Sub-acute rumen acidosis (SARA) is a prevalent problem for dairy herds as characterized by having more than 25 percent of cows sampled via rumenocentesis four to eight hours after a total mixed ration (TMR) meal with ruminal pH less than 5.5. Ruminal pH is largely a function of the balance between the production of volatile fatty acids from the fermentation of carbohydrates, their neutralization by salivary and dietary buffers and their removal by absorption across the rumen wall or passage from the rumen, and SARA is caused by the consumption of high amounts of ruminally-available carbohydrate, low amounts of effective fiber or both.

Laminitis, an aseptic inflammation of the dermal layers inside the hoof and a major source of lameness for dairy herds, has been linked to SARA. Despite diet formulation guidelines for neutral detergent fiber (NDF) from forage, total NDF and non-fiber carbohydrate (NFC), physically-effective NDF (peNDF) and starch along with the feeding of TMR, some degree of SARA may be inevitable in high-producing dairy herds because chewing time, and as a consequence salivary buffer flow, decline per unit of rumen-fermentable organic matter (RFOM) intake or volatile fatty acids production as RFOM intake increases.

An increased intake of RFOM as milk production increases is a normal consequence of high milk production because of increases in dry matter intake (DMI) and the feeding of higher-concentrate diets to increase dietary energy density. Further, bunk management and cow comfort have been implicated as risk factors for SARA and laminitis in dairy herds.

The foregoing discussion highlights the challenges we face in our efforts to minimize SARA and laminitis in today’s dairy herds and underscores the margin for error in our feeding programs is small. The emphasis of this [article] will be to discuss hot-topic areas.

Transition diets

Lead feeding is the practice of increasing the proportion of concentrate in the TMR during the last few weeks prior to calving. Intake of either excessive or minimal amounts of concentrate during the close-up dry period may increase the risk of SARA. Excessive lead feeding is probably not a common risk factor for SARA because low DMI during the transition period would reduce the rate and extent of post TMR-meal decline in ruminal pH and herd managers tend to be relatively conservative with concentrate amounts for dry cows.

Minimal lead feeding may increase the risk of SARA through failure to increase the volatile fatty acids’ absorptive capacity of the ruminal papillae and adapt the ruminal microbial population to starch during the close-up dry period prior to the feeding of high-energy (starch) milking cow diets. But these benefits are not well established in the literature.

Further, fresh cow transition diets and the relative composition of close-up dry cow versus fresh cow transition diets may influence the risk of SARA. Cook et al. suggested transition cows may be at an increased risk of developing laminitis following a SARA trigger.

Donovan et al. evaluated the influence of close-up dry cow and fresh cow transition diets on the incidence of laminitis and SARA. Cows were fed either low- or high-energy diets during the close-up dry period prior to calving for 24 days (on average), then post-calving were fed a common energy diet for five days (on average) followed by either low or high diets for 16 days (on average) followed by the common energy diet again from 21 to 70 days in milk (DIM).

Treatments were as follows:

•Pre High NEL, Post Low NEL

•Pre High NEL, Post High NEL

•Pre Low NEL, Post Low NEL

•Pre Low NEL, Post High NEL

Average rumenocentesis pH measurements were not affected by treatment. Lowest ruminal pH for samples taken at 8, 22 and 70 DIM was 0.2 units lower for cows fed Post High NEL (ruminal pH of 5.61 on average) than Post Low NEL (ruminal pH of 5.81 on average) diets. The percentage of cows with ruminal pH less than 5.8 at 8 and 22 DIM was greater (P < 0.05) for cows fed Post High NEL (62 percent on average) than Post Low NEL (34 percent on average) diets.

Using a more conservative cut-off for SARA, the percentage of cows below ruminal pH of 5.5 were 41 percent and 24 percent for Post High NEL and Post Low NEL, respectively. Hoof scores at 60 DIM were worst for cows fed the Pre Low NEL, Post High NEL treatment.

These results lend support to the practice of feeding fresh cow transition diets that stair-step energy content gradually to the high-cow diet, especially when low- to moderate-energy close-up dry cow diets are fed.

Heat stress

Mishra et al. reported lower ruminal pH for dairy cows in hot-humid (85°F and 85 percent relative humidity) than cool (65°F and 50 percent relative humidity) environments when fed either high-roughage (35 percent grain; pH of 6.1 vs. 6.4) or high-grain (65 percent grain; pH of 5.6 vs. 6.1) diets, possibly because of decreased rumination activity and increased slug feeding during heat stress.

Leonardi and Armentano reported extensive sorting away from long TMR particles, and this feeding behavior may increase during heat stress as cows attempt to reduce metabolic heat production by selecting away from forage toward concentrate. Stone recommended dietary peNDF content be increased and NFC content decreased during heat stress to reduce the risk of SARA.

Heat stress limits the amount of time cows spend in stalls which may increase risk of laminitis, since laminitis increased in cows that spent more time standing on concrete rather than lying in stalls. Managing facilities to optimize cow comfort and minimize heat stress is an important component of laminitis prevention.

Feed additives

There are several feed additives available for use in dairy cattle diets that may minimize SARA and laminitis. Rumensin® (monensin sodium) supplementation of dairy cattle diets for increased milk production efficiency has been approved. Subclinical ketosis was reduced in transition cows treated with a monensin controlled-release capsule. Monensin has been used to prevent lactic acidosis in cattle and monensin reduced lactic acid concentrations in vitro through inhibition of the lactic acid producer Streptococcus bovis, which suggests monensin may have a role in attenuation of SARA and laminitis.

In dairy cows, monensin increased ruminal pH in one study but did not influence ruminal pH in others. Monensin supplementation of feedlot cattle diets increased meal frequency and reduced average meal size in two trials reviewed by Milton. The efficacy of monensin for minimizing SARA and laminitis in dairy cattle remains to be determined.

Ruminal pH declines following meals with the rate of pH decline increasing as meal size increases and as dietary NDF content decreases. Dietary supplementation of sodium bicarbonate attenuates the decline in ruminal pH observed post-feeding and may attenuate SARA. The recommended inclusion rate for sodium bicarbonate is 0.75 to 1.0 percent of TMR dry matter.

Keunen et al. reported cows with experimentally-induced SARA did not attenuate SARA by consuming free-choice sodium bicarbonate. Researchers reported 20 milligrams per cow per day supplemental dietary biotin reduced laminitis-related hoof lesions, white-line separation and sole ulcers and improved sole ulcer healing. Dietary biotin supplementation did not influence ruminal volatile fatty acids or apparent total tract organic matter digestibility, and it is unlikely reductions in laminitis-related hoof lesions occur via an attenuation of SARA but rather via biotin’s role in keratization of hoof epidermis.

Nocek et al. reported reduced laminitis-related hoof lesions in five commercial dairy herds during the year of dietary supplementation with complexed trace minerals (zinc, manganese, copper and cobalt) relative to the year prior when these herds were not supplemented with this complexed trace mineral mixture. The role of trace minerals in keratization of hoof epidermis could explain this response.

Novel use of direct-fed microbials to enhance lactate utilization within the rumen has been reported with specific yeast strains. Controlled research trials in dairy cows are lacking. While some direct-fed microbials may have efficacy under situations of acute ruminal acidosis, their efficacy for controlling SARA is unclear.

Ration preparation and delivery

The extensive variation in DM, NEL and NDF concentrations found within bunker silos emphasizes the importance of meticulous face management with careful use of loader buckets or face shavers to minimize batch to batch variation and for obtaining samples representative of what is being fed. Switching between silo bags or even feed changes within bags can cause wide swings in nutrient delivery, especially if variation in DM content is unaccounted for.

Errors in nutrient delivery can occur because of failure to routinely determine the DM content of wet forages for adjustment of rations to maintain correct and consistent DM proportions of forage to concentrate. This becomes especially important after periods of heavy rainfall. To ensure consistent and accurate nutrient delivery and minimize SARA, effort should be made to assess and control these sources of error.

Mertens provided a minimum dietary peNDF guideline of 22 percent (DM basis). Particle size influences peNDF. Mixing the TMR for too long a period of time reduces particle size of the batch mix and was common in high-incidence laminitis herds.

Leonardi and Armentano reported extensive sorting away from long TMR particles which can reduce the peNDF content of the diet consumed relative to the TMR mixed depending on the proportion of long TMR particles, degree of sorting away from long particles and the weigh-back proportion. Procedures are available to evaluate forage, TMR and weigh-back particle size distributions in commercial testing laboratories or on the dairy to obtain a more accurate assessment of peNDF content of the diet consumed.

Beauchemin and Yang observed that increasing the peNDF content of diets increased chewing time, but increased chewing time did not necessarily reduce ruminal acidosis. Grain type (corn versus barley), harvest and storage method (dry versus high-moisture; DM content) and processing (rolled versus ground versus steam-flaked; fineness of grind) and corn silage DM content and processing influence ruminal starch degradability, which is a risk factor for SARA and laminitis. The guideline provided by Nocek for ruminal starch degradability was 60 to 70 percent of total dietary starch. However, analytical procedures for determining ruminal starch degradability of grains, corn or small-grain silages or TMR have generally been unavailable in commercial feed testing laboratories.

Blasel et al. reported on a laboratory method which provides an index of degree of starch availability (DSA) and ranked samples of corn grain appropriately for differences in particle size, DM content and vitreousness. Data from corn silage and TMR samples are limited. Research and field experience with DSA as a nutritional diagnostic tool is needed to determine its usefulness for minimizing SARA and laminitis.

SARA and cow comfort interaction and laminitis

SARA may or may not result in an increased incidence of laminitis. Environmental factors that influence lying and standing behavior may worsen laminitis-related hoof lesions triggered by SARA. Nordlund reported on the diagnosis of SARA without a high corresponding incidence of laminitis in three grazing herds and attributed this to the fact the cows were on dirt rather than concrete. Colam-Ainsworth et al. reported increased laminitis in cows that spent more time standing on concrete rather than lying in stalls.

Comparing his findings in grazing herds to diagnostic work-ups done in confined herds, Nordlund suggested the degree of SARA needed to trigger laminitis is greater for cows on dirt than for cows with significant exposure to concrete. Cow comfort and her environment must be evaluated when attempting to minimize laminitis in dairy herds. PD

References omitted due to space but are available upon request.

—From 2006 Four-State Dairy Conference Proceedings

Randy Shaver

Professor of Dairy Science

To contact Randy, e-mail him at rdshaver@facstaff.wisc.edu