The amount of milk removed from rats during 1 hour's suckling after a separation period of 8 hours was estimated by weighing their pups. In the 1st experiment litters of 9 pups were suckled by all rats. At test sucklings held daily on days 6-15 of lactation, pups were reallocated to give 3, 6, 9, 12 or 15 pups per dam. The largest amount of milk was removed at suckling by groups of 12 pups. In a 2nd experiment, rats were constantly sucked by litters of either 3, 6, 9, 12 or 15 pups from birth to slaughter at 15 days post partum. Rats with 12 and 15 pups yielded the most milk, nearly twice as much as those suckling only 6 or 9 pups. Litters of 3 pups consumed less than 1/3 of the amount consumed by litters of 6 or 9 pups.

No abstract

The biases inherent in the use of the growth curves of the litter and of milking devices to estimate milk yield in the rat are discussed. A method is described in which the pups are separated from their dams for a period of hours and are then allowed to suck for about 60 minutes. The increase in litter weight during suckling is used as an estimate of milk secretion during the period of separation. Various components of this method were examined and it was shown that (i) the rate of milk secretion in the rat is constant for at least 16 h; (ii) the administration of oxytocin at suckling does not influence the yield at that suckling, but does increase the rate of milk removal; (iii) gland evacuation is completed within 50 to 60 minutes of suckling; (iv) the emptying of the mammary gland prior to the secretion interval increases the sensitivity of the estimate of milk yield; (v) the yield after a standard period of separation is a function of the level of hunger of the litter prior to suckling; (vi) cross-fostering and frequent randomized reallocation of pups between treatment groups eliminate biases inherent in earlier methods. The problems associated with the technique are discussed and a laboratory routine is outlined. The use of the technique in cross-over designs is advocated.

The thermoregulatory responses of Israeli Holstein cows (33 kg milk/day) were determined during 4 days at 3-hr intervals in summer and in winter in their normal environment. The mean ambient conditions in summer and winter were 31.8 and 16.8�C black globe temperature, 32 and 43 % relative humidity, and 1.6 and 1.2 mlsec wind velocity respectively. Seasonal changes in cows from winter to summer were: 0. 11�C increase in tympanic membrane temperature, 0.37 and 2.6�C increments in rectal and mean trunk skin temperatures respectively, 57 % reduction in non-evaporative heat loss, 47 % increase in sweating rate, 47 % increase in respiratory frequency, and 23 % reduction in total heat production. This indicates that the lower summer heat production is not necessarily associated with depressed milk production. Distinct nychthemeral (24 hr) cycles in vasomotor, respiratory, and sweating activity were found in both seasons. Similar maximal values were attained in both seasons. This suggests a considerable seasonal shift in thermal sensitivity. Heat-dissipating functions were activated at lower levels than in comparable climatic chamber studies. These results cast doubt on the validity of climatic chamber studies for the assessment of thermoregulatory capacity.

SUMMARYSixty-six Awassi ewes were divided into two equal groups. The sheep were milked twice daily from lambing for 22 weeks. In one group (weaned) the lambs were separated from their dams within 6 h of parturition. In the second group (suckling) the lambs sucked the residual milk after milking for 15 weeks, after which they were weaned. Milking yields, fat tests, lactational persistency and udder health were not affected by the early weaning. The suckling ewes had greater amounts of residual milk and consumed more concentrates. Further data are presented that suggest that there may be a critical period of attachment of at least 10 days following lambing, during which weaning may effect subsequent milk production. The practical implications of these results are discussed in terms of flock management and the intensification of breeding programmes.