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1493
Relationship of days in milk to nutrient digestibility in lactating multiparous cows
The objective was to determine if differences in days in milk (DIM) affect apparent nutrient digestibility in lactating multiparous cows fed the same ration. Of 136 cows in a multiparous, high producing pen on a 1,000 cow commercial dairy, 24 were selected for sampling based on differences in DIM and milk yield (Low DIM: n=12, milk=64±6kg, DIM=85±6; High DIM: n=12, milk=55±4kg, DIM=158±15). Milk, fecal and TMR samples were collected once daily for 10 consecutive days. Samples were collected in the morning on 6 days and afternoon on 4 days. Individual cow milk and fecal samples were composited into one daily milk or fecal sample per DIM group and analyzed for nutrient composition. In addition, daily fecal composite and TMR samples were analyzed for apparent digestibility of potentially digestible NDF (PdNDFd), NDF (NDFd), protein (Protd), and starch (Starchd). Fecal nutrient composition and apparent digestibility data were analyzed using the Glimmix procedure of SAS. The model included fixed effects of group, sampling time and their interaction. Day was included as a repeated measure. The CORR and REG procedures were used for comparing daily variations in digestibility to milk yield and composition. For fecal nutrient composition, DIM × sampling time was significant for ADF and NDF, due to sampling time differences in the low DIM group (Table 1). There was a group effect on fecal DM in which DM was lower in low DIM cows. Time effects were also observed, with CP being higher in the AM and ADF, NDF and ash being higher in the PM. There was no effect of DIM on PdNDFd, NDFd, Protd or Starchd. There was an effect of DIM × sampling time on Starchd (P=0.03) due to higher Starchd at the AM sampling for high DIM cows (P=0.03). There were no significant correlations of digestibility measures with milk yield. A 1% increase in PdNDFd (P=0.02) and NDFd (P=0.006) was associated with a 0.04% and 0.05% increase in milk fat percentage, respectively. A 1% increase in Starchd (P=0.003) was associated with a 0.03% increase in milk protein percentage. Since DIM had minimal effects on digestibility measures, it is likely not necessary to consider DIM when selecting cows for estimation of nutrient digestibility within a high producing pen.
Table 1. Effect of DIM on fecal composition and nutrient digestibility.