The Limits of Managing Heat Stress

The heat and corn silage changes take a toll on milk production and dry matter intake at the Penn State Dairy Barn.
The Limits of Managing Heat Stress - Articles


Penn State Dairy Cows

July gave us a real taste of summer. The cows held steady until that week of high temperatures and humidity. It took its toll on the cows even in the barn with the sprinklers and fans. The various pens responded differently in dry matter intakes and milk production. The herd ration was analyzed, which yielded an unexpected result.

Figure 1 illustrates July's dry matter intakes for the various lactating cow pens. The free-stall barn with the sprinklers and fans contains pens 1 and 21 (left side of the barn) and pens 2 and 22 (right side of the barn). Typically this barn would contain two groups of 60, but because of a project being conducted, there were two groups of 40 (pens 1 and 2) and two groups of 20 (pens 21 and 22). Pen 15 is another free-stall barn that contains only fans. Pens 1 and 2 averaged 195 days in milk, pens 21 and 22 - 161 and pen 15 - 144. There were distinct pen differences in how cows responded to the heat and humidity based on their dry matter intakes and milk production drop (Figures 1 and 2). It is difficult to evaluate if there was a physical location affect or not. In the past, when cows are grouped differently than 60/60 in the main free-stall barn, we tend to see an unusual dynamics on intakes and production. This is the first time grouping the cows in this manner during the summer.

Figure 1. Dry matter intakes in pounds by pen and by day for July, 2013.

Figure 2. Average milk production pounds by pen and by week for July 2013.

Whenever the cows do not respond as expected, I always analyze the total mixed ration (TMR). I was surprised how much the cows dropped in milk, especially since we were feeding BMR corn silage. The sample analyzed at the beginning of the bag tested 39% starch compared to the conventional silage fed in June at 42.5% starch. The TMR fed with the conventional corn silage in June tested 14.9% protein and 28% starch on a dry matter basis, compared to the formulated diet of 15% and 26% respectively. When the BMR corn silage was started, the rations were formulated for 15% protein and 24% starch. The TMR results came back 14.7% protein and 20% starch. I was starting to see the milk protein percent slip under 3.0 towards the end of the month. This happened when dry matter intakes really dropped. I believe the cows got hit with a double whammy: they were not getting enough energy because of the low starch content and the protein level was not adequate when intakes declined.

At the end of July the herd was transitioned from the Ag Bag BMR to the bunk BMR, which was testing higher in moisture content. Until a sample is sent for analysis, the corn grain was switched to all fine ground corn and the protein level increased to 15.5%, which meets the metabolizable protein requirement at the current reduced intakes. I know from past experience it takes the cows about two weeks to bounce back completely from the effects of high heat and humidity. With the new diet already implemented the milk protein has already increased to over 3.0% and the cows are increasing in milk at the low end of the BST cycle, which should be when they are declining.

For the month of July the herd averaged 80.0 pounds with a 3.70% fat, 3.02% protein, 239,000 SCC and 7.4 mg/dl MUN.

IOFC Results

Month and YearNo Risk Mgt Gross Milk Price/cwtW/ Risk Mgt Gross Milk Price/cwtMilk income/cowFeed cost/cowIOFCAverage milk lbsLow BenchmarkHigh benchmark

Note: Correction to May 2013 feed cost per cow. An incorrect price was used for one of the ingredients and the actual cost/cow was $7.23 instead of $7.59 as originally reported in May's article.

IOFC Graph