This study was conducted to evaluate the effect of partial and total substitution of alfalfa hay (AH) by pistachio (Pistacia vera) by-product (PB) on counts of selected cellulolytic ruminal bacteria attached to AH by real-time PCR technique and to determine the effect of this replacement on degradability of AH in Baloochi sheep. Six Baloochi sheep, averaging 40.171.77 kg BW, fitted with ruminal cannula were assigned at random to three diets in a double 33 Latin square design. The dietary treatments were control (basal diet), low PB (LPB) diet (0.50 of AH in basal diet replaced by PB), and high PB (HPB) diet (all of the AH in the basal diet replaced by PB). The daily basal diet was 400 g AH dry matter (DM), 200 g wheat straw DM, 168 g barley grain DM, 24 g cotton seed meal DM, 6.4 g vitamin–mineral supplement DM and 1.6 g salt DM. Alfalfa hay was placed in to the rumen for in situ degradability of DM, organic matter (OM) and microbial attachment. Bacterial populations were assessed by DNA extraction of incubated alfalfa samples followed by real-time polymerase chain reaction analysis. The population of Fibrobacter succinogenes and Ruminococcus albus that were attached to AH decreased with increasing the level of PB in diets (Po0.05). The effect of dietary treatments on the population of Ruminococcus flavefaciens and total bacteria was not significant. PB replacement for AH decreased ruminal DM and OM degradability of AH (Po0.05). It was concluded that tannins from PB reduced the attachment of cellulolytic bacteria as well as in situ degradability of AH.

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