Big variations in the growth performance of young cattle are common in autumn. The most common causes of this (in order) are listed below:
Feed allowance: Autumn grown grass (that flush of feed we’ve had in most places after recent rain) is low in dry matter and it is common to overestimate how much animals are getting to eat, especially when pastures are short and still recovering.
The ‘magic number’ for pasture residual (the grass you leave behind when you take animals out of a paddock) for fast growth in young cattle is 1500kg drymatter per ha (kgDM/ha), or 3.5-4cm autumn pasture length.
Ideally weaners would be going into paddocks at 2000kgDM/ha-2500kgDM/ha or 6-8cm autumn pasture length. This can be hard to start after a dry spell.
Offering good quality supplement as part of the diet to let paddocks come away in front of weaners can be a good move at this time. Weaners that get a sudden feed change from drier summer pasture onto lush autumn grass can sometimes be affected by a spike of ammonia in the rumen – this can reduce their appetite for days and sometimes weeks; the addition of a good quality supplement in addition to this feed change can mitigate this.
Finally, the AMOUNT of feed a weaner needs to grow well is nearly double what it needs just to maintain itself. ‘Maintenance’ feed requirement for a 200kg weaner at 2% of its bodyweight is 4kg of drymatter per day. Weaners fed for high liveweight gain need to be eating somewhere around 3.5% of their bodyweight, or 7kg drymatter per day. If you’re rationing out grass on breaks, or feeding out a crop, make sure you get this allocation right, and remember to account for wastage.
Parasite larval challenge: Those autumn rains that cause a flush of grass growth also cause a flush of gastrointestinal worm availability in the sward – the moisture triggers egg hatching and movement of larvae out of the base of the pasture and up the sward to be ingested by hungry young cattle. This parasite challenge reduces feed conversion efficiency and appetite. Regular drenching will remove the adult or developing worm population in the gut of young cattle but will not mitigate this larval challenge to any great extent. Look for ways to create areas of lower worm challenge feed for weaners.
Pasture fungal toxins: In a survey of autumn ill-thrift on 50 NZ farms conducted in the mid 2000’s, pasture fungal toxins were the second most common non-feed related cause of poor growth performance in young cattle. Facial eczema (FE) is an obvious one but is often overlooked by beef farmers. A lack of clinical signs in animals is not evidence that all is well – FE toxin challenge is a very common cause of poor growth in beef weaners. Work in the Waikato showed that the most consistent improvement in liveweight gain under FE challenge was conferred by giving Zinc boluses. Trough treatment did not give reliable results, and pasture spraying was somewhere in the middle.
There are also a number of other fungal toxins that can run riot in autumn pastures, especially in the northern half of the north island. The management of these is not always easy but trying to graze pastures in the ideal cover range helps – the shorter the sward, the greater the concentration of toxins.
Trace element insufficiency: This was a rare cause of ill-thrift in the project mentioned above. Low copper levels can be a problem in some districts – this is probably the most common trace element deficiency we see in beef cattle. Liver biopsies provide a far more reliable indicator of status than do blood samples.
Ensuring that weaners have adequate trace minerals on board going into winter is great insurance, but make sure you’ve got those other major factors well covered if you’re chasing really good autumn and winter liveweight gains.
Ginny Dodunski – BVSc (dist) MACVSc