26 January 1996

How factors for ewe milk yield relate

THE time-old expression "milking off her back" is likely to conjure up a hilarious cartoon image within an imaginative townies mind.

But the biological practicalities lying behind the utterance are essential to a lambs early viability and growth.

Research by Dr John Robinson at the Scottish Agricultural College reveals how a ewes milk yield is directly linked to current intakes of feed, in the form of energy, as well as body-fat reserves. The latter are built up by correct feeding during pregnancy.

"An important feature of early lamb growth is the relatively large amount of milk required for maintenance," says Dr Robinson.

His work reveals that twin lambs, each weighing 6kg, require about 1.3kg of milk, daily, to maintain their weight in a temperate environment.

This increases to 1.7kg when lambs gain 150g a day and 2.9kg when daily liveweight gain is 300g a day.

Adequate protein

A trial demonstrated when 70kg ewes rearing twins received an adequate intake of protein in a diet supplying 20MJ of metabolisable energy (ME) – twice that required for maintenance – during early lactation, milk yields ranged from 2.1kg a day for those with 5kg of body fat to 2.8kg for ewes carrying 20kg of body fat (condition score 3.5).

These milk yields translate into daily lamb growth rates in each of the twin lambs of 180g and 280g a day, respectively.

Daily estimates of body fat lost by "thin" ewes were 105g against 360g for those which were better insulated.

When ME intake increased to 25MJ, milk yields improved but the influence of the ewes body fat reserves on milk yield reduced. The lambs daily liveweight gains were 260g and 310g, with 60g and 190g of body fat lost daily by thin and fat ewes, respectively. When ME intake increased to 30MJ, fat content on the ewes body had no significant effect on milk yield or lamb growth rate.

"These inter-relationships between current nutrition, body fat reserves and production provide a range of options for the allocation of feed resources during pregnancy and lactation," says Dr Robinson.