Currently there is a growing interest in steering livestock production towards more sustainable systems. The recent end of the milk quota system in Europe after 30 years has coincided with an increase in the price of agricultural commodities along with lower price of raw milk. This fact has forced dairy farmers to reduce costs, improving the efficiency in the use of their own resources. Maize silage provides a relatively low cost source of energy, in the form of starch and fiber which complements pasture (Kolver et al., 2001), because it provides a high proportion of grain and relatively digestible fiber. The high content of starch in maize has led to a high appreciation of its value mainly in dairy farming, and the low crude protein concentration of maize silage makes it an ideal component in protein-rich grass-based rations. From a livestock perspective, the benefit of maize silage can be achieved when it is used as a supplement to pasture, not as substitute for it (Macdonald, 1999), because the increase in maize cultivation area causes a large increase in greenhouse gas emissions by ploughing vast areas of grassland (Vellinga et al., 2004). The profitability of any supplementary feeding system is highly sensitive to the price of milk and supplements, and the different associated costs, especially labour (Macdonald, 1999).

The crop rotation maize-Italian ryegrass is continuously used in many of dairy farms located in areas that allow the mechanization of plots, due to their high potential of dry matter yield. Maize silage is the main constituent of lactating dairy cow diets in many American and European farms, representing between 300 and 800 g/kg of forage dry matter content of diets (Gallo et al., 2016). This rotation needs high amount of N fertilization that has negative effects on the soil (Heinze et al., 2011). The excessive use of N fertilization causes a huge change in ecosystems, incurring in soil degradation contamination of groundwater and atmosphere, which causes a progressive decrease of the soil organic matter content, affecting its physical, chemical and microbiological properties (Caravaca et al., 2002). The abuse of N fertilization has caused major environmental problems because the plants cannot assimilate all the N supplied and hence between 50 and 70% of N is transferred to the ecosystem, causing water pollution, eutrophication (Good & Beatty, 2011), and even generating biologically dead zones (Bristow et al., 2017). The efficiency of chemical fertilizer used in maize silage cropping has become a major concern, as the crop is often negatively connoted to N-aspects of surface and groundwater quality (Schröder et al., 2000). External inputs of N and phosphorus on the farms should be reduced for environmental and economic reasons. At present, the production of quality forages must be environmentally and ecologically sound and aligned with public values, because the livestock production account for about 9% of total anthropogenic greenhouse gas (GHG) emissions (IPCC, 2014).

Manure and slurry application to crop fields can recycle animal wastes and be a valuable source of nutrient. A large proportion of the dairy manure is applied to land in maize production for silage. The benefit of dairy manure application on maize silage production has been reported (Butler et al., 2008), and can be attributed to the improvement of physical and chemical edaphic properties (Butler & Muir, 2006) and to an increased P (19%) and K (21%) uptakes in maize (Singer et al., 2007). However, proper manure management is important when livestock densities are high and could potentially lead to high N-loading rates to agricultural land. The objective of this study was to evaluate the effects of the application of organic (manure) or chemical fertilization on maize yield, silage quality, cows' performance, as well as on the feeding costs, economic outcomes, and on the emission of nitrous oxide and enteric methane.

During maize growth (June to October 2011), daily average temperature was 17.7 °C (range: 12.0-23.5 °C) and the total rainfall was 277 mm with 53 rainy days. During the course of the animal trial (February to May 2012), daily average temperature was 10.8 ºC (range: 3.4-18.5 ºC) and total rainfall was 259 mm with 62 rainy days. The temperature data were similar to those historically recorded. However, the amount of rainfall was 27% lower than the average for the last 35 years for the same months.

The maize yield was a 20% higher in MnF than ChF treatment (13.3 vs. 10.7 t DM/ha respectively). The means of chemical composition and fermentative parameters are presented in Table 1. Both types of maize silage had a similar nutritive value, except the concentration of starch, that was higher ( p<0.05) in MnF (35.5%) than in ChF (31.0%). Ammonia-N concentration and acetic acid proportion were not affected by the fertilization. The lactic acid proportion was higher in MnF silage than ChF silage (5.2 vs. 4.3% lactic acid, respectively; p=0.095). The proportions of propionic and butyric acids fall below the limit of detection. The highest starch concentration in MnF allowed making a PMR with 5.4% less of concentrate when maize silage with organic fertilization was used, in order to formulate two isoenergetic (1.51 Mcal NE _l/kg DM) and isoproteic (13.3% CP) PMRs (Table 2). The pasture had 13.4% CP, 54.5% NDF and 1.39 Mcal EN _l/kg DM. The average nutritive value of both concentrates was 19.0% CP and 1.86 Mcal EN _l/kg DM.

Table 3 shows the total DMI and the intake of each ingredient included in the diet for both treatments. No statistical differences were observed in the total DMI between treatments; although that of MnF diet was numerically 10% lower than for ChF (17.7 and 19.8 kg DM per day, respectively). This is particularly due to the lower intake of grass during grazing by animals in MnF treatment (4.3 vs. 5.4 kg DM/d for the MnF and ChF treatments, respectively; p>0.05). However, there was a lower intake of concentrate included on PMR in treatment based on MnF silage than ChF silage (2.8 vs. 3.2 kg/d respectively; p<0.05) as a result of the lower inclusion of concentrate on PMR in MnF treatment as well as the lower intake of PMR in this treatment. The lower DMI of the animals in the MnF treatment was reflected in a decrease in the live weight throughout the experiment, although without differences between treatments (590 kg live weight in both groups at the beginning of the experiment, and 583 kg for the MnF treatment and 599 kg for the ChF treatment at the end of the experiment). The body condition score also changed over the experimental period (2.56 at the beginning in both treatments, and 2.45 vs. 2.65 at the end of the experiment for the MnF and ChF treatments respectively; p<0.05).

The daily averages of production and milk composition are shown in the Table 4. No differences were seen between treatments with respect to milk production (25.4 kg/d), fat (38.9 g/kg), protein (32.7 g/kg) and lactose (48.9 g/kg) contents.

The concentrate intake per kilogram of milk produced was of 142 g/kg in cows with MnF treatment, while it was of 160 g/kg in cows with ChF treatment ( p<0.05). Feeding costs and incomes from the sale of milk for the treatments based on MnF and ChF silages are shown in Table 5. The cost of each ingredient produced on-farm (maize silages grown using organic or chemical fertilization, grass silage and forage grassland) was calculated based on the cost of crop production, involving the whole process (seed, fertilizers, labour, machinery and facilities). The production cost of tonne of DM of the maize silage with chemical fertilization was 86.5 €, while with organic fertilization was 78.9 €. The grass silage costs 69.0 €/t DM and, the estimated cost of pasture was 4.9 €/t DM. The cost of feedstuffs purchased off-farm (barley straw and concentrates) was calculated per kg of DM according to the current market prices. The price of straw was 0.114 €/kg DM and the concentrates of PMR, S and L were 0.386, 0.365 and 0.444 €/kg DM respectively. The ChF diet was more expensive than the MnF diet (2.49 vs. 2.24 € per cow and day respectively). This difference reflects the higher DMI of cows feeding ChF PMR than MnF PMR, as well as the higher production cost of maize silage grown using chemical fertilizers and, especially, the higher inclusion of concentrate in the PMR based on ChF silage, increasing spending on purchases of feedstuffs off-farm (1.76 vs. 1.58 € per cow and day for cows in ChF and MnF treatments respectively).

The slightly higher fat content of cows milk in the MnF treatment (Table 4) causes a slight difference in the price paid by the dairy industry (0.299 €/kg for cow's milk in the MnF treatment vs. 0.294 €/kg in the ChF treatment). Although the daily gross income per cow was higher in the ChF than MnF treatment, with the different feed cost between managements, the overall net margin of profitability was 0.217 € per cow and day or 0.013 €/kg higher for cows in the MnF than for those in the ChF treatment (Table 5).

Greenhouse gas emissions per cow, per DMI and per kilogram of milk expressed as carbon dioxide equivalent are given in Table 6. More than 85% of methane emissions are due to enteric fermentation, being higher in ChF than MnF diet (817 vs. 714 L CH ₄/cow and day respectively). The difference was diluted when it refers to DMI (41.3 and 40.4 L CH ₄/kg DMI, respectively). The estimated N ₂O emissions due to soil management were similar in both treatments (13.3 g N ₂O/day). The prediction of total CO ₂eq emission in ChF treatment was higher than MnF (up to 13%; p<0.05). The difference observed in this study was due to the diet and not to the type of fertilization, because there were no differences in soil management nor manure excretion between treatments. There were no differences when GHG emissions were expressed to DMI, however a 10% higher production of CO ₂eq per kg of milk was observed in ChF than MnF (0.67 vs. 0.74 kg CO ₂eq/kg respectively, p<0.05).

In the present work, the different fertilization of maize with organic or chemical sources affected forage yield, being higher with manure fertilization than with synthetic fertilizers. The increase in maize silage production with organic fertilization agrees with that reported by other authors who examined dairy manure, chemical fertilizer and combinations of manure and chemical fertilizer (Butler et al., 2008). Manure increases soil fertility supplying K, nitrate-N and ammonia-N to aid crop production (Nevens & Reheul, 2005). The effect is higher with manure as fertilizer than slurry, because applications of slurry do not lead to such an increase of residual mineral N (Schröder, 1999). Manure N must be mineralized before it becomes available to plants (Klausner et al., 1994). Therefore, in the year of application, only a portion of it was available to the crop and the remaining was carried over to subsequent years. However, the silage maize takes up a relatively low amount of N owing to the short growing season and the poor root extension. In spite of this, the forage yield in MnF could have been increased because of the higher content of organic carbon (Loveland & Webb, 2003) or available water, due to an important water retention, in manured soils (Arriaga & Lowery, 2003).

Butler et al. (2008) have reported higher concentrations of NDF and ADF in maize silage with organic fertilization than inorganic fertilization. However, these differences were not consistent among years. Wachendorf et al. (2006) reported higher concentration of CP and lower net energy in maize silage with organic fertilization, apparently because of the possible effects on competition for water and nutrients from the grass understory. In any case, all these differences were small and biologically insignificant. The nutritive value of maize silage is largely determined by the cob-stover ratio. Although the proportion of cob was not measured in this experiment, previous work carried out reported a higher percentage of cob in maize silage produced with organic fertilization than chemical fertilization (Martínez-Martínez et al., 2009). There have been also reported higher maize grain yield from manure as compared to fertilizer (Eghball & Power, 1999). In the present work, the application of organic fertilization did not change the nutritive value of maize silage, except the starch concentration, an indicator of the high cob production, which was higher in MnF silage than ChF silage. A higher concentration of starch means more lactic fermentation capacity (Mogodiniyai Kasmaei et al., 2013), which would explain the higher concentration of lactic acid in MnF silage than ChF silage, which is a guarantee in the fermentation process. The higher concentration of starch in MnF leads to slightly higher energy values. This allowed making a PMR with 5.4% less of commercial concentrate in MnF treatment than ChF treatment. When the cost of concentrate is high relative to the price of the animal product, one of the potential benefits of including alternative forage is the potential to maintain animal performance whilst reducing concentrate feed level. This saving in feeding costs with the MnF diet comes in addition to a substantial saving in the cost of maize silage from organic fertilization, circa 9%. The maize silage is produced at 20% higher costs than 3-cut grass silage (Keady et al., 2012) in agreement with our results for cost of maize and grass silages. In the present study, both treatments had no effect neither on milk production nor milk composition. The absence of any difference in milk yield might be explained by the adequate net energy intake of the cows, which is further confirmed by the absence of variation in live body weight.

Grazing has been proposed as an essential strategy for the efficient use of pastoral resources, which are abundant in wet temperate areas. In these climatic conditions, grazing is allowed all year round. This fact allows savings in the cost of feed in the dairy farms, which could provide an increase in the profitability. However, in this experiment, grazing occurred only for 6 hours daily as a result of drought that year, since rainfall was almost one-third less than the average of historical records (Infomet, 2015). Despite this, the intake from grazing reached over 26% of the total DMI, and only accounted for 1% of the total feeding costs. The concentrates used, bought off-farm, represented 21% of the total DMI, but account over 66% of the total feeding cost. The concentrate intake per kilogram of milk produced was lower in cows with MnF than ChF treatment. The expenses on concentrate in MnF treatment represent 58 €/t of milk while the ChF treatment spent 64 €/t of milk. The difference between MnF and ChF treatments reflects a saving of 25.3 eurocents in the cost of feed per cow and day, and represents the creation of 21.8 eurocents in added value. Given these conditions, for the average herd with 40 dairy cows grazing, using maize silage produced with organic fertilization in the diet could bring increased incomes of over 3000 € per year.

The higher total DMI in the MnF treatment led to greater daily enteric CH ₄ emission than by the ChF fed cows, because of the level of DMI is the main driver on methane emissions in cattle (Bannink et al., 2010). In addition, this was favored by the differences, although not significant, toward a higher grass intake. The pasture contains a high concentration of structural carbohydrates that increase the rumen retention time and affect the fermentation pattern, which results in a greater methanogenic capacity (Janssen, 2010). In both treatments, the enteric CH ₄ emissions estimated were higher than the estimated values by Legesse et al. (2011) or measured in respiratory chambers by Brask et al. (2013). However, the proportion of forage in all these studies was 60% or less. Aguerre et al. (2011)studied the effect of forage-to-concentrate ratio in dairy cow diets on GHG emission. Increasing the proportion of forage from 47% to 68% in the diet increased CH ₄ emission from 0.538 to 0.648 kg CH ₄ per cow and day. In our study, the diets had a 79% forage, and therefore, this could explain our higher estimated GHG emissions. Nitrogen oxide emissions generated by soil management were similar between diets because both chemical and organic fertilizers had equivalent amounts of N and crop residues were similar. On the other hand, N ₂O is also produced directly through nitrification and denitrification, and indirectly by the volatilization and leaching of the manure's N. Urinary N is more labile than fecal N and it is considered the main contributor to NH ₃ and total N losses. Therefore, it is important to reduce the urinary N losses and/or derive N excretion through the faeces (Hristov, 2013). Our results demonstrate that is possible reducing the CO ₂eq emissions with the use of manure as own source, without lowering the milk production.

On the basis of the results obtained, it could be concluded that using organic fertilization in the studied conditions saves costs of maize crop for silage. The silage produced with this management had higher starch content, reducing 11.3% the intake of concentrate per kilogram of milk in grazing dairy cows, without increasing the voluntary intake of grass. This fact makes significant feed cost savings per cow and day, increasing profit margins. The use of maize silage grown with organic fertilization does not alter the milk yield and raw composition. The results show that using organic fertilization on maize culture is possible reducing the GHG emissions with regard to chemical fertilization without lowering the production.

The authors would like to thank the staff of Laboratory of Animal Nutrition (SERIDA) for undertaking laboratory determinations.