EFFECTS OF MILKING FREQUENCY ON MILK PRODUCTION AND HISTOLOGICAL STRUCTURE OF UDDER IN ZARAIBI DAIRY GOATS

ABSTRACT
This work was carried out on 32 Egyptian Nubian (Zaraibi) goats to investigate both the effect of the milking frequency on milk yield and milk composition and the effect of stage of lactation on histological structure and histochemistry of the secretory mammary cells and its relationship with milk production in Zaraibi goats. Biopsies were taken surgically from the mammary gland from 3 does milked once daily (1x) and 3 does milked twice daily (2x) at the three stages of lactation, early, mid and late, for histological and histochemical studies. The histological structure showed clear differences due to milking frequency and lactation stage, being more developed at early and mid stages and twice daily milking compared to late stage of lactation and once daily milking. The number of the alveolus secretory cells per alveolus increased from the early to the mid stage of lactation by17.6% and then reduced at the late stage by 25% compared to mid stage, while no difference noticed between twice and once daily milking. Once daily milking (1x) reduced milk yield by 6%, and increased fat percentage to 4.0% compared to 3.67% in twice daily milking group (2x). Milk of once milking (1x) group contained higher percentage of total solids 11.38% than twice milking (10.93%), but milk protein and lactose did not differ between 1x and 2x milking. Lactation curve showed 32.2% increase in yield during mid stage than early stage, while late stage attained 61.3% reduction in milk yield compared to mid stage. Protein and lactose percentages did not change throughout different stages of lactation, while fat and total solids percentages showed the highest values at early stage of lactation (4.0 and 11.5%, respectively), and the lowest at mid and late stage of lactation. The total sectional areas (u/plate) of the alveoli were the smallest during late lactation (495399 u /plate) compared to that during early and mid stages of lactation (705206 and 759901u/plate, respectively). 
Numerous loci of alcaline phosphatase (AP) were apparent on the outer surface of the alveolar secretory cells at the early and mid stages of lactation—reflecting high activity of this enzyme at these two stages. This was accompanied by a high level of milk secretion reaching1778.2±38.9 and 2351.4±68.4 g/head/day, respectively.
In contrast, at the late stage of lactation, the size of alveoli was reduced and few alveoli showed weak AP activity. This coincided with the reduction in milk yield (910g/head/day). It could be concluded that stages of lactation influence the cell number and activity of mammary epithelial cells.