Animal as the solution: use of dairy cows divergent for milk urea nitrogen breeding values as a potential mechanism for reducing the environmental impact of pastoral dairy production practices: A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

The purpose of the work described in this thesis was to demonstrate an animal-based solution to an environmental problem currently facing the pastoral agricultural sector. High levels of nitrogen loading as a result of high concentrations of urea in the urine of dairy cows have been associated with widespread environmental degradation, such as reduced water quality and N2O emissions. Known linear relationships exist between the concentrations of urea in urine and urea in milk, with milk urea nitrogen (MUN) concentration considered a moderately heritable trait. The primary hypothesis of this research was that animals considered low for milk urea nitrogen breeding values (MUNBV) would have reductions in urinary urea nitrogen (UUN) concentration that could be associated with reductions in environmental impact. The primary objective of this research was to provide supporting evidence for the primary hypothesis. Multiple experiments were conducted, and are detailed across several chapters in this thesis, testing the primary hypothesis and providing supporting evidence as to potential physiological mechanisms that may explain any observed relationships. Chapter 3 presents an experiment using 48 lactating Holstein Friesian × Jersey cows considered divergent for MUNBV in a grazing setting and found that per unit decrease in MUNBV the concentration of UUN decreased by 0.67 ± 0.27 g/L. Chapter 6 reports the results of an experiment conducted as a metabolism crate study using 16 lactating Holstein Friesian × Jersey cows considered divergent for MUNBV. The experiment detailed in Chapter 6 reported a consistent 28% reduction in average UUN concentration between high and low MUNBV cows when consuming a ryegrass diet and was thus in agreement with the findings of the experiment presented in Chapter 3. Chapter 7 reports a plot of the diurnal pattern of the results from the experiment presented in Chapter 6 and suggests that low MUNBV cows when consuming ryegrass consistently for all hours of the day had a lower concentration of UUN than high MUNBV cows on the same diet. The results reported in Chapters 3, 6, and 7 provide supporting evidence to accept the main hypothesis of this thesis and conclude that low MUNBV cows do have reductions in UUN concentrations. Reductions in UUN concentrations from low MUNBV cows can be associated with reductions in N losses to the environment and therefore lower environmental impact from low MUNBV cows compared with high MUNBV cows when consuming ryegrass diets. The experiment presented in Chapter 4 investigated the grazing behaviour of the cows used in the experiment presented in Chapter 3. The results presented in Chapter 4 suggest that a one-unit decrease in MUNBV resulted in a 0.05 ± 0.02 increase in mastications per prehension, indicating that low MUNBV cows have a greater level of oral processing than high MUNBV cows and have altered grazing strategies and nutrient acquisition. Increased oral processing from low MUNBV cows is likely to alter rumen fermentation dynamics by resulting in faster rumen outflow rates. The hypothesised differences in rumen function as a result of different oral processing is considered the likely explanatory factor behind the observed phenotypical differences between dairy cows considered divergent for MUNBV reported in this thesis. Dietary interactions presented in Chapters 3, 6, and 7 explored the effects of grazing either a plantain (Plantago lanceolata L.) or ryegrass (Lolium perenne L.) based diet and the interactions with MUNBV. Inconsistent results were found by diet where the effects of MUNBV were detected for cows consuming ryegrass diets but not detected when cows consumed plantain diets. Chapters 4 and 7 provide evidence that the lack of effect from MUNBV on plantain diets is likely due to the ability of plantain to be easily masticated, therefore allowing the high MUNBV cows which are less efficient at oral processing to achieve similar oral processing and subsequent rumen function to the more efficient low MUNBV cows. It is speculated in Chapter 6 that the MUNBV values are based on a genetic model whose parameters are based on cows grazing predominately ryegrass white clover pastures and therefore may need re-parameterisation for different diet types. The acceptance of the main hypothesis of this thesis and identification of a likely mechanism causing the observed differences in MUNBV achieves the purpose of these studies to demonstrate an animal-based solution to an environmental problem.