The process of pressing the silage to extract the juice removed not only moisture but also WSC and nonprotein nitrogen, creating a drier forage with proportionally greater fibre content than the original silage. The diets used in the present study were formulated to have similar forage NDF, aiming to reduce the difference in filling effect of the diets on intake regulation. However, even with greater inclusion of silage compared to SP, the diets showed important differences that have to be considered when comparing the complete substitution of silage for SP in the dairy cow diets. When compared to the silage-based diet, the SP-based diet was drier and with greater content of fibre components, such as NDF, ADF and forage NDF, which might have affected DMI and consequently milk production of dairy cows [7].

In the current study, the dietary DM varied from 533 g/kg in the silage-based diet to 671 g/kg in the SP-based diet and DMI was numerically greater for cows fed silage than for cows fed the SP-based diet, suggesting that DMI might have been affected by the DM content of the diet. However, the effect of dietary DM concentration on DMI and performance of dairy cows has previously been evaluated and results do not suggest that DM contents of diets play a major role on DMI and milk production. Leonardi et al. [18] evaluated two diets varying in DM contents (808 vs. 644 g/kg) for lactating cows and concluded that dietary DM did not affect DMI or milk production. However, Miller-Cushon and DeVries [19] claimed that the previous study tested diets that were much drier than regularly used in production dairy herds, and proposed a new study, in which the dietary DM concentration was compared in a more common range (576 vs. 479 g/kg). The authors observed that the drier diet showed a greater DMI, but milk production did not differ. Later, Fish and DeVries [20] compared diets that ranged in DM concentrations from 617 to 519 g/kg, and concluded that dietary DM had no effect on DMI or milk production of lactating cows.

On the other hand, the dietary forage NDF concentration was 9.1% higher in the SP-based diet (310 g/kg DM) compared to the silage-based diet (284 g/kg DM), while the DMI and milk production decreased at similar proportions, where DMI decreased by 8.2%, MY by 8% and ECM yield by 12% for cows receiving the SP-based diet compared to cows receiving the silage-based diet. In a recent meta-regression study, Allen et al. [21] evaluated the effect of dietary filling factors on lactating cows and observed that dietary forage NDF concentration was negatively correlated with DMI and MY. According to Allen [7], the dietary forage NDF concentration is the main factor limiting intake when intake is regulated by fill capacity of the rumen due to its slow passage rate. The effect of treatments on intake and milk production was more related to the diet formulation than to the forage source itself, suggesting that if SP and silage were included in the diet at the same forage NDF concentration cows would likely present a similar intake and milk production.

Silage pulp is a byproduct and its utilization in diets of lactating dairy cows should be performed with caution. In case of complete substitution of silage for SP, the inclusion of forage should be based on the dietary forage NDF concentration, otherwise intake can be compromised. If the forage to concentrate ratio cannot be reduced to meet a proper dietary forage NDF concentration in the diet, as in the present study due to the organic production regulations, the complete replacement of silage for SP is not recommended. Another alternative to use SP in the diet of dairy cows is to partially include it by mixing with silage. Savonen et al. [5] compared diets that substituted 0.25 or 0.50 (DM basis) of the silage with SP to a diet containing only silage as forage. The authors observed that the substitution of 50% of the silage for SP did not affect DMI or MY, but there was a tendency of lower ECM when compared to the silage-based diet. However, when 25% of the silage was substituted by SP, no difference was reported on MY or ECM but intake was greater, reducing its feed efficiency when compared to the silage-based diet. The authors concluded that the reduction in performance was considered mild and SP can be partially included in the diet of dairy cows if there is a reduced feed cost associated.

The milk composition between treatments was similar throughout the current study, however, as MY was generally greater for cows fed the silage-based diet, yields of milk components were greater than when cows received the SP-based diet. Body weight and BCS were not affected by the treatments throughout the study, suggesting that milk production was supported by the diets only and body reserves were preserved. Researcher hypothesized that the mechanical pressing during silage juice extraction would increase the digestibility of the fibre in the SP, resulting in greater NDF digestibility that could compensate for the greater NDF content. However, there was no difference in NDF digestibility between silage and SP, and the greater in vitro OM digestibility observed in the silage was basically related to the greater concentrations of CP and WSC, and greater proportion of protein Fraction A, which has rapid and complete rumen degradation, compared to SP. Similar results were observed by Savonen et al. [5] when SP replaced 50% (DM basis) of the dietary silage. The authors evaluated the apparent total tract digestibility in dairy cows using indigestible NDF as internal marker and reported that there was no difference in NDF digestibility between forages, but OM digestibility was greater for silage compared to the silage-SP mixture [5].

The smaller concentration of CP observed in SP was caused by the loss of nonprotein nitrogen (Fraction A) during the mechanical pressing, which increased the proportion of rumen undegradable protein. Nonprotein nitrogen represents the majority of CP in silage [22], which easily and extensively can be hydrolyzed to ammonia in the rumen [23], reducing the efficiency of nitrogen utilization that increases urinary excretion by ruminants [24]. Thus, the reduction of nonprotein nitrogen concentration in SP compared to the original silage might benefit not only the ruminant nutrition, but also the environmental concerns related to livestock waste.

In conclusion, the mechanical pressing process did not increase fibre digestibility in SP, thus, the complete substitution of silage for SP reduced the milk production of dairy cows over time.