Genetics of feed intake and efficiency in grazing dairy cows : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science
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2017
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Massey University
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Abstract
Feed efficiency in dairy cows is widely acknowledged as a highly desirable
characteristic to improve because of its well-documented impact on production costs.
Traditional measures of feed efficiency have used ratio traits, specifically energy
conversion efficiency, but these have undesirable statistical properties. Alternative
measures of feed efficiency are those based on the residuals from regression-type
statistical models, the most common of which is residual energy intake (REI). Residual
energy intake is defined as the difference between actual and predicted intake and is
usually derived from least squares regression models. The general objective of this
thesis was to quantify phenotypic and genetic (co)variances between the feed intake
complex, performance, and fertility traits in lactating Holstein-Friesian dairy cows. A
total of 8,199 feed intake records from 2,693 lactations on 1,412 grazing lactating
Holstein-Friesian dairy cows from experimental farms were used. Several alternative
efficiency definitions were developed, each with their own respective strengths and
weaknesses. Exploitable genetic variation was demonstrated to exist for the range of
alternative efficiency traits, and the magnitude of this variation was sufficiently large to
justify consideration of the feed efficiency complex in future dairy breeding goals. The
heritability estimates for the different efficiency traits estimated using repeatability
models varied from 0.06 to 0.21. Variance components, however, differed across
lactation when estimated using random regression models; for example, the heritability
of REI varied from 0.04 (34 DIM) to 0.11 (280 DIM) across lactation. Phenotypic
correlations among many traits including REI and energy balance (EB) differed not only
by stage of lactation but also by cow parity. Moderate to strong genetic correlations
existed between REI and EB across lactation (ranging from 0.45 to 0.90). Albeit
associated with large standard errors, estimated genetic correlations between feed
efficiency and reproductive performance were either neutral or favourable suggesting
greater genetic merit for feed efficiency does not appear to be antagonistically
genetically correlated with reproductive performance. Selection index calculations using
the current economic weights in the Irish Economic Breeding Index, and genetic
(co)variances estimated in this thesis, indicate that the inclusion of REI in the index
with an economic weight of €0.078/UFL will generate animals with improved REI.
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Dairy cattle, Holstein-Friesian cattle|, Feed utilization efficiency, Genetics, Research Subject Categories::FORESTRY, AGRICULTURAL SCIENCES and LANDSCAPE PLANNING::Animal production::Animal breeding