Comparing dairy farm performance and heat stress abatement strategies in the United States using summer to winter ratios (S:W ratio)

Heat stress abatement is a challenge for many producers around the world. Dairy producers could benefit by utilizing a simple metric to measure heat stress abatement strategies with the goal of motivating improvement in heat stress management on farm. The summer to winter ratio (S:W ratio) was developed by Extension Service of the Ministry of Agriculture and Israel Cattle Breeders Association to evaluate the seasonal effects of heat stress on performance. Summer performance variables are divided by winter performance variables and the closer the ratio is to one, the less effect heat stress is having on the variable of interest.


Our study was performed to explore the use of the S:W ratio, by comparing farm performance variables [energy-corrected milk (ECM), SCS, milk fat and protein %, heat detection rate (HDR), conception rate, and pregnancy rate] among regions of the United States. Cow performance data recorded monthly by the Dairy Herd Improvement Association (DHIA) from 2007 to 2016 for all US DHIA herds processing records through Dairy Records Management Systems (DRMS), were obtained and averaged by farm. A total of 105,279 performance records included a total of 16,573 herds [Northeast (n = 7,955), Midwest (n = 6,555), Northern Plains (n = 305), Southeast (n = 1,370), and Southern Plains (n = 388) regions]. The results identified an effect of summer heat on performance among regions of the US, particularly in the Southeast and Southern Plains as performance is not maintained at the same level during the summer as in the winter compared to all other regions. Summer to winter ratios were lower for ECM, heat detection rate, conception rate, and pregnancy rate for the Southeast and Southern Plains compared to the Northeast, Northern Plains, and Midwest. For milk fat % and milk protein %, the two southern regions had higher S:W ratios. For somatic cell score (SCS), the Southern Plains had the highest S:W ratio, indicating higher SCS in the summer months. These results correspond to the Southeast and Southern Plains having the highest mean temperature-humidity index (73.5 ± 1.7 and 73.6 ± 1.9, respectively) compared to the other regions (64.8 ± 3.4 to 67.9 ± 2.2), indicating a higher level of heat stress in those regions.

A second study investigated data collected from an on-farm survey given to 122 herds in Virginia (n = 63), Kentucky (n = 40), and Tennessee (n = 19) participating in the Southeast Quality Milk Initiative projects. Survey questions included management practices related to milk quality and heat stress. Summer to winter ratios for performance variables were calculated based on survey responses. Responses to the question of what temperature fans are turned on were grouped into > 70°F or < 70°F. Summer to winter ratios showed significant differences in ECM when turning fans on at temperatures > 70° F or < 70° F (0.93 ± 0.008 vs. 0.88 ± 0.02; P = 0.017) but showed no differences for other variables. Somatic cell score S:W ratios were higher for farms that did not have fans in the holding pen compared to farms that did have fans in the holding pen (1.11 ± 0.03 vs. 1.04 ± 0.01 P = 0.04). The opposite effect was seen for SCS S:W ratios for using sprinklers in the holding pen with a higher SCS S:W ratio for farms that used sprinklers in the holding pen (1.09 ± 0.02 vs. 1.03 ± 0.01; P = 0.03). Responses were grouped into the categories of using fans and sprinklers combined or no cooling systems at all in the holding pen. Summer to winter ratios were statistically higher ratios for farms using both in the holding pen compared to none (0.87 ± 0.04 vs. 0.67 ± 0.06; P = 0.02) and (0.94 ± 0.03 vs. 0.80 ± 0.05; P = 0.04) for conception rate and HDR S:W ratios, respectively. Results from this study showed that S:W ratios were closer to 1.0 when cooling systems were implemented, or no differences.

The results of these studies demonstrate the use of the S:W ratio to potentially benefit producers and consultants as a tool to assess heat stress levels in specific herds or regions, with the goal of encouraging improvement of on-farm heat abatement.

Text: Jenna Guinn



Jenna Guinn completed her undergraduate work in Animal Science with a dairy focus at the University of Kentucky. She returned to the University of Kentucky after working in industry for two years and defended her masters in April 2018 with her work focusing on heat stress and milk quality. She now works in the research department and technical support for Alltech.




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