Dietary dry matter variation affects lactating dairy cows

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Although many researchers, dairy nutritionists and producers intuitively know that cows perform best when a consistent diet is fed every day, there is very little research in this area. Researchers in Ohio set out to determine the effects of short-term variation of dietary dry matter (DM) on the performance of lactating cows. At the Tri-State Dairy Nutrition Conference held in Indiana in April 2013, Bill Weiss presented the findings of three studies conducted at The Ohio Agricultural Research and Development Center. For this article, we will focus on the experiment involving varying the dietary dry matter of forages.

Forage dry matter can vary significantly if the forage gets wet due to weather during storage and feed out. Having a rapid change in forage dry matter that is not accounted for in the ration balancing will result in the total mixed ration (TMR) being higher in concentrates than accounted for. In addition, the amount of dry matter feed in front of the cows will be less due to the increased weight from water, despite the same pounds on an as fed basis. If cows run out of feed, there is no doubt that DM intake and milk yield will decrease. Another source of variation is if the diet is prematurely adjusted based on the overly wet sample, when forage DM content will soon return to normal.

Researchers conducted a 21-day experiment to determine whether short term changes in silage DM affected cows and if rations should be adjusted to account for short term DM changes in silage. The control group was consistent at 55:45 forage to concentrate ratio on a DM basis. The second treatment (wet, unbalanced) was fed the same diet as control except during two three-day periods in which water was added to silage to reduce DM concentration by 10 percentage units. During the two three-day periods, the wetter silage was substituted at an equal as-fed amount resulting in a reduced forage to concentrate ratio of 49:51 on a DM basis, reduced Neutral Detergent Fiber (NDF) concentration in the diet and increased the starch concentration. It is essential to note that in this study, the researchers increased as fed pounds of the TMR when wet forages were fed and did not allow the cows to run out of feed. The third treatment (wet, balanced) was the same as treatment two, except the amount of as fed (lower DM) forage was increased so that the forage to concentrate ratio remained at 55:45 and dietary nutrients were unchanged.

Over the 21-day experimental period, DM intake did not differ between treatments. Milk yield was significantly increased for treatment two (wet, unbalanced) compared to control (87.6 lbs./cow/day vs. 86.6 lbs./cow/day) and milk fat percentage was significantly lower (3.33 percent vs. 3.46 percent for wet unbalanced vs. control respectively). This is likely because of the decreased forage to concentrate ratio resulting in more grain intake. When cows were fed the wet silage, as fed intake immediately increased – which the researchers planned for – and offered more TMR. On the second day after wet silage was introduced as fed intakes continued to increase and DM intake returned to normal. When switched back to the normal diet, cows continued to eat more feed (as fed and DM) that day. This implies that extra feed should be offered to cows for at least one day after feeding wet silage.

Researchers concluded that for their study, rebalancing diets for a short term (1-3 day) change in silage DM is not necessary. Cows will adapt to the wetter feed and consume more on an as fed basis. Producers must anticipate this increase in TMR consumption and offer extra TMR at the bunk during the wet silage feeding period and at least one day following. Michigan State University Extension recommends testing forage DM on a routine basis, rebalancing rations if the change in silage DM lasts more than three days, closely monitoring groups of cows sensitive to additional concentrate in the diet, and implementing best management practices for silage storage to reduce the effects of weather.



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