The primary constraint to extended milking intervals of dairy cattle without loss of solids production is the storage capacity of the udder needed to accommodate the milk produced over the extended period. This constraint may be overcome by the production of more concentrated milk as the volume of production is a major determinant of udder capacity. The production of more concentrated milk is being explored by two approaches. The first exploits the natural variation in milk solids' concentration present in commercial dairy herds. The herd test records from LIC, NZDB were used to identify 2 groups of 19 high breeding index Jersey cows with either high or low milk solids content, which were subsequently established at Ruakura in June 1988 along with 19 high BI Friesians. Mean fat concentration in November 1988 was 6.61, 5.32, and 4.74 for high and low solids and Friesian groups respectively. Corresponding values for protein content were 4.34, 3.72 and 3.42%. Effective hours worth of udder capacity in October-November 1988 was 27.4, 24.8 and 22.9 hours for high solids, low solids and Friesian groups respectively. The second approach involves the use of recombinant DNA techniques to modify the expression of the a-lactalbumin gene, a key component in water secretion in milk. Alpha-lactalbumin, one of the major whey proteins and a component of the enzyme lactose synthetase, plays a major role in regulation and synthesis of lactose, which in turn is the major osmole of milk. Antisense RNA and RNA enzymes can be used for highly specific inhibition of gene expression. These gene blocking agents bind to the 'natural' mRNA and block translation of the message into a protein. An antisense RNA construct with RNA enzyme cleavage activity against the a-lactalbumin gene has been constructed and is currently under evaluation. This construct will be placed downstream to a mammary specific promoter region and inserted into the bovine genome by gene transfer techniques to produce a transgenic animal. These techniques have considerable potential as tools in physiological studies and for producing animals of superior genotypes.
Proceedings of the New Zealand Society of Animal Production, Volume 49, , 29-34, 1989
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