Achieving animal performance targets at pasture

Irish beef production is largely pasture-based where collectively grazed and conserved pasture account for almost 90% of the lifetime feed consumption.

As spring-calving predominates nationally, suckler and dairy-bred progeny taken through to slaughter spend at least two seasons at pasture and in most cases have two indoor winter periods. As levels of animal growth in one period of the production cycle can have an influence on gains at a later stage (compensatory growth), optimisation of animal performance at the various stages of the cycle is important.

An efficient grass-based beef system depends on maximising individual animal performance from grass forage, and this requires excellent grassland management, encompassing both grazing management as well as the production of high-digestibility (>70 DMD) grass silage for the indoor winter periods. The strategic integration of both grazing and silage production is critical in grass-based beef production systems. As grazed pasture is invariably the cheapest feed resource for beef production, much attention must be focused on performance at pasture. 

Maintaining optimum grazing conditions

Planned early turnout of cattle to pasture in spring has been shown to improve farm profitability, where cheaper pasture replaces more expensive winter forage and a further saving in slurry storage and spreading costs are achieved. In grassland management terms, preparation for early turnout, and having high-quality pastures available in spring, starts in the autumn of the previous year, where paddocks are ‘closed’ in a sequence and rested for the winter. The use of an autumn and spring rotation planner is a beneficial tool in guiding grassland management. If managed properly, pasture quality is invariably high from turnout in spring until mid-summer. However, when grass seed heads appear, sward quality deteriorates and, if not correctly managed, nutritive value can become sub-optimal later in the season.

Managing pasture to maximise animal performance

Post-grazing sward height

Grazing pasture to a relatively low post-grazing sward height is desirable to increase grass utilisation; however, grazing too tightly can adversely affect individual animal intake and reduce animal performance at pasture. General grazing guidelines are based on data from lactating dairy cows, where output per hectare rather than per animal is the primary focus, and suggest a target post-grazing sward height of approximately 4 cm.

However, in beef production where individual animal performance is essential to produce a commercially saleable product, a balance between output per unit area and per animal is needed. A series of studies at Teagasc Grange has investigated the effects of grazing management practices, namely contrasting pre and post-grazing sward heights on beef cattle live weight gain at pasture.

The effect of post-grazing sward height on animal performance was compared over two years using yearling beef × Holstein-Friesian steers turned out to pasture in spring and grazed to a post-grazing sward height of either 3.5 or 5.0cm in a rotational grazing system from mid-March until late-October. By the end of the grazing season, those grazed to 3.5cm were 30kg lighter than those grazing to 5.0cm. It was concluded that excessively tight grazing (to a sward height of 3.5cm) was not recommended.

In another study with lactating suckler cows and their calves, animals grazed to a post-grazing sward height of either 4.0 or 5.5cm. At the end of the grazing season, cow body condition score tended to be lower and calf live weight gain at weaning was 8-10kg lower for the 4.0cm compared to the 5.5cm sward height. It was concluded that grazing to 4.0cm had negative effects on the performance of beef suckler calves.

More recent research at Teagasc Grange compared post-grazing sward heights of 4 and 6cm using suckler-bred steers in a weanling-to-beef system. Yearlings were turned out to pasture in spring and grazed to their respective post-grazing sward height in a rotational grazing system. The grazing season lasted over 200 days and they were housed in November. Animals were offered a finishing diet of grass silage only plus minerals and vitamins until slaughter in March at 24-months of age.

Grazing to a post-grazing sward height of 6cm rather than 4cm resulted in animals being 29kg heavier at housing at the end of the grazing season (Figure 1), and having a 15kg heavier carcass - demonstrating that the live weight advantage at housing was retained during the finishing period. As there was no difference between the two post-grazing sward heights in the digestibility of the herbage offered during the grazing season, this suggests the additional growth performance was achieved through higher individual animal intake.

The net effect of grazing to 6cm was to reduce the stocking carrying capacity by 15% and produce ~0.5 t less DM per ha; however, there was no difference in live weight gain per hectare. In practical terms, higher weights at housing can translate into an earlier slaughter date, thereby reducing feed costs and potentially reducing the carbon footprint of beef systems. To reach the same carcass weight as the cattle grazing to 6cm, those grazing to 4cm would require an additional month of ‘finishing’. Collectively, these studies indicate that grazing too tightly (~4.0 cm compared to 5.5-6.0 cm) negatively impacts individual animal growth rate, most likely mediated through reduced herbage intake.

Pre-grazing herbage mass

The mass of pre-grazing herbage offered to animals can affect animal performance. Two experiments were conducted at Teagasc Grange to evaluate the effect of pre-grazing herbage mass on animal live weight gain.

In the first study, yearling steers, again part of a suckler weanling–to-beef system, were allocated a pre-grazing herbage mass of 1,500 or 2,000 kg DM/ha in spring and were rotationally grazed as described above for a grazing season of over 200 days duration. Animals were housed in November, offered grass-silage only plus minerals and vitamins, and slaughtered at ~24 months of age. Pastures were topped once in mid-summer. Increasing pre-grazing herbage mass from 1,500 to 2,000 kg DM/ha resulted in animals being 14kg heavier at the end of the grazing season and having a 5kg heavier carcass at slaughter. As the digestibility of the grass offered in the two pre-grazing herbage masses was similar, this suggests that increased individual animal performance associated with the higher pre-grazing herbage mass was due to a higher intake.

Figure 1. Effect of post-grazing sward height (left) and pre-grazing herbage mass (right) on animal live weight

Concentrate supplementation at pasture

Table 1. Effect of concentrate supplementation (GC ‘X’ kg daily) at pasture in autumn on growth and carcass traits of finishing steers compared to unsupplemented animals (G0)

¹NS= not significant, * = P < 0.05; ** = P < 0.01; *** = P < 0.001

This paper by Edward O’Riordan, Peter Doyle, Aidan Moloney and Mark McGee first appeared in the Beef 2022 - Supporting Sustainable Beef Farming open day book.