|dc.description.abstract||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 (¹⁴N and ¹⁵N) 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 (∆¹⁵N) 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) (r² = 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 ∆¹⁵N was related to both Feed conversion efficiency (negative) for the whole population (r² = 0.35; P<0.001), and repeatable for the subset of animals over four time points in beef heifers (r² = 0.47, 0.56, 0.64. 0.56 respectively; all P<0.001) (chapter 4). Plasma δ¹⁵N 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 δ¹⁵N or ∆¹⁵N after taking account of this effect. Milk Milk δ¹⁵N (‰) 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 Δ¹⁵N (mean 0.64; SD = 0.44) in period 2. There were weak negative correlations between NUE, milk δ¹⁵N and Δ¹⁵N because NUE was heavily driven by a large excess of rumen degradable protein. Chapter 7 demonstrated a highly significant relationship between NUE and plasma δ¹⁵N (r²=0.23; P<0.01) and ∆¹⁵N plasma (r²=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 feed efficiency and ∆¹⁵N results between beef and dairy cattle which were attributed to differences in the efficiency of utilisation of amino acids for growth and lactation respectively, absolute maintenance requirements and the dilution of maintenance for production. There were high levels of Rumen Degradable Protein (RDP) in pasture in most studies which was responsible for the weak relationship between N isotopic fractionation and feed efficiency in some studies (Chapter 5 and 6). Results suggested that Δ¹⁵N may be an indicator of the genetic variation in animal efficiency of amino acid utilisation in body tissues, but it was not strongly related to NUE because of the dilution effects of excess RDP. Preliminary evidence also suggested that differences in N isotope fractionation are a result of genetic between-animal variations in feed utilisation (Chapter 6 and 7). However, further investigation is required with more complex models to evaluate sire differences and relationships between parents and progeny. High Breeding Worth (BW) was associated with more N efficient animals at lower intake levels. Selection for cows based on BW may indirectly increase feed efficiency; in particular NUE, because protein yield is an important trait in the BW index and has a high economic weighting, however this process may still be slow because of genetic correlations with other traits in the index.
The main conclusions of this thesis were the reliability of using NUE as a measurement of feed efficiency in dairy cows, and the highly significant repeatable negative relationships between plasma ∆¹⁵N and feed efficiency (measured as FCE; g live weight gain/ g intake) in growing beef heifers (Chapter 4) and in lactating cows (measured as NUE) (Chapter 7). ∆¹⁵N in plasma has potential to be a used as a diagnostic tool in breeding programmes, evaluating feed efficiency without measuring feed intake (and diet composition). However, this approach may be limited by the ability of using isotope fractionation to detect variation for diets rich in RDP and requires further study with diets containing lower levels of nitrogen.||en