Evidence of a genotype by feeding level interaction in grazing
Holstein-Friesian dairy cattle at different stocking rates
J.R. BRYANT, C.W. HOLMES, N. LOPEZ-VILLALOBOS, K.A. MACDONALD
AND I.M. BROOKES
Massey University, Private Bag 11-222, Palmerston North
E-Mail:
NZSAP 2003 Abstract No. 19
Proceedings of the New Zealand Society of Animal Production
63: 73-76
Lactation yields and estimated breeding values (EBVs) for
protein, fat and milk were obtained for individual cows in a farmlet
trial carried out at Dexcel in Hamilton, using Holstein-Friesian cows,
to test for the existence of a genotype by environment interaction
across the range of feeding levels, and EBVs typically found on New
Zealand pasturebased dairy farms. At very low levels of feeding, with
yields of 267 - 307 kg milksolids per cow (kg MS/cow), regression
coefficients of lactation yield on EBVs were not significantly different
from the expected value of 1. At high levels of feeding, with yields of
383 - 411 kg MS/cow, most of the regression coefficients for yields of
protein, fat and milk on EBVs were significantly greater than 1. The
results of this study show that at low levels of feeding the actual
differences in production between cows, which differ in EBVs correspond
to the actual differences in EBVs, whereas at high levels of feeding the
difference in milk yields between genetic groups are greater than the
difference in EBVs. This constitutes a form of genotype x environment
interaction, in which genotype is measured as breeding values, and has
important practical and economic implications for dairy farms, and for
the expected value of genetic improvements.
Keywords: NZSAPAB;
genotype; environment; interaction; Holstein-Friesian;
pasture-based; breeding values; genetic merit
Last Updated 8/07/2004
