# Distribution of seasonality of milk yield, adjusted for parity and days in milk, according to heat stress intensity in the United States.

## A. P. S. Franzoni

## Events

### 06-22-2020

## Abstract:

** M89 **** Distribution of seasonality of milk yield, adjusted for parity and days in milk, according to heat stress intensity in the United States. **

A. P. S. Franzoni*^{1}, F. C. Ferreira^{1}, J. S. Clay^{2}, A. De Vries^{3}. * ^{1}University of California Davis, CA, ^{2}Dairy Records Management Systems Raleigh, NC, ^{3}University of Florida Gainesville, FL. *

The objective of this study was to describe the distribution of seasonality in milk yield, adjusting for parity and days in milk, under different intensities of heat stress (HS) in the United States (US). We used 2015 DHIA lactation records from 5,005 herds located in 41 states of the US. Weather data of closest station of each herd was retrieved from the National Oceanic and Atmospheric Administration archives. Linear regression models with sine and cosine functions adjusted for days in milk and parity were used to describe seasonality in average milk production per herd, and to find the annual low (minimum) and peak (maximum) of the curves. Annual low-to-peak (LP-ADJ) ratios were measures of maximum seasonality. Sine and cosine functions were also used to model hourly and daily temperature humidity indexes (THI) for each weather station. For every herd, we calculated the average daily THI and the number of hours per day above a THI of 68. Calculated measures of HS intensity were the yearly heat load (HL), the number of days per year with average THI above 68 (DAT), and the number of hours per year above 68 (HAT). For each HS intensity measure, herds were classified as exposed to LOW (HL <73, DAT <50, HAT < 1108), MEDIUM (HL >73 and <299, DAT >50 and <86, HAT > 1108 and < 1778), and HIGH (HL > 300, DAT > 87, HAT >1779) intensity HS groups. The PROC CORR procedure (SAS 9.4) revealed correlation (<0.001) between LP-ADJ and HL (r = 0.19), DAT (r = 0.16), and HAT (r = 0.17). Distribution of seasonality (25, 50, and 75 percentiles) for LOW, MEDIUM and HIGH HS intensity were: HL (0.88; 0.92 and 0.94), (0.88; 0.92 and 0.95) and (0.85; 0.90 and 0.94); for DAT (0.89; 0.91 and 0.95), (0.89; 0.91 and 0.95) and (0.86; 0.89 and 0.94); and for HAT (0.89; 0.91 and 0.95), (0.89; 0.92 and 0.95) and (0.86; 0.90 and 0.94). Herds exposed to HIGH HS had a greater variation in their seasonality, but HS intensity was not a major effect in seasonality variation among herds. Even within herds exposed to HIGH HS, seasonality can be managed and reduced, for instance, through efficient heat abatement strategies.

**Keywords:** seasonality, dairy cow, sine and cosine functions.

**Biography:** Ana Paula Franzoni is a Postdoc Employee at University of California, Davis, in the department of Population Health & Reproduction. She got her DVM, MS and PhD degrees in Brazil, where she also worked for the government for 3 years before coming to the US.