There is evidence of the ability of dairy cattle to alter eating behavior or diet choice based on rumen environment, this study looks at the influence of Subacute ruminal acidosis on diet preference and DMI of lactating dairy cows fed diets differing in corn silage particle size and hay availability.

Cereal grains are used to provide high-producing dairy cattle require large amounts of dietary energy. These highly fermentable carbohydrates can result in build-up of organic acids in the rumen and reduce rumen buffering causing a depression in rumen pH. Subacute ruminal acidosis (SARA) occurs when rumen pH is below 5.5 for more than 3 hours per day. Based on reduced milk yield and efficiency of production, premature culling, and death loss it has been estimated that SARA costs the US dairy industry $500 million to $1 billion per year, with a cost per affected cow of $1.12 per day.

Simulating SARA conditions by withholding feed for 12 to 24 hours or reducing intake has been studied to find solutions to this costly issue. Previous research suggests that ruminants will select a diet to help maintain a healthy ruminal environment when given the opportunity to do so. Growing evidence indicates that dairy cows will select feeds with high ruminal buffering capacity in an attempt to attenuate effects of very low ruminal pH.

Despite evidence of the ability of dairy cattle to alter their eating behavior or diet choice based on their rumen environment, no research has been published observing the influence of SARA on the diet preference and DMI of lactating dairy cows fed diets differing in corn silage particle size and hay availability. Therefore, the objectives of this experiment were to induce a bout of SARA in lactating dairy cows that had ad libitum access to four distinct high starch diets that varied in corn silage particle size and supplemental hay and to determine how SARA affects TMR feed preference in lactating dairy cows under this situation.

The experimental protocol was reviewed and approved by The Pennsylvania State University Institutional Animal Care and Use Committee.

Cows were offered 1 of 4 diets that were similar but varied in corn silage particle size and supplemental second cutting orchardgrass hay:

1) short corn silage total mixed ration (TMR; ST)

2) short corn silage TMR with 5.6% supplemental hay (SH)

3) long corn silage TMR (L), and

4) long corn silage TMR with 5.6% supplemental hay (LH)

Cows were allowed to adapt to this feeding scheme for 14 days, and cannulated cows were then subjected to a rumen challenge to induce a bout of SARA by restricting feed before the challenge and providing 4 kg of ground wheat via the rumen cannula. Although baseline pH was low, the SARA challenge lowered ruminal pH further for all cows regardless of diet. Daily average rumen pH decreased from 5.44 and 5.45 to 5.33 and 5.38 for ST and SH, respectively, and from 5.64 and 5.54 to 5.47 and 5.39 for L and LH, respectively, from baseline to challenge phase.

Decreasing corn silage particle size significantly increased TMR and total DMI during all phases of the model. Feeding short corn silage TMR increased milk, protein, and lactose yields. Cows fed supplemental hay had increased fat yield and protein concentration in the milk and responded minimally to the effects of particle size selection when challenged with SARA. Cows consuming short corn silage TMR changed feed preference for longer forage particles during the course of the SARA challenge. It is important to note that during the recovery phase, however, cows refused an average of 13.5% of the offered TMR and 78.7% of the supplemental hay. These results indicate that cows preferred eating shorter particle size diets, and did not prefer the supplemental hay regardless of corn silage particle size during a bout of SARA, which may have been due to the possible adaptation of a low ruminal pH throughout the study. In addition, the data from this study show that using a TMR particle separator with a 4.0-mm sieve may be a better indicator of physical effectiveness.

Authors: A.D. Kmicikewycz,  A.J. Heinrichs

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