*2.2. Feedstock Yield*

The farms simulated in this study were selected based on their ability to collectively provide a full representation of the Irish dairy industry, which consists of mainly small to medium-sized farms, as illustrated in Figure 2. The study used a co-digestion feedstock of both dairy cow manure and grass silage. Grass silage was selected because of its popularity in Ireland, where 80% of agriculture land is devoted to pasture, hay and grass silage [24]. Furthermore, Ireland has ideal climate conditions for grass production, experiencing mild and moist conditions, an abundance of rainfall and a lack of extreme temperatures [25,26].

**Figure 2.** Distribution of total farms and dairy cows by farm size in Ireland. Estimated, based on data received from [27].

Five dairy farm sizes were selected, these relate to the assumed herd sizes of 50 dairy cows (Scenario 1); 100 dairy cows (Scenario 2); 150 dairy cows (Scenario 3); 200 dairy cows (Scenario 4); and 250 dairy cows (Scenario 5). The number of dairy cows refers to the number of female bovine dairy cows, which had already calved and were kept exclusively to produce milk.

The dairy enterprise is based on a self-contained Holstein–Friesian herd [28], retaining pure-bred replacements and selling beef crosses at three weeks. Dairy cows are culled, on average, after five lactations (i.e., annual replacement rate of 18%), which is common in Ireland [29,30]. Manure is predominantly collected from the milking parlour and the cattle housing units (mainly slatted sheds) [31]. The quantity of manure produced per adult cow, heifer, and calve is presented in Table 1. Over the 16 week winter period, it was assumed all manure produced was collected for digestion as the cows are housed [32]. It was more difficult to estimate manure collection over the grazing period (remainder of the year) as collection mainly occurs when the cows are being milked. Based on a milking rate of two times per day and the increased metabolic rate during this period, a 20% manure collection rate was assumed in comparison to Table 1 figures, i.e., 10.4 kg fresh weight (FW) day−<sup>1</sup> for adult cows, 7.44 kg FW day−<sup>1</sup> for heifers, and 3.72 kg FW day−<sup>1</sup> for calves.


**Table 1.** Characteristics of dairy livestock.

a Assumed, based on reports for livestock weight by [32]; b Ultimate analysis presented in [33] for dairy cow manure production.

In the model, it was assumed that the dairy enterprise was the primary source of income, with revenue from biogas production being a supplementary income stream. Consequently, the needs of the dairy herd were prioritised, with only surplus crops used for biogas production. The area of farmland available to grow feedstock was estimated by subtracting Ireland's mean farm size (based on herd size) from the area of land required to sustain the dairy herd. The mean farm sizes for the scenarios considered corresponded to 43.51 (Scenario 1), 68.74 (Scenario 2), 93.96 (Scenario 3), 119.19 (Scenario 4), and 144.41 hectares (Scenario 5) [27]. The area of farmland required to sustain the dairy herd was based on a recommended ratio of 2.8 cows per hectare with an additional 20% margin of safety added, to account for seasonal variations and unusable land [32]. Silage yields in Ireland are typically between 11 and 15 t dry solids (DS) ha−1; yields are generally higher in the southwest and decrease towards the northeast [25,34]. The model assumed an average yield of 13 t DS ha−<sup>1</sup> to enable it to represent the majority of Irish dairy farms.
