Optimal selection responses under varying milk prices and margins for milk production

In The Netherlands the price a farmer receives for milk is formed by the price per kg for fat and protein and a negative base price for milk. The price ratio between fat and protein is about 1:1, and the base price about — Dfl 0.12 kg⁻¹ milk. Feed costs for producing fat are Dfl 3.34 kg⁻¹, for protein Dfl 1.70 kg⁻¹ and for carrier (milk-fat-protein) Dfl 0.0622 kg⁻¹.The net economic returns (sales-feed costs) are Dfl 0.1822 kg⁻¹ for carrier, Dfl 6.28 kg⁻¹ for fat and Dfl 7.92 kg⁻¹ for protein. These values are used for the breeding goal. An index (INET) was constructed for combination of breeding values for kg milk, % fat and % protein. Weighting factors were 0.316, 260 and 500, respectively.Correlations of the optimal indices to be used in the different circumstances with INET and the responses for the milk characteristics were calculated to study the sensitivity of INET. Correlations were almost one when the base price per kg milk was varied from Dfl 0.00 to − Dfl 0.20, the margin from Dfl 0.30 to Dfl 0.40 kg⁻¹ milk and the fat/protein price ratio varied from 2:1 to 1:2.When production level increased to 6000 kg milk with 3.6% fat and 3.2% protein, the correlation with INET declined to 0.96 and to 0.93 when the production level was decreased to 4000 kg milk with 6% fat and 4% protein. It was concluded that INET is an efficient selection criterion for a broad scale of aggregate genotypes, which may be valid in the future in The Netherlands.