Implementing selection index for nitrogen efficiency in Italian Holstein population

ANAFIBJ has been working on improving environmental sustainability, as well as animal welfare and biodiversity. The aim of the present study was to develop a new selection index for Italian Holstein population, e.g. nitrogen efficiency, that will help farmers to breed more efficient animals. For this index, protein (P%) and urea (U%) content in milk were used. From them, a compound index P/U was derived as selection objective: P% and U% weights were derived following selection index theory, resulting in a combined indicator. Data ranged from 2017 to 2022 milk recording and included records between±3 DS from the mean and at least 4 records per cow; after editing, the dataset consisted of 20,824,821 P/U records on 1,908,431 cows belonging to 9,324 herds. Breeding values (EBV) were estimated using a repeatability animal model which included as fixed effects: days in milk (DIM – 10 classes of 30-d each, ranged from 5 to 305 d), parity (first, second and third lactation), age within parity (9 classes within parity), interaction between DIM and parity, and herd-test-day (328,654 classes) as contemporary group. The random effects were additive genetic animal, permanent environment and the residual. Phenotypic means for P% and U% were respectively 3.4%±0.4 and 0.024%±0.007 and heritabilities were 0.35 and 0.15, respectively. Genetic correlation between P/U genetic index and its phenotypic value is 0.42 and EBVs reliability averaged 0.94±0.05. As expected, bulls with higher EBVs had higher mean of P/U due to lower U%, while lower EBVs were associated with a decrease in P/U. On average, bulls with EBV equal to 100 have daughters’ phenotypic mean equal to 141.46±10.42 P/U. ANAFIBJ will release this index to aid in the selection in minimizing nitrogen waste and maximizing milk protein output. Study supported by ‘Latteco2 project, sottomisura 10.2 of the PSRN-Biodiversity 2020-2023’.