6 minute read
Genetic and Genomic Studies on Production, Composition and Processability Characteristics of Dairy Sheep Milk
The New Zealand dairy sheep industry has had significant growth over the last seven years and is an alternative to traditional dairy cow farming.
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The high nutritional value of sheep milk is mainly attributed to the high content of unsaturated fatty acids, minerals, and protein . The higher content of milk solids (fat, protein, and lactose) in sheep milk also makes it ideal for processing into dairy products such as cheese . The lower carbon footprint and the smaller nitrogen leaching effect produced by dairy sheep when compared to that of dairy cows are also important aspects to consider as New Zealand aims to market environmentally sustainable farming systems . The characteristics of milk prior to processing is determined by several factors such as the animals` feed and genetics, farm management, and animal health . However, there has been very limited research done on the molecular genetics (identification of genes that determine traits of economic interest) in dairy sheep milk . Most dairy sheep breeding programmes have focused mainly on improving milk yield, leaving aside other traits of economic relevance such as milk composition, milk processing value, ewe udder morphology, milk somatic cell count, ewe machine milking ability, and ewe resistance to diseases . Selection of dairy sheep solely for milk yield can indirectly result in reduced fertility, impaired udder morphology and reduced milk solids . Udder characteristics must be considered in the breeding programme as selection of dairy ewes for high milk yield has been related to pendulous udders, which is not a desirable trait for machine milking nor for suckling lambs . Furthermore, udder scores were reported to have high heritability and can affect coagulation traits which can be exploited in selection indexes . Milk processability traits have not yet been included in selection schemes (for most ruminant species) although heritability of coagulation traits (important indices of product yield and quality i .e ., cheese yield and quality) in sheep milk has been reported to be moderate and could be included in selection schemes . Heritability of somatic cell counts, lactose content and milk pH were moderate and were significantly correlated with coagulation traits in sheep milk, so there is also potential in including these traits .
Black ewe and her lamb in the yards at Kingsmeade.
PhD student Ana
Carolina at the Food
Characteristics Lab,
Massey University, for analysis of milk coagulation traits.
Overall, the New Zealand dairy sheep industry needs a breeding program for selection of genetically superior animals for production and for milk characteristics . Although application of molecular genetics is costly and recording of production data/traits is time demanding, using genes to assist in selection for traits in a small number of animals (nucleus) for further expansion of genetics through breeding and transfer to commercial flocks can result in faster genetic gain, which is attractive for the industry . Therefore, the aim of this study is to develop a model for genomic selection of dairy sheep for milk production, composition and processability . This study will also investigate a faster and cheaper method to achieve coagulation traits for inclusion in genetic schemes . The calibration of the spectra generated by infrared spectroscopy equipment could enable indirect measurements of coagulation traits and of fatty acid profiles from individual milk samples . It is also important to enhance the need for more accurate and efficient milk sampling and recording systems at the farm level as most milk meters designed for sheep or goats collect a very small percentage of the daily milk volume which is insufficient for research purposes that usually require 100-200 mls of milk .
This study is being conducted at Kingsmeade, Masterton, New Zealand . Whilst most New Zealand dairy sheep farms have relied on the use of new imported genetics from European countries (mainly East-Friesian and Lacaune), Miles and Janet King, have been selecting from an original flock of cross-bred East-Friesian sheep since 1998 . They have registered New Zealand’s first dairy breed of sheep as Dairymeade . Their selection has been based mainly on temperament, health, and lactation length on a pasture-based system with low supplementation . Therefore, these animals are genetically adapted for milk production under the New Zealand environment (i .e ., resistant to diseases, efficient in converting feed, and adapted to the climate) when compared to the new imported European genetics . They are considered ideal for the current study . The current study forms the PhD of Ana Carolina Marshall, a postgraduate student at Massey University . A total of 225 ewes have been enrolled in the current study and milk samples are being collected from each ewe three times during the production season of 20212022 . The milk samples are taken to the laboratories at Massey University, where they are analysed for composition and for processability characteristics . A small skin notch sample was collected from each animal and DNA was isolated and scanned . Animal information (i .e ., year born, pedigree, coat colour, udder score), test-day records for milk volume, milk composition (fat, protein, and lactose), live weight and milk processability characteristics will be used to perform genome-wise association studies (GWAS) for the identification of chromosomal regions affecting these traits . The genome-wide association study (GWAS) aims to identify genetic variants (genotypes) that are associated with specific traits (phenotypes) . It enables the investigation of genetic markers across the whole genome of a large number of individuals and predicts genotype-phenotype associations by statistical analysis at the population level . This knowledge will be used to increase the accuracy of genetic evaluation of dairy sheep, which will allow the identification of the best animals at early age creating the opportunity to increase the rate of genetic gain in the right direction for the traits of economic importance in dairy sheep . From the data obtained so far from over 300 test-day records, it is already possible to see variation and a large gap between low and high producing animals in levels of milk yield, milk fat content and in milk processability characteristics which is evidence that there is potential for improvement of the herd through selection of the ewes with the best production and milk processibility traits . This project is part of the MBIE-funded New Zealand Milks Mean More (NZ3M) Endeavour programme which aims to obtain a better mechanistic understanding of natural variations in New Zealand bovine and nonbovine milks .
Opposite Page: Test buckets attached to the milking line for milk sampling and recording of milk yield on test-day.
Black Dairymeade sheep after being milked, in the milking shed. PhD student Ana Carolina performing DNA extraction from sheep skin samples. Miles (right) and farm assistant Andrew (left) during normal milking routine at Kingsmeade.
For more information on the NZ3M programme, please contact Professor Warren McNabb at w.mcnabb@massey.ac.nz or +64 6 951 7742.
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