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Estimation of daily maximum temperature-humidity index thresholds affecting milk traits in Italian Brown Swiss.

A. Maggiolino

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06-22-2020

Abstract:

61
Estimation of daily maximum temperature-humidity index thresholds affecting milk traits in Italian Brown Swiss.
A. Maggiolino*1, G. Dahl2, N. Bartolomeo3, A. Rossoni4, P. De Palo1. 1Department of Veterinary Medicine, University of Bari �A. Moro,� Bari, Italy , 2Department of Animal Sciences, University of Florida Gainesville, FL, 3Department of Biomedical Science and Human Oncology, University of Bari �A. Moro,� Bari, Italy , 4Italian Brown Swiss Breeders Association Verona, Italy.

It is known that heat stress affects dairy cows' production and reproduction affecting both physiological and behavioral patterns. The aim of the work was to detect the temperature-humidity index (THI) thresholds, from the day of test-day sampling until 15 d before, for multiple milk production traits in Italian Brown Swiss dairy cows. A 10-yr data set (2009 — 2018) of test-day records was used. After the editing according to different criteria, the final set provided 202,776 test-day records of 23,296 Brown Swiss cows. All data were matched with the maximum daily THI calculated by temperature and humidity hourly recorded. First, a mixed linear model was fitted to obtain least squares estimates of THI effect on production traits. In a second set of analyses, the solutions for the THI class per parity class effect in the first model were used as the dependent variables to estimate change points in the relationship between production parameters and heat load, applying a 2-phase regression analysis. Results showed that there were no THI thresholds for milk yield in Brown Swiss, in all parities categories. On the contrary, FCM was affected by heat stress with average THI thresholds over 75. Protein yield and cheese production were affected by heat stress with average THI over 74. Brown Swiss showed higher thermal tolerance aptitude than Holstein cows. Heat stress negatively affect cow's performance, although in Brow Swiss it occurs at daily maximum THI higher 2 to 3 points more than Holstein cows. The lack of fitting of the 2-phase regression model on milk yield in Brown Swiss highlights a different metabolic pathway of this breed than Holstein during heath stress. With the raise of THI, Brown Swiss cows tended to produce the same amount of milk, but with a decreasing quality, mainly for protein yield, and then with a significant effect on cheese yield, the most important use of this milk. Further studies are necessary to estimate the heat tolerance genetic component that could and should be included in the selection procedures.

Keywords: heat stress, temperature-humidity index (THI), Brown Swiss.