Two considerations to manage a high corn silage dietPrint
By Dr. David C. Weakley
Last year’s drought coupled with continuing weather challenges this year, including alfalfa winterkill in the Midwest, has many dairy producers and nutritionists considering high corn silage rations.
High corn silage diets are appealing for many reasons and, depending on feed inventories, may be one of the most cost-effective forage feeding options. It also pulls more corn into the ration, which means potential for fewer ground dry corn purchases.
However, feeding a high corn silage ration doesn’t come without challenges. Understanding these challenges and how to manage them can improve the success rate of a high corn silage diet.
The first challenge with a high corn silage diet is the amount of starch available in the rumen. Starch digestibility increases the longer a crop is ensiled. For example, when a silage pile is first opened the amount of starch digested in the rumen will be much lower than in 6 months.
This change in starch digestibility presents a challenge, as a time may come when ruminal starch digestibility is too high. Too much starch digested in the rumen can cause a shift in energy partitioning more towards body energy deposition, decrease dry matter intake and reduce neutral detergent fiber (NDF) digestibility. As a result there is potential for negative consequences including milk fat depression, intake depression and milk production losses.
The amount and availability of starch in the diet is an important consideration. There are two ways to measure starch content of the ration – total starch and rumen degradable starch.
• Total starch measures how much starch is present in a feedstuff or ration.
• Rumen degradable starch measures how much starch is actually available to the rumen microbes when the feedstuff or ration is fed.
Total starch does not mean that all of the starch captured by this measurement is available in the rumen.
Monitoring rumen degradable starch levels on an every-other-week basis can assist dairy producers and nutritionists in determining how much starch is available in the ration.
Figure 1 shows the impact of increasing ruminal starch degradability from high moisture corn and corn silage, resulting from increased storage time, in a high corn silage diet. For this example the herd started feeding high moisture corn and corn silage in November. At that time the carbohydrate ingredient amounts, as demonstrated in the first column of Figure 1, supported a normal amount of rumen degradable starch as shown by a relative score of 106. As the ensiling time lengthens the amount of available starch from the corn silage and high moisture corn increases as represented in the second column. The resulting diet had a greater amount of rumen degradable starch, as shown by a higher score of 121.
Left uncorrected, potential losses in intake, milk fat and production could result. If the dairy monitors starch degradability, the opportunity is there to reformulate the ration and avoid any potential losses in milk fat, intake and production by removing 3 lbs. of corn, as shown in the third column. Therefore, the opportunity may also exist to reduce the cost of the ration.
The second challenge when feeding a high level of corn silage is getting enough rumen fill in the diet. If an appropriate amount of rumen undigested NDF (RUNDF) or rumen fill can’t be achieved, the passage rates out of the rumen will be too high and feed digestibility and efficiency can suffer.
Furthermore, the shorter particle length in high corn silage diets can allow for a higher rate of passage as well. In most cases, an additional source of forage of longer particle length will be needed. Alfalfa makes sense, because it not only has a longer particle length, but has a higher energy and protein content than most other longer stem forages.
Let’s look at two situations dairy producers might find themselves in when feeding a high corn silage diet, and considerations they might make.
1) No available alfalfa. In the first column of Figure 2, about 10 lbs. of dry matter was coming from alfalfa. This diet has a normal amount of rumen undigested NDF, or rumen fill, as shown by a relative score of 100. However, what would happen if alfalfa hay or silage is no longer available on the farm? If the alfalfa was removed entirely from the diet and no other adjustments were made, this farm would be looking at a ration that provides a low fill rate, as shown by the much lower fill score in the second column, resulting in a poorly digested ration and lower feed efficiency.
Another option would be to increase the amount of wheat straw, or another forage of lower NDF digestibility, in the diet. If an additional 6.5 lbs. of dry matter from wheat straw were included in the diet as shown in the third column, the ration would regain the same fill score of 100. Digestibility would likely be maximized, but cows would not be consuming enough energy and protein to meet their requirements, due to the excessive amount of wheat straw in the diet. Therefore, a better dietary compromise is needed, as shown in Figure 3.
2) Reduced amount of alfalfa available. In Figure 3, a better dietary formulation compromise is demonstrated in the third column. To stretch the alfalfa supply, the amount of alfalfa in the diet is reduced from 10 lbs. of dry matter to 5 lbs. of dry matter. However, as shown in the second column, rumen fill is still lost, as shown by the lower rumen fill score of 84. To regain adequate rumen fill, digestibility and feed efficiency in this situation, an additional 3 lbs. of dry matter from wheat straw can be added, as shown in the third column. While this diet is still likely lower in energy and protein than feeding 10 lbs. of alfalfa, from a feed efficiency and economic stand point, it may be the better option, as less supplemental protein and energy would be needed than removing all of the alfalfa from the diet.
Regardless of the feeding situation that one might be faced with, keeping careful watch on rumen degradable starch and the amount of rumen fill will assist in better achieving dietary success.