Advances in forage genetics, harvesting, ensiling, management practices and forage analysis – in addition to dynamic ration-balancing software – have allowed dairy farms to progressively feed increasing amounts of forage to their lactating cows.

High-forage, lactating cow rations are typically defined as those containing more than 55 percent forage in the TMR or more than 25 percent forage neutral detergent fiber on a dry matter (DM) basis. It is common practice for today’s TMRs to contain two-thirds of their forage DM from corn silage, either BMR, conventional or a combination of the two types of hybrids.

Moving toward diets with higher levels of forage can reduce the need for dairies to purchase additional feed and promote milk component yield when the quality of the homegrown forage is high.

However, the main fraction in high-forage TMRs is usually corn silage. What makes corn silage unique is its combination of kernel-processed corn grain along with chopped corn stover.

On a whole-plant corn silage DM basis, corn silage with 30 percent starch content typically contains 40 to 45 percent of kernel-processed corn grain and 55 to 60 percent chopped corn stover.

How the cow sees high-forage TMRs

Every 1 pound of good-quality corn silage DM is equivalent to 0.4 pounds of kernel-processed high-moisture shell corn (kp HMSC) DM plus 0.6 pounds of corn plant stover DM. In reality, corn silage is like high-moisture shell corn on a corn stover stick.

Take the ration shown in Figure 1 as an example. This TMR may at first appear to be a high-forage ration, but the corn silage fraction will contain 40 percent of DM as kp HMSC.

Therefore, the cow’s rumen will respond to two grain fractions in this “high-forage TMR” – the dry grain blend plus kp HMSC equivalent from the corn silage. On paper, this appears to be a high-forage ration.

In reality, the hidden kp HMSC in the corn silage means the cow’s rumen will be responding to a 45.2 percent true forage component plus 54.8 percent total grain fraction.

These types of real-world situations mean rumen function is at risk for disruption even with higher forage inclusion rations.

Subacute ruminal acidosis

It is often mistakenly believed that such high-forage rations will not expose the lactating herd to subacute ruminal acidosis (SARA). This can prove to be a costly misconception for the cow’s rumen function and consequent milk yield and components.

SARA is the period of time the cow’s rumen environment spends three hours or more below a pH of 5.8 during a 24-hour period. A pH of 5.8 or lower creates negative effects on ruminal cellulolytic bacterial activity and fiber digestion.

Since lactating dairy cows consume multiple meals (six to 10 per 24 hours), the rumen pH can – and will – vary throughout the day. A cyclical pattern of rumen pH occurs, which simply means SARA is an occupational hazard for the modern lactating dairy cow.

Trials confirmed a cow may experience multiple bouts of SARA within a day, and the number of daily combined hours the rumen environment was exposed to SARA ranges from 6.4 to 11.8 hours over a three-day period.

SARA is present on every dairy farm but often hard to identify. The hidden incidence rate of SARA can be as high as 40 percent, and the cost to the U.S. dairy herd is estimated conservatively at $1.12 per cow daily.

This cost primarily includes losses of milk production and milk components; it would likely be higher if any health or fertility costs were included.

One challenge with SARA is that individual cows do not necessarily show any immediate or overt signs. There may also be a time delay between the original SARA insult and any observable changes in milk production, behavior or health.

While the incidence of SARA is recognized as a herd syndrome, the risk for SARA within a herd is not the same for all cows. There is considerable variation in exposure to SARA within herds, even when cows are fed and managed similarly.

There are a number of inherent factors that will predispose fresh cows, early lactation, high-yielding and mid-lactation cows to SARA in any herd. This is further complicated by a wide array of on-farm feed and management factors that can exacerbate daily inconsistencies and, therefore, increase the risks of SARA greatly.

Preventing SARA

Keeping a cow’s rumen pH above the 5.8 threshold for as much of the day as possible is critical to preventing SARA. Producers can achieve this goal through several feeding and management strategies, including:

• Monitoring DM content in ensiled forages, moist grains and moist feedstuffs every week
  
  
• Testing for total starch content, starch digestibility and the starch degradation rate on ensiled corn silage and HMSC every month
  
  
• Monitoring neutral detergent fiber and physically effective neutral detergent fiber content of the TMR
  
  
• Ensuring particle size distribution of the TMR is as expected
  
  
• Making sure total oil and polyunsaturated fatty acid contents in the TMR are not excessive
  
  
• Checking that the salt and buffer content are sufficient within the TMR and that the dietary cation-anion difference value is adequately positive
  
  
• Ensuring the TMR is dispersed evenly along the feedbunk and regularly pushed up
  
  
• Watching fecal consistency and texture, rumination rates and milk component inversions
  
  
• Minimizing heat stress
  
  
• Including a rumen-specific active dry yeast probiotic

One active dry yeast probiotic – Saccharomyces cerevisiae CNCM I-1077 – has been shown to help maximize rumen function in all life stages of dairy cattle.

Research shows cows fed an active dry yeast specifically selected to maximize rumen function spend significantly more time above the SARA threshold and produce 2.1 pounds more per day of 3.5 percent fat-corrected milk per cow per day.

Including a probiotic active dry yeast can help minimize pH fluctuations when feeding high-forage diets. One study showed a greater fluctuation of pH over six days for cows eating a 25.6 percent neutral detergent fiber TMR compared to cattle receiving the same diet supplemented with S. cerevisiae CNCM I-1077 (Figure 2).

In this study, the group fed S. cerevisiae CNCM I-1077 had significantly higher average rumen pH and spent 90 percent of the day in non-SARA conditions, whereas the control group supplemented with only sodium bicarbonate spent 237 more minutes per day with a pH of 5.8 or lower.

Helping cattle maintain a more consistent rumen pH can optimize rumen function – avoiding SARA and making the best use of a carefully formulated ration which, in turn, leads to greater productivity. Work with your nutritionist to establish feeding the appropriate forage content for your operation and help your cows perform at their optimal level.  PD

References omitted but are available upon request. Click here to email an editor.

• Tony Hall

• Ruminant Technical Service
• Lallemand Animal Nutrition
• Email Tony Hall