Total starch is a common term thrown around when discussing rations, but it does not tell the whole story. The amount of rumen degradable starch – or starch available to the rumen microbes – is a critical element in cow performance.
"A diet lacking in rumen degradable starch can negatively impact cow performance," noted Dr. David Weakley, director of dairy forage research for Calibrate® Technologies. "Sudden changes in the amount of rumen digestible starch in the ration can affect milk production, milk components and even cow health."
Ruminal starch digestibility in corn silage varies by year and location, and is often influenced by many factors, including stage of corn maturity at harvest, kernel processing, storage time, hybrid selection and growing conditions.
Timing of harvest
A fresh corn fodder study conducted in central Texas last year revealed wide swings in starch and fiber digestibility in relation to harvest time and hybrid selection.
During year one of a three-year study, 16,000 acres of non-irrigated corn from 56 farms was harvested over a 15-day period.
"We have a two-week window to harvest corn in central Texas before the weather gets too hot," said Dean McMahon, dairy nutritionist at Purina Animal Nutrition. "With 16,000 acres to harvest, we can't afford to wait until the first acre is mature because by that point, the last field will be overly mature. If we wait until July, the corn starts burning up."
Corn silage harvest started in late June, prior to any exposure to 100' F. days, when the corn was rather green and dry matter was about 28%. Within two weeks, dry matter increased to between 36% and 37%, with the bunker averaging 33% to 35% dry matter.
Weakley and his team compared 31 corn silage hybrids, measuring moisture, starch percent, starch digestibility, neutral detergent fiber (NDF) and NDF digestibility of 348 fresh cut corn plant samples. Using proprietary GPN™ technology measuring how digestible and available starch will be to the rumen microbes, the samples receive a score based on an indexed scale of 1 to 11, where one equals slow-digesting, and 11 equals fast-digesting.
In the corn silage samples, starch content averaged 34%, but individual samples ranged from 18% to 48%. The starch digestibility rating averaged 6.8, while individual samples ranged from 5.8 to 7.8.
"We saw a clear linear decrease in rumen starch degradability as crude starch content increased," said Weakley. These results showed starch content and digestibility are inversely related, which means waiting to let the ear fill completely to maximize total starch content may not provide optimum starch digestibility for dairy cows.
"Some samples high in starch proved to be unacceptable for cows," reported McMahon. "For example, a sample measuring 40% starch with low digestibility is probably a hard kernel meant for the elevator and not to be fed to a cow, as it will pass right through her rumen. The most desirable corn hybrid is one offering both high starch and high digestibility."
The GPN™ score increased as moisture content increased, supporting the notion that harvesting more mature corn silage to capture more starch and total yield may not be the best strategy for dairy producers. "As the plant matures and kernel starch fill continues, the rumen digestibility of the starch declines," explained Weakley. "Although less mature corn silage contains lower quantities of starch, the starch is more digestible in the rumen, the kernels process better, the higher moisture results in more favorable packing and provides better aerobic stability after opening the bunker."
McMahon said they discovered considerable variation between plant varieties concerning starch degradability and starch percent, demonstrating corn variety may be just as important as harvest timing.
"Samples that were less than 30% starch were not always a result of excessive moisture, and those above 38% starch were not always showing poor degradability of the starch," he noted. "We found some of the early-harvest varieties to be just as good as the late-harvest varieties, and some of the late varieties were better than we anticipated. When the three-year study is complete, we hope to determine the most effective varieties to plant."
When is the ideal time to harvest corn silage? According to Randy Shaver, professor of dairy science and Extension dairy cattle nutritionist at the University of Wisconsin-Madison, a good rule of thumb is when the kernel is at half milk line maturity – when half the kernel is milky and the other half is doughy.
"Once the kernel is dented, it's time to start measuring kernel movement and dry matter," he explained.
Starch digestibility changes over time, increasing during storage as corn silage ferments, with the greatest change typically occurring over the first few months of ensiling.
Therefore, routine testing of rumen degradable starch is recommended to ensure the ration cows eat contains appropriate amounts of digestible starch to maximize performance.
"Conservatively speaking, a twopound increase in daily individual milk yield can be expected when starch is efficiently utilized as an energy source," said Shaver. "It can also help save money by reducing the need for starch byproducts."
The ongoing Texas study, with sampling continuing every other week to assess the impact of storage duration, confirms there is an optimum time to harvest to ensure the best feed-out results. Work with your nutritionist, agronomist and harvest team to determine which varieties and harvest schedule will work best for your individual farm.
Stacey Smart is a freelance journalist based in Waukesha, Wis.