AgriLife Research study: Harvest management can enhance sorghum silage value

AMARILLO – Forage sorghum silage is an alternative to corn silage due to its drought tolerance, but management is key to making the silage valuable, according to a Texas A&M AgriLife Extension Service specialist.

Jourdan Bell, Ph.D., agronomist jointly appointed to both AgriLife Extension and Texas A&M AgriLife Research, Amarillo, planted 92 sorghum hybrids along with some corn for a side-by-side silage comparison in her research trials northeast of Bushland.

In general, if water is not limited, corn will yield better, which is why corn silage is often the silage of choice, Bell said. But, corn loses yield and quality when water is limited. If the crops are drought stressed, sorghum will “shut down” at peak water demand periods, while tasseling corn will lose quality more quickly than with sorghum.

“Our research has shown high-yielding, high-tonnage forage and grain sorghums can yield a high-quality silage,” she said. “And with proper harvest timing and processing of the grain, you can improve the quality of that silage.”

Bell said her forage sorghum trials are about more than the farmer looking at yield differences. Part of her educational message is the importance of harvest timing and how that forage is managed after it is harvested.

“If we can promote optimal management, we can increase the adoption of forage sorghums,” she said.

Research study

Bell and a graduate student, in a collaborative research study sponsored by the Texas Grain Sorghum Board, evaluated forage sorghum silage quality. The silage study included two harvest stages – soft-dough and hard-dough, with and without a kernel processor, and four ensiling durations: 0, 30, 60 and 120 days.

“We wanted to evaluate how ensiling duration affects silage quality,” Bell said. “Ideally, producers should harvest forage sorghum at a soft-dough grain stage when the forage moisture is 65%-70% and use a kernel processor to optimize forage quality. But many challenges can delay harvest such as equipment breakdowns, precipitation and even delayed forage dry down due to the stay-green trait.

“Additionally, producers are generally paid by the ton, and they do not receive a premium for forage quality,” she said. “So, it may be in the best interest of the producer and the feeder to discuss a premium to compensate for reduced tonnage if an earlier harvest is desired by the end-user.”

Optimizing quality

The study data confirmed that silage stabilizes quicker when harvested at soft-dough, Bell said. Silage harvested at a more mature grain stage took 120 days to stabilize. She said a fermentation analysis can give producers an indication of silage quality.

Silage pH is a measure of the silage acidity. The pH of fresh forage sorghum is about 5.5 while the pH of ensiled sorghum is between 3.6 and 4.2.

“Because ensiling is a fermentation process, we want the pH of the silage to drop quickly,” Bell said. “If the drop in pH is slow, spoilage can occur as a high pH environment favors the growth of bacteria and molds. Silage pH is also related to protein breakdown in the pit. When the pH of silage is greater than 4.2, protein will break down to ammonia resulting in nitrogen and protein losses.”

She said by evaluating a fermentation analysis, they saw that the pH of the silage harvested at soft-dough, both with and without a kernel processor, dropped to 4.0 by 30 days. The pH of the silage harvested at hard dough was still greater than 4.5 after ensiling 30 days. The hard-dough harvested silage had a gradual decline in pH and took 120 days to reach the upper level of the ideal pH.

Lactic acid is another key characteristic of quality, Bell said. Because lactic acid is the primary acid of fully fermented silage, it is responsible for the final drop in pH and responsible for stabilization. The lactic acid levels of soft-dough silage quickly spiked to optimal levels, but did not reach desired levels until after 120 days of ensiling with the hard-dough silage.

“From an end-user standpoint, knowing the growth stage of the forage sorghum provides information on the necessary ensiling duration for forage sorghums harvested at different stages,” she said.

Other considerations

Another concern for some may be optimizing the starch from the forage, Bell said. In looking at starch digestion in situ, the research group confirmed harvesting sorghum at soft dough, whether cracked or whole berry, had greater than 80% of the starch utilized. Silage harvested at hard dough had closer to 60%-70% of the starch utilized and required a longer period in the pit to ensure fermentation and ideal access to the starch.

“So, while the starch is higher at hard dough, it is it not fully available,” she said. “The berry is harder and less starch is digested by the animal. If we want to optimize silage cut at the hard-dough stage, producers need to use a kernel processor.”

Other management considerations she said include silage cut too early and wet will have low starch, low energy, high acetic acid and end up with seepage. Silage cut too late risks reduced digestion, poor aerobic stability and shrink.

As harvest timing is important, so is the chop length, Bell said. Long, dry material is difficult to compact, and it traps oxygen. Also, if it is too long, the sugars won’t be released from the plant to begin the rapid pH decline needed to start the fermentation.

“A feed yard is more forgiving than a dairy, but a half inch is the ideal chop length at high dry matter,” she said. “Increase that to a 1-inch chop length as it gets wetter, up to 65% moisture. Above that, 75%-80% moisture may need to be chopped at 2-3 inches long.”

In the end, though, the customer pays for pounds delivered not pounds fed, Bell said. As dairies and feed yards try to meet their needs, producers may see some very specific contracts on what is to be delivered.

To optimize the silage delivered, she said, they can harvest at the recommended maturity stage and use a kernel processor. Other tips are to pack the silage in layers as it is being delivered and inoculate with lactic acid. Covering the pile or bunker also will help maintain quality.

Bottom line

One issue between harvesting at the optimal soft–dough stage and waiting to the hard-dough stage is the money is the same, she said. There’s not a premium paid for premium silage.

“The producer who is looking at getting paid by the ton gets more tonnage and money per acre by cutting it at the hard–dough stage,” Bell said.

“They are not getting compensated for the higher starch and digestibility that may be available at the soft–dough stage. There can be a $50-$90 per acre loss from soft to hard dough for the producers, so they are looking for compensation for bringing it in at the soft–dough stage.”