Focusing on Pasture Quality in Late Summer/Autumn.
by Brian Hockings, Issue 4 Autumn 2007
With an increasing awareness that 18 month dairy heifers have an increasing total feed requirement during the summer and autumn period, and a recent recommendation for heifers to reach 90% of their mature weigh by 22 months of age, what is required by pasture managers to achieve enough nutrition from pasture to supply these needs?
It is well recognised, and expected, that as the year moves to late summer and autumn animal productive performance drops – whether it be milk production, liveweight gains, or fibre production.
This is generally attributed to seasonal factors, the generally hotter and drier conditions making the livestock more lethargic and less energetic.
Producing milk, meat or fibre is hard work, and just like us, under hot conditions, animals tend to avoid as much hard work as possible – generally preferring to lie in the shade and watch the world go by.
This inclination is abetted by the fact that, at this time of the year, the animals maintenance requirements are at their minimum, as there is little need to burn energy to maintain body temperatures, and in breeding animals, generally pregnancy will not be sufficiently advanced to be making any appreciable nutritional demands on the dam.
High Dry Matter %
In addition, for the pasture fed animal, due to the drier conditions, and greater maturity of the pastures, dry matter levels in the pasture will be significantly higher, with a 25% level being quite possible in what would still be considered green grass.
In effect then, with spring pastures possibly as low as 12% DM, a grazing animal could intake twice the dry matter from a mouthful of autumn pasture than from a mouthful of spring pasture.
Thus the paradox is, that the late summer and autumn is theoretically the time when the grazing animal has the greatest potential for the highest intake of feed allocatable to productivity – because of this low maintenance requirements and the higher DM in the feed.
Unfortunately however, these factors are counteracted by the livestock having reduced intakes due to the warmer drier conditions, so that the feed intake above even the minimal maintenance requirements is limited, and productivity consequently reduced.
Why do nothing?
However, there is, in my view, too much of a tendency to accept the inevitability of all this – so that the elements of the self fulfilling prophecy start to appear.
We expect productivity to decline over late summer and autumn , so that when it does, it is just taken as the way things are, and little or nothing done, to try and minimise the effects.
Significant increase possible
In many cases autumn animal production can be increased significantly above the low levels than may farmers are prepared to accept.
The critical factor in achieving this will be pasture quality, and the characteristic of high quality pasture is a high proportion of leaf in the sward, and a good clove content.
Offering an adequate volume of high quality feed will have a major benefit in two ways;
- Its greater palatability will encourage the stock to eat more so that DM intake is increased.
- Its much higher digestibility will also mean that, of that intake, more nutrients per kgDM will be effectively extracted and utilised.
Spring vs Autumn pasture
It would be unrealistic to expect late summer and autumn pastures to match the quality standards of spring, when predominantly vegetative growth is occurring, and it is also important to realise that the quality factors can differ significantly.
A typical analysis of a ryegrass/white clover spring pasture on a per kgDM basis would be;
- carbohydrates 14%
- protein 24%
- Metabolisable Energy (ME) 12.0 MJ
- digestibility 86%.
For dry, stalky summer/autumn pasture, this will drop to;
- carbohydrates 10%
- protein 10%
- ME 8.9 MJ
- digestibility 62%.
The differences between these levels is very significant for feed quality considerations – particularly those of the proteins and carbohydrates.
Generally, a practical guide for desirable protein levels in a pasture is;
ü adequate for maintenance 8%,
ü adequate for growth 10 – 14%
ü and adequate for lactation 15%.
It can thus be seen that inadequate protein will never be likely as a limiting factor the spring production.
High protein issue
Indeed, the opposite is true, in that high levels of protein in spring pasture can mean an imbalance in the carbohydrate /protein ratio, meaning that energy has to be diverted away from production to break down and process the unwanted protein.
It is the carbohydrate levels that will be critical in the spring, and these will be lower under long periods of cloudy and overcast conditions, where a lack of sunshine will be limiting the formation of plant sugars by photosynthesis.
Low spring carbohydrate levels will limit animal production, severely when very low – the classic "gutless" grass.
While lowered milk production is usually the popular indicator of this, growth rates, and general thrift of young growing stock can also be significantly affected.
One has to go back no further than last spring to find very good examples of this.
The downside of this is that because protein levels are so high in the spring, and so far above that required, the mindset is engendered that protein considerations will never be a limiting factor to animal production. Not true.
Low protein issue
From the above figures, if protein levels have dropped to 10% for late summer /autumn pastures, and the animal requires 10 – 14% for growth, insufficient protein will be extremely likely to begin to inhibit productivity.
This is not an extreme example, as a browntop dominant pasture ( of which there are still more than a few around), can actually drop to as low as 5% protein.
As proteins are the main building blocks for meat and muscle it is thus only to be expected that the young, growing animal will exhibit poor liveweight gains when offered only this type of low quality feed.
Key factors to focus on
Keeping pastures in good quality over late summer/autumn will thus be a key factor in maximising liveweight gains over this period. There will be no simple, one size fits all, method of doing this, but the focus will be on maintaining the maximum amount of leafy grasses in the sward, encouraging as much clover growth as possible, (clovers still grow mainly leaf over summer and autumn and more importantly, have a higher protein content than grasses), and minimising the amount of weeds and poorer grass species in the sward.