(a summary of an article by Sue M.Macky, BVSc MRCVS.)
With the rising of the daily temperatures in late spring and early summer, the composition of the available feed changes considerably and with that the behavior the grazing animals.
With the apparent climate change (global warming) rye grasses appear to be being pushed out of their temperate climate comfort zone and go into survival mode -run to seed- earlier.
Motivation to eat is necessary to maximise performance, it is both a management and an animal issue.
While generally milk production in lactating ewes with lambs will help drive appetite, young growing lambs may not have any motivation to eat much more than is necessary for performance (keeping alive).
Regardless of feed available performance, milk production, growth, reproduction etc. depends on the relative influence of four main areas:
All are inter related - outcome in one area will be influenced by others.
In any given condition the results will be effected by genetics, age, condition, size etc. but optimum feed conversion depends on starting with a healthy animal.
Heat stress and its effects on intake and feed conversion efficiency depends on the relative influence of breed, age, genetics, temperature, humidity, activity, shade, water, endophyte/fungi intake, feed composition, physical characteristics and animal performance.
Heat stress depression of performance can be seen at air temperatures as low as 20 ºC.Back to the top
On an all pasture diet dry matter intake and its digestibility is generally the single most important factor determining performance.
Grass is a high volume, low nutrient feed. Animals must eat a lot of it to get sufficient intake of carbohydrates and protein and be motivated to keep eating, especially if the potential heat stress factors are becoming more significant.
In spring when grass is leafy and wet, it is easy to break off. As summer approaches the digestibility falls, grass gets tougher and each mouthful takes more time and effort to collect. How tough grass gets can be easily demonstrated by the grab test, literally grab a handful of grass like the animal would. As its gets tougher there will be more stalk, less leaf with a higher fibre content. These changes are associated with grass entering its reproductive/seeding phase and leaf will start to break off above the stalk leaving an apparent surplus, yet performance may decline. It is not a genuine surplus just grass too tough to collect and animals that run out of time to take the necessary extra mouthfuls. At its worst the digestibility fall will be caused by an increase of the lignin or "wood" fibre content in the grass. This makes its tough to collect and will take longer with smaller mouthfuls. Less nutrients are collected yet there may appear to be plenty of feed.
These changes occur on all farms regardless of management, because climate is the single most significant influence. Ground temperature, moisture, wind and air temperature dictate the severity of the changes. In the most adverse conditions lignification occurs more rapidly and to a greater degree.
Changes will be registered by animal performance more quickly than can be visually recognised.
So what can you do about it?
To get the best performance, feed needs to contain as near as possible the basic nutrients that can be used to make the end product e.g. milk, meat and bone.
Minimum amounts of protein, sugar, starch (carbohydrates), minerals, water etc. are necessary for maintenance, with more required for each kg of weight gain.
Pasture high in legumes will entice lambs to eat and reduce the effects of internal parasites.
Pasture with high tannin content enhances protein supply and will prevent protein breakdown before it reaches the small intestine.
Pasture that is digested quickly moves faster through the gut of the animal.
As grasses mature and run to seed, not only do they become less easy to eat, but each mouthful contains less energy, less protein and carbohydrates especially in hot conditions. Less is eaten yet more is needed to maintain performance if metabolic rate is being affected by heat.
Production declines because the total dry matter intake falls and total nutrient intake per kg eaten declines - a double loss.
Fresh clean drinking water easily accessable to all animals becomes more important as performance increases and/or as heat stress increases.
In times of compensatory growth, weight gain will be a disproportionate amount of fat if adequate balanced nutrients for skeletal growth are not present.Back to the top
Getting enough feed down an animals throat still does not mean that the animal will be well fed or will produce as expected. What happens in the rumen determines which nutrients actually get into the animal and may be available for production.
Too much fibrous, poorly digestible feed slows down throughput. More time needs to be spent processing for lower return, less time available for eating, more energy wasted as heat. Where lignification is complicating the fall in digestibility, the decline in feed conversion efficiency is 2.4 times as great as expected, hence a very rapid decline in performance in the summer. Where a fall of 10% in digestibility might be expected to produce a 10% fall in productivity, the loss is actually 24% because of the woody cells. At the worst, especially in warmer weather, completely undigested pieces of fibre pass right through the animal actually dragging fluid and other nutrients with it. If there is enough dry matter available, smaller and younger animals tend to be more severely affected - lower absolute gut capacity easily filled with junk - in terms of total production.
Indigestible feed and not much of it is a big problem for all.
For further reading: "400 plus": A guide to improved lamb growth. Edited by Peter Kerr. A Sheep council publication, available from Meat NZ or Woolpro.Back to the top