Supplementation strategies and dairy cow genotypes influencing milk production, composition and processability and nitrogen utilisation efficiency in lactating dairy cows

The objective of this thesis was to investigate the effects of supplementation strategies and dairy cow genotypes on milk production, composition and processability and nitrogen utilisation efficiency (NUE) in lactating dairy cows. In Chapter 3, higher milk genotype cows had an increased milk response to concentrate supplement (CS) compared to lower milk genotype cows. This result highlights the potential of genotype and CS to improve milk production where cows are pasture-based and are in late lactation. Chapter 4 investigated effects of CS and genotype on milk composition and processability in late lactation, spring calving grazing dairy cows. Results show that offering cows CS decreased milk fat and protein concentrations but increased milk ethanol stability. In summary, offering cows extra supplemental energy through CS may improve milk colloidal stability in late lactation. Chapter 5 found that decreasing the supplementary crude protein (CP) concentration offered to cows, from 18% to 14%, had no negative impact on milk production over the main grazing season. However, NUE was reduced in cows that were offered the 14% CP CS due to the increased pasture dry matter intake in cows offered the 14% CP CS. These results are important considering the current European Union Nitrates Derogation rules limiting CS to a maximum of 15% CP. In Chapter 6, cows that were offered a 13% CP CS had decreased milk production and milk urea N concentration compared to cows offered a CS with 18% CP despite both CS having equal PDI (protein digestible in the small intestine). Decreasing the supplementary CP concentration had a positive impact on N partitioning. These results show that while milk production is decreased, N partitioning to urine can be reduced with lower supplementary CP, which is a positive result from an environmental viewpoint. Chapter 7 concluded that offering cows the decreased supplementary CP concentration of 13% did not affect milk fat or protein concentration or milk processability compared to cows offered the increased supplementary CP concentration of 18% CP. Under the conditions of this study, results show that on farm management strategies such as genetic selection for higher milk genotype and decreasing the supplementary CP concentration do not impact milk processability in late lactation. In conclusion, the research conducted in this thesis shows that milk production, composition and processability and N partitioning can all be influenced by supplementation strategies, whilst milk production and composition can be influenced by dairy cow genotype.