Abstract
Effects of 4 different milking intervals (8, 12, 16, and 24h) on milk yield and milk composition were studied in Tunisian Maghrebi dairy dromedaries (n=6) at late lactation [240±14 days in milk (DIM), 5.84±1.62 L/d]. Camel-cows suckled their calves for 2 mo, were hand milked while suckling until mo 4 of lactation (calf weaning) and machine milked thereafter. Intravenous injection of oxytocin was administered before machine milking at each experimental milking to induce complete milk ejection and to avoid carryover effects of milking intervals. Cisternal and alveolar milk were measured at 380±16 DIM for a 24-h milking interval. Milk accumulated logarithmically (R2=0.95) in the udder from 8- to 24-h milking interval without reaching a plateau. Consequently, milk secretion rate decreased exponentially (R2=0.93) according to milking interval. Compared with 12-h milking interval (6.1 L/d), estimated daily milk yield was 113, 87, and 70% for 8-, 16-, and 24-h intervals, respectively. Total milk solids, milk fat content, and milk pH decreased with increasing milking interval, showing the greatest value at 8-h intervals (14.1±0.4%, 4.6±0.5%, and 6.66±0.05, respectively) and the lowest at 24-h intervals (12.3±0.9%, 2.9±0.6%, and 6.54±0.02, respectively). Milk protein (3.9±0.1%), lactose (4.5±0.2%), ash (0.84±0.01%) and density (1.028±0.01) remained constant for all milking intervals. Milk K, Ca, and Mg contents increased as milking interval increased, but Na content did not change (0.06±0.01%, on average). Milk Na:K ratio tended to decrease from 0.35 (1:2.9) to 0.22 (1:4.5) for the extreme milking intervals. Plasma lactose concentration steadied from 8- to 16-h (67±32 μmol) but increased dramatically at 24-h intervals (338±118 μmol), indicating that mammary tight junctions became permeable after 24h of milk accumulation. Camel udders showed small cisterns (19.3% of total milk in the udder at 24h) when compared with other dairy animals; we recommend the use of prestimulation for machine milking and selection for larger udder cisterns. Alveolar milk contained more fat (5.16 vs. 1.75%; SEM, 0.39%) and protein (3.23 vs. 2.73%; SEM, 0.15%) than cisternal milk. Despite the increase of plasma lactose during tight junction leakiness, the tendency for the Na:K ratio to decrease may be indicative of a camel's specific regulatory mechanism for controlling Na and K concentrations in milk and delaying the inhibitory effect of milk stasis on milk secretion rate. In conclusion, this short-term study proved the low storage capacity of the Tunisian Maghrebi camel udder but also showed their moderate ability to adapt to extended milking intervals at late lactation.