Genetic and genomic studies of production, composition, and processability characteristics of milk from dairy sheep : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Manawatū, New Zealand

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dc.confidentialEmbargo : No
dc.contributor.advisorLopez-Villalobos, Nicolas
dc.contributor.authorMarshall, Ana Carolina
dc.date.accessioned2025-03-24T21:21:12Z
dc.date.available2025-03-24T21:21:12Z
dc.date.issued2024-10-20
dc.description.abstractThe main objective of this study was to perform genetic and genomic studies of sheep milk production, composition, and processability, particularly concerning cheese-making aptitude (milk coagulation properties) of sheep milk. The milk traits were investigated in a flock of 169 lactating ewes producing on a pasture-based system in Masterton, New Zealand. The total yield during the 2021-2022 season was 86.1 kg/ewe in 130 days of milking after the suckling period. The effects of animal factors on the lactation curves, and on milk composition and processability (milk coagulation properties, individual laboratory cheese yield, and heat coagulation time) throughout the season were investigated and discussed. Stage of lactation significantly (p < 0.05) influenced processability. In late lactation, the rennet coagulation time was longer, the curd at 30 minutes after rennet addition was softer, and milk heat stability was lower. In addition, milk protein polymorphisms were shown to influence milk composition and protein composition. Particularly, heat stability was affected by β-lactoglobulin polymorphism. The milk processability traits were also significantly associated with protein composition. The heritability estimates for milk production, composition, and processability traits ranged from 0.12 to 0.48. The genetic correlations obtained indicate that genetic improvement of this flock for higher yields of fat, and protein, and for lower somatic cell score, should indirectly improve milk coagulation traits in this flock. However, other traits such as milk pH, percentage traits (protein, casein, and lactose percentages), ratio of casein to protein, calcium, and ratio of casein to calcium were more strongly correlated with processability. The genome-wide association study performed on 149 dairy sheep genotyped with 50K SNPs Bead Chips, revealed a total of 87 SNPs and 55 candidate genes across Ovis aries autosomes 2, 3, 6, 16, 18, 20, 25, and 26. The genetic architecture of milk coagulation traits was similar to that of the ratio of casein to calcium, pH, lactose, and the ratio of casein to protein. The genetic correlations and identification of potential genes associated with the control of these milk traits provide valuable insights for the selection of superior dairy sheep in New Zealand. The findings of this thesis need to be validated with a systematic large-scale recording scheme before developing a selection index for dairy sheep populations in New Zealand.
dc.identifier.urihttps://mro.massey.ac.nz/handle/10179/72594
dc.publisherMassey University
dc.rights© The Author
dc.subjectdairy sheep, milk production, milk composition, milk processability, milk coagulation properties, genetics, genome-wide association
dc.subject.anzsrc300302 Animal management
dc.subject.anzsrc310509 Genomics
dc.titleGenetic and genomic studies of production, composition, and processability characteristics of milk from dairy sheep : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Manawatū, New Zealand
thesis.degree.disciplineAnimal Science (Animal Breeding and Genetics)
thesis.degree.nameDoctor of Philosophy (Ph.D.)
thesis.description.doctoral-citation-abridgedMrs. Marshall investigated the genetic bases and genomic factors influencing milk production in dairy sheep and the suitability of milk for processing into dairy products like cheese. The results indicated that selecting dairy sheep for higher fat and protein yields would indirectly enhance the cheese-making aptitude of the milk. This insight could be applied to animal breeding programmes aimed at improving dairy sheep production in New Zealand.
thesis.description.doctoral-citation-longThe New Zealand dairy sheep industry is relatively new and requires animals that are both profitable for the farming business, and that are suited to the local climate and farming systems. Currently, no formal breeding programmes for dairy sheep exist at an industry-wide level. However, informed breeding decisions are essential to manage undesirable genetic correlations such as undesirable consequences on processability. This study examined the genetic bases and genomic factors influencing milk production, milk composition, and cheese-making aptitude of milk (milk coagulation properties). Research on 169 lactating ewes in a pasture-based system in Masterton, New Zealand (2021–2022 season) found an average yield of 86.1 kg/ewe estimated for 130 days in milk. It was found that late lactation reduced milk coagulation efficiency. Milk protein polymorphisms influenced milk heat stability and protein composition. The heritability estimates (0.12–0.48) suggested genetic improvement potential for the traits investigated. The genome-wide association study identified 87 SNPs and 55 candidate genes linked to milk production, composition, and processability traits. These insights support the development of dairy sheep breeding programmes in New Zealand, though large-scale validation is needed before implementing a selection index.
thesis.description.name-pronounciationAna Carolina Marshall
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