Diet effect on nitrogen partitioning and isotopic fractionation (δ15N) in ruminants

Abstract

The main objective of this PhD study was to investigate the relationship between nitrogen (N) partitioning and isotopic fractionation (δ15N) in ruminants consuming diets with varying levels of N and/or water soluble carbohydrate (WSC). Nitrogen balance studies were conducted for all studies with measuring δ15N from various N sinks (e.g., milk, plasma, urine and faeces). Concentrations of blood (BUN) or milk urea N (MUN) and urinary excretion of purine derivative (PD) were also measured to provide indications to N metabolism in the whole body and rumen levels. Treatment effects were tested using ANOVA; the relationships between measured variables were analysed by linear regression analysis.

Studies of lactating cows (Chapter 4) and lactating goats (Chapter 5) explored the manipulation of WSC/N effect on N-use efficiency (NUE; milk N output/N intake), whilst studies of non-lactating sheep (Chapter 3) and lactating goats (Chapter 5) explored the manipulation of WSC/N effect on urinary N output proportion to N intake (UN/NI) and urinary N output proportion to faecal N output (UN/FN).

The increase in WSC/N increased NUE of lactating cows by 56 % through increasing energy intake and milk N output (MN), and decreasing N intake. On the other hand, the increase in NUE and MN were not shown in lactating goats study. It is believed the lack of response was mainly due to the high dietary N concentration and very high dietary WSC concentration in conjunction with unchanged DMI, compared with lactating cows study. No change in MCP was detected in lactating goats study; but a possibility of rumen acidosis resulted from excessive WSC intake should not be excluded. No change in UN/NI was observed from both the non-lactating sheep and lactating goats studies. Little dietary effect on UN/NI in lactating goats may be explained by similar reasons (i.e., high dietary N and WSC concentration and fixed DMI) led to lack of N partitioning response to dietary treatment. Calculated microbial crude protein production per unit of fermentable ME (MCP/FME) and PD analysis suggested no improvement in MCP was achieved in non-lactating sheep study. No change in UN/FN was observed in both lactating goats and non-lactating sheep studies. The opposite effect from dry matter digestibility (increased) and N digestibility (decreased) resulted in little changes in non-lactating sheep FN when WSC/N increased. On the other hand, lack of changes in MCP synthesis resulted in no changes in UN in lactating goats study.

Plasma, milk, faeces, muscle, wool and liver were enriched in δ15N compared with feed, while urine was depleted in δ15N relative to feed. It is suggested that the main reason for the enrichment of δ15N in faeces is the presence of enriched endogenous material. The majority of N in wool, liver, muscle, milk and plasma exists as true protein, which has been reported to be enriched in δ15N. In contrast, urea is the main N source of urine and is reported to be depleted in δ15N. Regression analysis demonstrated that urine and faecal δ15N were related to feed δ15N (Chapter 5), milk δ15N and plasma δ15N were related to each other (Chapter 4), and muscle δ15N - feed δ15N was related to urine δ15N - feed δ15N (Chapter 3). It is believed that these observed relationships related to N isotopic fractionation during digestion and absorption of feed components, such as WSC and N or a possible common effect of the endogenous N contribution to various N sinks from feed N.

A major observation from this PhD project was the linear relationship between N isotopic fractionation and N partitioning from both non-lactating sheep (Chapter 3) and lactating cows studies (Chapter 4). Two plausible sites (i.e., liver and rumen) of N isotopic fractionation that may be related to N partitioning were discussed in Chapter 6. Overall, under the condition of these studies (high dietary N); the relationships between N isotopic fractionation and N partitioning was believed to be mainly driven by deamination or transamination in the liver, rather than MCP synthesis in the rumen.

Results confirmed there is a potential of using N isotopic fractionation as an easy to conduct indicator of N partitioning. However, further research on the accuracy of using N isotopic fractionation predicting N partitioning at different feeding system (e.g., low vs high N intake); animal variations contribute to changes in N isotopic fractionation; and the contribution from different body parts (e.g., rumen, liver and intestines) are needed.