AbstractsBiology & Animal Science

Nitrogen isotopic fractionation and nitrogen use efficiency in beef and dairy cattle

by Nicole Wheadon

Institution: Lincoln University
Year: 2014
Keywords: stable isotope; nitrogen use efficiency; feed conversion efficiency; energy conversion efficiency; nitrogen metabolism; rumen degradable protein; metabolisable protein; urea nitrogen; milk protein; sire; breeding index; body reserves; dilution of maintenance; phenotype; cow; heifer
Record ID: 1297394
Full text PDF: http://hdl.handle.net/10182/6393


Future rises in world population, increased demand for food production and greater concern for environmental emissions means that new strategies are required for sustainable growth of ruminant industries. Improvement of feed efficiency in cattle is a major solution to increasing production at lower costs; however, identifying between-animal variation requires markers to predict their phenotype. The studies reported in this thesis investigated a new approach to predict feed efficiency based on measuring the differential fractionation of the stable isotopes of N (?????N and ?????N) in plasma and milk. The main objectives were to evaluate the advantages of using nitrogen-use efficiency (NUE; g milk N/g feed N) as a measurement of feed efficiency in dairy cows, investigate the relationship between plasma and milk N isotopic fractionation (????????N) and feed efficiency in beef and dairy cattle, and lastly to understand the genetic factors involved in these relationships. Studies were carried out in Ireland and New Zealand, using growing beef heifers and lactating dairy cows in a number of herds, diets based on grass silage or grazed grass, as well as a range of diet composition and production levels. Chapter 3 showed a highly correlated relationship between NUE and an energy based measure of efficiency (ECE) (r?? = 0.90; P<0.001), but NUE was less affected by the short term changes in body reserves over lactation so was a more reliable and stable measurement of feed efficiency in dairy cows to investigate the relationship between NUE and N isotope fractionation. Plasma ????????N was related to both Feed conversion efficiency (negative) for the whole population (r?? = 0.35; P<0.001), and repeatable for the subset of animals over four time points in beef heifers (r?? = 0.47, 0.56, 0.64. 0.56 respectively; all P<0.001) (chapter 4). Plasma ???????N measurements from the same animals in the subset were significantly correlated over adjacent time points (P<0.001 correlation between all days) (average r = 0.96). Further development of the use of N fractionation to predict NUE was extended to free grazing dairy cattle in chapter 5-7. There was pasture and N isotope variation in the 9 treatment groups in chapter 5, with no relationship between NUE and plasma ???????N or ????????N after taking account of this effect. Milk Milk ???????N (???) was measured in chapter 6, and the high N isotope signature 7.28 (SD = 0.50) and 7.06 (SD = 0.44) for periods 1 and 2 resulted in a low enrichment of milk ???????N (mean 0.64; SD = 0.44) in period 2. There were weak negative correlations between NUE, milk ???????N and ???????N because NUE was heavily driven by a large excess of rumen degradable protein. Chapter 7 demonstrated a highly significant relationship between NUE and plasma ???????N (r??=0.23; P<0.01) and ????????N plasma (r??=0.45; P<0.001). There were no significant relationships between NUE and any urine analytes within groups apart from a negative relationship with uric acid (mmol/l) (P<0.05). There were differences in…