Ensuring silage supply meets demand

This article supplied by SAC Consulting Ruminant Nutritionist, Lorna Shaw, looks at how to calculate silage demand before fields are shut for silage.

Silage demand for many farms can be a difficult thing to predict with multiple factors affecting how much silage is harvested and how much will be used over the winter period.

However, it can be useful to do a rough calculation of demand before fields are shut away for silage production to ensure enough allocation has been made.

It is important that this decision also comes without detriment to summer grazing availability which should come as a first priority to ensure the performance of lactating stock at mating and growing/finishing stock on farm. 

Take a 120-head suckler cow herd, for example, selling all calves as suckled stores in the autumn. Allowing for housing in early November, calving March/April and turnout in May, the demand for forage would be as follows (please note some assumptions must be made as forage quality will not be known at this stage):

• Number of cows: 120
• Average weight of cows (kg): 700
• Daily dry matter intake (DMI) when dry (kg): 10.5
• Daily DMI in lactation (kg): 14
• Total DM/cow over 5-month dry period (t/152 days): (10.5 × 152)/1000 = 1.596 t
• Total DM/cow over 1-month lactation while housed (t/30days): (14 × 30)/1000 = 0.420 t
• Total DM while housed/cow: 1.596 + 0.420 = 2.02 t
• Total DM (whole herd): 2.02 × 120 = 242.4 t
• Total DM required allowing for 20% wastage/surplus: 242.4 × 1.2 = 290.88 t
• Total fresh weight required (assuming 30% DM): (290.88 ÷ 30) × 100 = 969.6 t fresh weight

Assuming an average yield of 5 t DM/ha for the first cut, this would require 58 ha of silage ground to meet demand.

Typically, second-cut yields can be around 80% of the first cut (4 t DM/ha), so based on this assumption it would require around 32.5 ha of silage ground cut twice.

Another way to calculate supply is to work back from last year's (or an average of a few years) silage cuts to predict an average yield into the pit or look at bales per hectare using an average weight of bales, yields are likely to vary year-on-year depending on weather, nutrient applications etc.

Actual silage quality will not be known at this stage, so it is recommended to analyse forage once ensiled after a minimum of six weeks, look at winter rations based on the analysis and carry out a full feed budget in the autumn to ensure sufficient supplies.

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