Feeding Fodder Beet
Fodder beet has a low phosphorus content, low fibre level and less than optimal protein content. On the other hand it has a high sugar content, making it very palatable.
Cows grazing fodder beet ingest more soil than cows grazing pasture or other non-bulb crops. New Zealand soils contain reasonable levels of iron, so when cows are grazing fodder beet, their iron intake increases. Iron is an antagonist for many trace elements - in other words, it interferes with the normal metabolism of those trace elements.
Copper absorption in particular is decreased in the presence of high dietary iron, most likely as a result of insoluble compounds forming in the rumen.
Role of phosphorus
Phosphorus is a major component of the skeleton. It also plays a key role in a number of metabolic processes, including those that take place in the rumen. It's needed for bone strength, energy metabolism and milk production. It's also involved in buffering the rumen, which helps to minimise the risk of acidosis.
Cows that are fed a low-phosphorus diet for an extended period of time may develop sub-clinical phosphorus deficiencies. Symptoms of low-grade phosphorus deficiency include reduced appetite and rapid weight loss; in some cases, blood may be seen in the urine.
The effects of a low-phosphate diet are not immediately obvious, as in the short-term cows will mobilise their existing phosphorus reserves to maintain blood phosphorus levels.
Lactating cows require a dietary phosphorus intake of between 0.3% and 0.4% (i.e. 3.0 - 4.0 g P/kg DM). Dry cows require a slightly lower level (0.27 - 0.35%).
Phosphorus supplementation has been associated with an increase in fertility. However, the mechanism underpinnning this is uncertain. It may be a result of weight gain, reduced weight loss, or a direct effect of the phosphorus itself.
Heifers that have a phosphorus deficiency may experience delayed oestrus. In addition, cows with a prolonged sub-optimal phosphorus intake may show a reduction in milk yield.
The on-farm experience
Animals are typically wintered on fodder beet for 60 to 100 days. Grazing on a predominantly low-phosphorus diet for this length of time could result in phosphorus deficiency manifesting at calving or early lactation, which would have implications for the health of young stock and for the milk production potential of dairy cows.
While some cows that experience phosphorus deficiency will recover, others will go down. The classic presentation of a cow that has gone down as a result of phosphorus deficiency is a 'creeper cow' - the cow is down but alert. Such cows will try to crawl around, especially on their forelimbs, but will be unable to rise. The fact that these cows are alert and energetic distinguishes them from the sluggish downer cow seen in milk fever.