Assessing climatic effects on milk yield traits using sinusoidal modelling in dairy cows

The effect of climatic variations on dairy cows is a current hot topic in scientific research. Aims of the study were: (i) to test the adaptation of the sinusoidal function (SIN-F) to describe the circannual monthly variation in climatic traits and (ii) in the cows' production traits; (iii) to compare the SIN-F applied to plains and hills/mountains dairy farms and to cows of different breeds; and (iv) to assess mid-term (summer) deviations from the SIN-F as possible indicators of the effects of (heat) stress. We used data from 39 weather stations and 65 883 test-day records from 117 herds to test the effects of the circannual SIN-F and mid-term monthly deviations on daily milk yield (dMY, kg), and fat (FAT%) and protein (PROT%). Herd (percentage of the sum of squares attributed to the source: 29.6-59.8%) and stage of lactation (18.3-53.4%) were major sources of variation of milk traits, while parity was important (27.1%) only for dMY. Year represented 0.9-2.6% of total variation in milk traits, long-term SIN-F 2.4-12.8%, and mid-term monthly deviations from SIN-F 0.4-2%. The altitude effect was modest (0.1-3.3%), and the breed effect represented 1.9-4.3%, with a significant interaction with SIN-F but not with altitude. The milk FAT% and PROT% reached their zenith values in December-January, and nadir values in June-July, one month earlier than the temperature zenith, and with an opposite pattern with respect to photoperiod. The SIN-F of dMY showed nadir values delayed by three months (in September-October) compared with photoperiod zenith, and by two months compared with temperature zenith. The difference between dMY zenith and nadir, larger in plains (2.84 kg/d) than in hills/mountains (2.06 kg/d), possibly reflects a greater climatic stress of plain herds. Compared with Holsteins, Simmental and Brown Swiss exhibited a smaller amplitude of milk yield SIN-F, which may be partly due to their lower dMY, but also to a potential greater resistance to climatic variations, and a delay of one month of milk yield SIN-F, potentially attributable to greater resilience to climatic variations. Significant decreases in dMY, especially between June and July, compared with SIN-F, could be an expression of heat stress. Circannual SIN-F and their variations proved to be a promising tool for assessing the effects of climatological conditions on milk production traits at the level of individual herds and of different breeds and farming systems.