep, requiring modifications in the milking machine settings, milking routines and varying their tolerance to extended milking intervals. Effects of genetics (i.e., breed and selection) and feeding (i.e., use of high concentrate diets and supplementation) on milk composition play an important role since sheep and goat milk are mostly transformed into dairy products. Genetic selection, especially in dairy goats, resulted in vast improvements in milk yield and lactation length but, in most cases, accompanied by decreased milk composition and higher risk of fat-protein contents inversion. Milk protein content is hard to modify in both sheep and goat, the casein being regulated by the same genes (CN locus) in both species, although goats show a greater degree of polymorphisms which are markedly related to curd and cheese traits (i.e., α_S1-CN, κ-CN). An interaction between CN genotype and protein content of the diet has been reported in dairy goats. On the contrary, milk fat content increases rapidly and fatty acid profile varies after fat supplementation (e.g., oilseeds, calcium soaps and vegetal oils) in sheep and goats, with low or not significant effects on milk protein. Feeding fresh forages and vegetal oils to sheep and goats markedly increase milk c9, t11CLA, the effect being not significant when whole oilseeds are fed. Use of t10, c12 CLA reduces milk fat synthesis and increases milk yield in lactating goats and sheep, without altering milk protein and lactose contents. Although sheep and goats are moderately tolerant to harsh environments, milk fat and protein contents decrease markedly by effect of heat stress, which also impairs curd formation and cheese-yielding. 

Keywords: Goat, Sheep, Lactation physiology