3.1. Dairy Cows Fed Potato By-Product (Experiment 1)
The chemical composition and feed values of the feeds in the dairy cow experiment are shown in
Table 2. The basal grass silage was of good nutritional and hygienic quality. The values for the PBP reflected closely the values of whole potato given in Luke [
16] and Feedipedia [
28] and indicate that in the peeling process, a lot of potato was included in the PBP. The PBP had a much higher starch content than the concentrates and a high energy value, but the protein values were lower due to the low CP content of it.
The microbial quality of feeds and TMRs fed to dairy cows is presented in
Table 3. The microbial quality of the individual feeds was numerically better than the final TMRs offered to cows. The microbial quality of the TMR containing PBP was similar to Control immediately after preparation. In the production trial, the development of hygienic quality of the TMR or the quality of the PBP over the one-week storage period were not determined. Based on Experiment 2, storage time may pose a risk in terms of faster deterioration of the TMR with PBP, which might be reflected in decreased intake. However, in this case a formic acid-based preservative was used to prolong the shelf-life of PBP, and no fluctuations in feed intake nor subjective signs of deterioration were observed either in respect to TMR storage time or within the one-week storage period of each PBP batch.
The inclusion of PBP in dairy cow diets resulted in lower (
p < 0.05) feed and nutrient intakes (
Table 4). Consequently, the daily nutrient intakes of OM, CP and NDF were lower (
p < 0.05) than in the control diet, but due to the high starch content of PBP, the starch intake was higher (
p < 0.05) in the PBP diet. The OM, CP, NDF and iNDF intakes were lower in the diet containing PBP, while the starch intake was higher (3.91 vs. 2.54 kg/day;
p < 0.05). The reduced intakes might have been due to a greater starch content in the diet containing PBP, as reported by Lechartier and Peyraud [
29]. This contrasts with Onwubuemeli et al. [
30], who reported no difference in dry matter intake when feeding dairy cows with wet potato processing waste up to 20% of the diet on a dry matter basis. The lower CP concentration was due to the lower CP concentration in PBP compared to the cereals it replaced in the diet. We decided not to compensate for that by adding more rapeseed meal into the PBP diet and thus confound the effects of PBP and rapeseed meal inclusion. A similar MP concentration in the diet and a clearly positive PBV value for the PBP diet suggest that the diets were not deficient in protein and the small difference in diet protein concentration probably did not influence the results obtained from this experiment.
The cows received formic acid as part of the PBP. Additionally, the silage contained a formic acid-based additive so that the daily intakes of formic acid were 154 and 194 mL (5.4 and 7.4 mL formic acid/kg of TMR) per cow for the control and potato groups, respectively. The amount was somewhat higher for the potato group, but this was probably not a major factor affecting the outcomes of the experiment. Kara et al. [
31] reported that no significant differences were observed between 4 and 8 mL of formic acid/kg of TMR on ruminal fermentation parameters. Additionally, based on the recommendation from EFSA [
32], the formic acid supply to cows was lower than the maximum proposed dose (10,000 mg (8.2 mL) of formic acid/kg of TMR). Furthermore, considering the fast perishability of the potato by-product, it was necessary to extend the shelf life of this ingredient by using a formic acid-based preservative.
The apparent total-tract digestibilities estimated by using iNDF as an internal marker are presented in
Table 5. The DM and OM digestibilities were greater (
p < 0.05) in the diet with inclusion of PBP. No statistical differences (
p > 0.05) were found for CP, starch and NDF digestibilities. The greater DM and OM digestibilities for the diet containing PBP were probably due to its higher starch content, which has greater digestibility, and at the same time, lower NDF content, which has lower digestibility. The digestibility values were low which is also highlighted by the lower diet ME concentration when estimated from OM digestibility compared to the one based on feed values. As a methodological challenge, incomplete recovery of iNDF leads to underestimation of the digestibility values, but it may still be assumed to detect the differences between the dietary treatments (see, e.g., Savonen et al. [
17]).
Starch degradation may vary according to its source, and thus the risk of acidosis should be considered when including potato in ruminant diets. Monteils et al. [
33], comparing the ruminal degradation of wheat and potato starches, found that the concentrations of total VFA and ruminal pH were more variable for wheat than for potato in grass silage-based diets. Additionally, they identified that wheat can be completely replaced by potato in maize silage-based diets without risk of acidosis nor negative effects on digestion.
In our experiment, milk production in kg/day was lower (
p < 0.05) for cows fed PBP, but the difference did not reach significance for ECM (
p = 0.12), as the milk solids content was higher (
p < 0.05) in the milk of cows receiving the PBP (
Table 6). In contrast to our work, Onwubuemeli et al. [
30] reported that milk production was not affect by incremental levels of wet potato processing waste up to 20% on a dry matter basis as a substitute for high-moisture maize in dairy cow diets. Literature results are very variable, as Mosavi et al. [
10] reported minor decreases in milk production, while Tavares et al. [
13] found slightly greater milk production and Eriksson et al. [
11], Zunong et al. [
12] and Jurjanz et al. [
34] found no difference in the performance of cows fed potato by-product. The differences in responses are likely to be due to the quality of potato-based feed in relation to the dietary ingredients it is replacing in the diet, but in none of the experiments cited previously were clearly adverse effects detected.
Daily protein and lactose yields were lower (
p < 0.05) for cows fed PBP (
Table 6), while no difference was found (
p > 0.05) for fat production. The milk fat content tended to be higher (
p = 0.06) while no statistical differences (
p > 0.05) were found in milk protein and lactose concentration. The increased fat concentration may be linked to the higher starch content of the PBP diet and a similar effect was observed by Jurjanz et al. [
34]. Jurjanz et al. [
34] studied the effect of starch sources in dairy cow diets, such as wheat and potato peelings, and did not observe any effect on DM intake and milk production, but milk fat concentration was higher when potato peelings were fed at a higher starch concentration.
The slightly improved (
p < 0.05) efficiency of nitrogen utilization in response to PBP inclusion merely reflects the decreased CP intake on that diet, as nitrogen use efficiency is directly related to CP intake [
35]. No difference was found (
p > 0.05) for energy-corrected milk in kg/kg DM intake.
The reason for the reduced feed intake and milk production in response to PBP inclusion in the diet could not be unequivocally explained. Subjective observations during the experiment did not give reason to suspect hygienic problems of the PBP. In addition, no problem related to palatability was observed. Schneider at al. [
36] emphasized that potato is a palatable alternative to dairy cows with approximately the same energy value as ground maize, at least when offered as a meal. The main difference between the diets was the higher starch intake of PBP-fed cows, but it did not impair fiber digestion and was still at a moderate level, so problems related to rumen acidosis, for example, should be unlikely.
Although milk production was slightly reduced, the economic output of the system depends on the costs of the feeds in relation to each other as well as milk price. A viable option would also be to use the by-product feeds in the diets of other livestock groups, such as growing cattle. The environmental benefits depend on the alternative uses of the by-product, but in general, feed use can be considered more useful than other uses, e.g., in soil amendment. The net food production efficiency can be improved by the use of non-human-edible by-products [
37] and it can be assumed to improve the acceptability of livestock production. The use of by-products can also decrease the carbon footprint of animal products, but it is partly dependent on the allocation of CO
2 equivalents to the main products and by-products.
3.2. Aerobic Stability of Total Mixed Ration and Potato By-Product (Experiment 2)
The heating of TMR may be a serious practical problem as mixing various feed components provides inoculation, aeration and multiple substrates for microbes, accelerating spoilage, and moist by-products may be particularly challenging [
38]. The chemical composition and microbial quality of the ingredients used to prepare the TMRs in the aerobic stability trial at laboratory-scale are presented in
Table 7. The silage DM concentration used in the TMR recipes was slightly higher than in the dairy cow experiment, while the DM of PBP was similar. Starch and OM contents were higher in PBP than in silage and concentrate, while its contents of ash, CP and NDF were lower. The microbial quality parameters of PBP were closely comparable to those in the silage, except for lactic acid bacteria, which was higher in PBP. The presence of enterobacteria and molds were higher in the concentrate than in silage and PBP. Due to the low DM content of PBP, water was added in TMRs 1 and 2 (with lower inclusions of PBP) to be able to compare the TMRs at a similar DM level. The achieved DM levels were 356, 352 and 385 k/kg for TMR1, TMR2 and TMR3, respectively.
There were no statistically significant interactions between the main effects in Experiment 2 such that the results are presented by the main effects in
Table 8. To show the individual treatment effects for aerobic stability,
Figure 1 is included. The effect of PBP inclusion on aerobic stability was quadratic (
p < 0.05) (
Table 8) as it drastically reduced from TMR1 to TMR2 (117 vs. 58 h) but further reduction in TMR3 was minor (54 h). The addition of FPA linearly increased (
p < 0.05) the aerobic stability of the TMRs. For the salt-based preservative, a quadratic pattern (
p < 0.05) was observed, in which the improvement in aerobic stability was modest at low dose and greater at the high dose.
Figure 1 shows that the responses to preservatives were greater in the most stable TMR1, and their efficacy decreased in the more challenging conditions of TMR2 and TMR3. A similar phenomenon was demonstrated by Seppälä et al. [
39] using brewers’ grains or low-quality ingredients to challenge the TMR stability.
The visually observed first spot of mold and complete spoilage were quadratically affected (p < 0.05) by the inclusion of PBP in the TMR. The TMR1 spoiled later than TMRs 2 and 3. Both preservatives improved (p < 0.05) the shelf-life of TMRs with increasing levels of PBP by delaying the first appearance of visible mold and complete spoilage.
The aerobic spoilage of the PBP as such and the effects of the preservatives were also evaluated (
Figure 2). Both preservatives delayed (
p < 0.05) the appearance of mold and complete deterioration of PBP, but Salt was more effective (
p < 0.05) than FPA. Rinne et al. [
15] also noticed a clear delay in visual spoilage of a moist carrot by-product in response to a formic acid-based preservative. Chemical preservatives seem to be efficient in preserving moist by-products and the clear dose response to chemical preservatives indicates that in case of spoilage problems in practice, increasing the dose is a recommended strategy. Some researchers have also successfully ensiled potato or other moist vegetable by-products mixed with higher DM feed components to alleviate the storage challenges [
13,
40].