Genotype by environment interactions: experience from Langhill

Genotype by environment interaction was investigated for a range of traits. Selection (S) and control line (C) cows, housed and managed at the Langhill Dairy Cattle Research Centre, were offered ad libitum complete mixed diets, with proportions (in total dry matter) of concentrates, silage, brewers’ grains of either 20:5:75 (LC; 1.0 t concentrate per annum) or 45 :5 :50 (HC; 2.5 t concentrate), over a full lactation. No diet X genetic line interactions were observed for a range of traits (production, food intake, live weight and condition score) when compared as means. However, regression coefficients of protein yield (P < 0.05), milk yield (P < 0.10) and protein percentage (P < 0.10) on pedigree index for fat plus protein yield were different between LC and HC. Genetic correlations between HC and LC were high for most traits, and only for fat yield was the value below 0-80. Because the rate of increase in energy output from selection is unlikely to be matched by the rate of increase in intake during the first part of the lactation, it was expected that intake capacity is likely to become more important in the future, regardless of the feeding system. For this reason, it is suggested that selecting for a lower live weight whilst simultaneously selecting for increased yield (an approach which has been advocated in some countries) should be treated cautiously. Especially on low input diets increases in yield as a result of continued selection (on higher input diets) may be limited by the rate, or extent of tissue mobilization; under these circumstances health and fertility problems might be the first indicators of such limits being approached.