Achieving High Yield and High Digestibility With First-Cut Silage

Keep silage costs down
Grassland-based systems of beef and milk production in Ireland will continue to rely on conserving surplus summer grass as silage, and feeding this to cattle when adequate grass is not available in winter. However, commercial realities dictate that farmers feed housed cattle at progressively lower costs, and each opportunity to reduce the cost of producing beef or milk during winter needs to be carefully considered. On some farms, alternative crops such as whole crop maize, wheat or barley may provide attractive opportunities, while on others grazed grass may be able to provide some of the animals daily feed requirements. On most farms however, the emphasis in the medium term will be to reliably produce grass silage at a cost compatible with the livestock systems on those farms, and maximise the intake of these silages during the winter. In all cases, high yields of grass of the appropriate quality allied to minimal conservation losses and restrained input costs are vital.

Yield
The yield of grass harvested has a huge impact on the cost of feeding silage – light yielding crops make expensive silage whereas heavy-yielding crops can spread the costs over a greater tonnage. On some farms, such as those with spring calving beef suckler cows where the progeny are sold as weanlings in autumn, high yields of grass may be produced at the expense of grass digestibility (68 to 70% DMD) by delaying the harvesting date from late May to early June. On other farms, such as those involved in winter milk production or beef finishing where it is important that silages have a higher digestibility (75+% DMD), the earlier harvesting that will be required will be at the expense of some grass yield. In all cases, however, it is important that the optimum balance between yield and digestibility that the particular livestock system needs be achieved, and that all previous management permits the attainment of the best yield and best digestibility feasible, at that time. Guidelines for consistently achieving high first-cut yields are:

1. Well-drained, non-compacted soils
2. Adequate soil fertility (P, K and lime) based on regular soil analysis and full cognisance of animal manures spread and inorganic fertiliser applied. Although additional sulphur may be required in some situations, benefit from such application is most likely for second cut silage from sandy soils.
3. Total nitrogen (N) inputs of up to about 125 kg N/ha (100 units N/acre), from the combined input of inorganic fertiliser (e.g. CAN or urea) and slurry, applied evenly and as early as feasible. This average rate may need to be increased or decreased depending on the type and age of sward, soil organic matter content and previous management. Nitrogen is the input under the farmer's control that can have the fastest short-term beneficial effect on yield, of which an adequate input is essential, if commercially viable yields are to be achieved. However, in many situations total N inputs much in excess of 125 kg N/ha (100 units N/acre) are unlikely to yield economically attractive marginal yield responses. Furthermore, excess inputs of N can harm silage quality (dealt with later). The nitrogen value of slurry can vary widely. Cattle slurry collected undiluted from slatted-floor sheds and spread evenly on grassland in mid March should provide the crop with about 5 to 8 kg N/4500 litres (i.e. 10 to 15 units per 1000 gallons). Thus, 3000 gals per acre should provide 30 to 40 units per acre, and requires that the inorganic N fertiliser input be reduced to 60 to 70 units/acre.
4. Perennial ryegrass swards, preferably of mid to late season cultivars. Ryegrass dominant crops such as those have the potential to produce excellent yields of top quality grass. Considerable increases in silage yield and quality can ensue where an agronomically inferior old pasture (due to poor plant species, severe soil compaction, etc.) is optimally replaced by a new reseed. However, for the full benefits to accrue, the new reseed must be managed in a manner that will ensure its long-term dominance in the sward and also in a manner that will efficiently convert its output to beef or milk. Many productive permanent grassland swards that still have a high content of ryegrass would be difficult to justify replacing by a new reseed. Indeed, some agronomically productive old swards of low ryegrass content can produce as good a yield as reseeds when considered over a number of years (Table 1). Certainly for less intensive farms where fertiliser N inputs and animal stocking rates are relatively low and where only a single cut of silage is being taken each year, the case for replacing an agronomically productive old permanent grassland sward by a new ryegrass reseed would be difficult to sustain.
5. Consistent previous management for grazing severity and time of closing. Grazing swards to different severities in autumn or spring will significantly influence the yield of grass available to harvest in late May (Table 2). Thus, considerable variation from year to year in the timing and severity of the final grazing before swards are finally closed for first-cut silage will contribute to inconsistency in the yield of first-cut silage.
6. Harvest at the optimal grass growth stage for the particular livestock enterprise (Table 3).
   

Quality
Whether the intention is to harvest first-cut silage in late May or early June, it is important that the best digestibility that can be attained at the prevailing yield at that stage be achieved – i.e. if a crop will produce 30 tonnes settled silage per ha (12 t/acre) on 1 June, it is better in virtually all cases if this can have a dry matter digestibility (DMD) of 72% rather than 68%. Guidelines for consistently producing silages of superior digestibility are:

1. Perennial ryegrass swards, preferably of mid to late season cultivars. Table 1 summarises a large-scale experiment conducted at Grange over a number of years. Among the sward types being compared was a new perennial ryegrass reseed and an old permanent pasture of low ryegrass content (15%) that had been well managed for many decades previously. A clear benefit from the reseed was the major increase in silage digestibility from 68 to 74% DMD. This more than anything else supported the major increase in the production of saleable animal product per hectare. A separate benefit of the ryegrass swards was the much higher concentration of sugars it contained compared to the old pasture, with the resultant effect that the ryegrass was much easier to preserve properly as silage.
   
2. Harvest when seed heads begin to emerge from the grass plant if top quality (75+% DMD) is required. This is much safer than targeting to harvest at a particular calendar date. Assume that throughout late May and early June the grass DMD will normally decrease by about 2.5% units per week as the crop gets stemmier, heads out and finally flowers (Table 3). Consequently, if qualities other than top quality are required, the expected stage to harvest can be gauged from this.
   
3. Limit the total nitrogen input to 125 kg N/ha (100 units/acre) to avoid the major quality losses associated with crop lodging (up to a 9% unit drop in DMD in a single week in wet weather), and to avoid making the crop more difficult to preserve. The data in Table 4 show that increasing rates of N fertiliser applied on 31 March progressively made grass harvested in May and early June harder to preserve properly by a combination of reducing the concentration of grass sugars and increasing its buffering capacity.
   
4. Graze silage swards short in late autumn, or graze them in early spring, to avoid the accumulation of an aged low quality butt on the crop. The data in Table 2 indicate that the effect of an old butt of low quality vegetation at the base of a relatively new ryegrass crop can be to reduce the digestibility of the grass at harvesting by up to almost 7% units DMD.
   

Conclusion
Some farmers find it difficult to consistently achieve high yields of satisfactory quality first cut silage. However, the fact that many farmers can attain these goals each year indicates that it can be done on commercial farms. In all such successful cases, it is the effective pursuance of all of the guidelines outlined above that delivers continued success. Achieving these high yields of satisfactory quality grass, together with subsequently minimising losses during ensilage and feedout and restraining input costs, are the four essential components of producing a cost competitive home-produced feedstuff for cattle and cows during the winter.

Table 1. Ryegrass sward versus productive old pasture (annual values). Source: Grange Research Centre

Old GRassland

Silage yield (t DM/ha): 10.5
Grass Sugars (%): 1.7
Silage DMD (%): 68
Carcass gain(kg): 742

Perennial Ryegrass

​Silage yield (t DM/ha): 10.8
Grass Sugars (%): 2.4
Silage DMD (%): 74
Carcass gain(kg): 850

Table 2. Previous management effects yield and digestibility of grass at ensiling. Grazed high = 10 cm stubble, grazed low = 5 cm stubble. Source: Grange Research Centre

Date closed for silage: December 2nd, status: ungrazed

Yield (t DM/ha) on May 18: 7.0
Yield (t DM/ha) total to July 6: 12.0
DMD % on May 18: 75.4

Date closed for silage: December 2nd, status: grazed high

Yield (t DM/ha) on May 18: 6.2
Yield (t DM/ha) total to July 6: 13.2
DMD % on May 18: 82.4

Date closed for silage: December 2nd, status: grazed short

Yield (t DM/ha) on May 18: 5.4
Yield (t DM/ha) total to July 6: 13.4
DMD % on May 18: 82.1

Date closed for silage: march 16th, status: grazed short

Yield (t DM/ha) on May 18: 4.7
Yield (t DM/ha) total to July 6: 12.6
DMD % on May 18: 82.4

Table 3. Average yield and digestibility patterns in first-cut silage swards. Source: Grange Research Centre

april 30th

Yield (t DM/ha): 2.5
DMD (%): 83.1

May 10th

Yield (t DM/ha): 5.0
DMD (%): 79.1

May 20th

Yield (t DM/ha): 6.8
DMD (%): 75.8

MAy 30th

Yield (t DM/ha): 8.1
DMD (%): 72.3

June 9th

Yield (t DM/ha): 8.8
DMD (%): 68.5

Table 4. Impact of rate of fertiliser N application on grass ensilability. Source: Grange Research Centre

  Rate of fertiliser N applied (kg N/ha): 0

Dry matter (%): 22.5
Sugars (% juice): 3.1
Buffering capacity (mEq/kg DM): 212
Crude protein (% DM): 11.1

  Rate of fertiliser N applied (kg N/ha): 50

Dry matter (%): 18.4
Sugars (% juice): 2.4
Buffering capacity (mEq/kg DM): 223
Crude protein (% DM): 13.6

  Rate of fertiliser N applied (kg N/ha): 100

Dry matter (%): 17.5
Sugars (% juice): 2/0
Buffering capacity (mEq/kg DM): 248
Crude protein (% DM): 15.1

  Rate of fertiliser N applied (kg N/ha): 150

Dry matter (%): 16.0
Sugars (% juice): 1.7
Buffering capacity (mEq/kg DM): 264
Crude protein (% DM): 17.9

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Phil Rogers MRCVS, Lucan, Dublin, Ireland​
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