Snow Cows (IMAGE) Penn State Caption In general, among all herds studied, peak yield, fat concentration and protein concentration occurred in winter months when days are shorter, and lowest in summer months, when days are longest. On average, milk yield peaked in April, fat and protein yield peaked in February, fat concentration peaked in January, and protein concentration peaked in December. Credit Issac Salfer, Penn State Usage Restrictions non commercial use only License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.