Final Report Summary - NUEMARKER (Nitrogen isotope fractionation as a marker for Nitrogen-use efficiency in dairy cows.)
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 project investigated a new approach to predict feed efficiency based on measuring the differential fractionation of the stable isotopes of N (14N and 15N). 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 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 compositions and production levels.
The studies showed that NUE was less affected by the mobilisation of body reserves in early lactation and so was a more reliable and stable measurement of feed efficiency in dairy cows compared to other, energy-based, measures. 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. Results suggested that Δ15N may be an indicator of the genetic variation in animal efficiency of amino acids 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 between-animal (genetic) variation in feed utilisation. 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 observation from this project was the highly significant repeatable negative relationships between plasma ∆15N and feed efficiency in growing beef heifers and in lactating cows. ∆15N has potential to be a used as a diagnostic tool in breeding programmes, evaluating feed efficiency without measuring feed intake (and diet composition) and comparing nutrient use efficiency of different diets. This would also accelerate the collection of feed efficiency data for large numbers of breeding stock to further improve feed efficiency in the agriculture industry.
The work has focussed on dairy cattle; studies with sheep have not been conducted, but we are planning to evaluate the ∆15N approach with pigs – which may be advantageous because the rumen in cattle and sheep may complicate the relationship between NUE and ∆15N.
The studies showed that NUE was less affected by the mobilisation of body reserves in early lactation and so was a more reliable and stable measurement of feed efficiency in dairy cows compared to other, energy-based, measures. 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. Results suggested that Δ15N may be an indicator of the genetic variation in animal efficiency of amino acids 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 between-animal (genetic) variation in feed utilisation. 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 observation from this project was the highly significant repeatable negative relationships between plasma ∆15N and feed efficiency in growing beef heifers and in lactating cows. ∆15N has potential to be a used as a diagnostic tool in breeding programmes, evaluating feed efficiency without measuring feed intake (and diet composition) and comparing nutrient use efficiency of different diets. This would also accelerate the collection of feed efficiency data for large numbers of breeding stock to further improve feed efficiency in the agriculture industry.
The work has focussed on dairy cattle; studies with sheep have not been conducted, but we are planning to evaluate the ∆15N approach with pigs – which may be advantageous because the rumen in cattle and sheep may complicate the relationship between NUE and ∆15N.