MILKING

Milk yield was estimated by the so-called "oxytocin method" described by McCance and Alexander (1959). The technique involves emptying the udder by milking with the aid of oxytocin and repeating the milking procedure a measured time later, at which time the milk yield is measured. The lambs are separated from the ewes during the intervening period. Advantages and constraints associated with this method will be discussed in Chapter 7.

2.3.1 PREPARATION AND RESTRAINT

Lambs were separated from their mothers between 0830 and 0900 h each morning and kept separate until after the afternoon milking. Ewes housed indoors were milked in their pen. Each ewe was restrained by one person while being milked by another. Ewes grazed at pasture were brought into the shed and milked on a raised platform holding 4 or 6 ewes. In this case 2 ewes could be restrained by one person while being milked. It was found necessary to have a person to restrain the ewes during milking because they were not able to be trained, as dairy animals are, to accept the milking process calmly.

2.3.2 OXYTOCIN

To ensure milk letdown it was necessary to use oxytocin. The recommended dose of 10-15 i.u. was tested and found to be excessive, resulting apparently in severe discomfort or pain in the area of the mammary gland as evidenced by the abnormal behaviour of the ewes. Successively lower doses were tried until letdown was observed to be compromised at a dose of 0.5 i.u.. It was also difficult to ensure accuracy of delivery of such a low volume (0.05 ml) with a 1 ml syringe. Accordingly 1 i.u. (0.1 ml) was adopted as the effective dose. This was diluted with 0.9 ml of physiological saline to facilitate injection. Injections were administered into the jugular vein 0.5-1 min before milking commenced.

2.3.3 THE MILKING MACHINE

The milking machine was a small, portable, electrically powered unit, designed to milk one cow or two sheep or goats, manufactured by OTENZ, (Otorohanga, New Zealand). The pulsator frequency was set at 60 pulses/min and the vacuum at -40 kPa. Milk lines were adapted to collect the milk from each ewe, and in some trials, from each gland, in separate containers. The machine and milk receptacles were mounted on a trolley for mobility. After each milking the machine was rinsed with cold water and, following the afternoon milking, hot water and detergent followed by a hot water rinse were used.

2.3.4 MILKING PROCEDURE

Milking was carried out only by experienced milkers, since the milking machine could not be relied upon to extract all of the available milk. Despite the use of oxytocin and the machine, differences in letdown, ewe behaviour and gland anatomy (especially teat shape and placement as well as streak canal size) led to large errors in milking if hand-stripping was not correctly done. Milkers learned the peculiarities of each ewe in order to ensure that all the milk was removed. When in doubt, completeness of milking was checked by another milker. If still in doubt at the afternoon milking, the previous day's records were consulted to ensure that the amount obtained was about the expected weight. On a few occasions, when the amount obtained was far below that expected, another dose of oxytocin was administered and the milking process repeated. Due to the potential for error, milking and oxytocin injections were done only by skilled milkers, and the number of such staff was kept to a minimum (usually 3 in each trial) to ensure consistency of technique. The milking routine was as follows;

Morning milking:

First, the lambs were separated from the ewes. About 1 minute before milking each ewe was injected intravenously per jugular with 1 i.u. (0.1 ml) oxytocin made up to 1 ml with physiological saline. The teat cups were then put on and the ewe machine-milked for approximately 100 seconds. During milking the cups were held on by hand and moved rhythmically in time with the pulsation to assist milk letdown, and aid blood flow to the teats. This was, in fact, a mild form of machine stripping. Following the removal of the cups each gland was hand-stripped until empty. The time at which milking ended for each ewe was recorded. Ewes housed indoors remained in their pen with access to water while their lambs were kept elsewhere. Ewes on outdoor trials were returned to pasture between milkings, while their lambs remained penned indoors.

Afternoon milking:

Following oxytocin injection, ewes were milked as above. Following the completion of milking, the milk from each gland or from each ewe, depending upon the objectives of the particular trial, was weighed. Milk weights were recorded to the nearest gram and this value was adjusted according to the time interval between milkings (recorded to the nearest minute) to obtain an estimate of the daily milk yield. The milk was subsampled for analysis of fat, protein and lactose content (see section 2.6.4). The remaining milk was saved for feeding lambs. Ewes housed indoors were then fed, and lambs were returned to them. Ewes on outdoor trials were returned to pasture with their lambs.