20 March 1998

RIGHT NUTRITION AT TURN-OUT

Our dairy vet John Dawson

gives his views on managing

nutrition and fertility at

turn-out

COWS and herdsmen love turn-out. The cow races out to an abundance of appetising grass, while the herdsman breathes a sigh of relief as the scraper tractor can be parked for the summer. However, it is a period of immense change and nutrition must be carefully balanced to maintain good quality milk production and safeguard fertility.

The most likely nutritional factor to influence fertility is energy balance, especially in early lactation, although other nutrients and minerals can play an important part in fertility failure.

Usually the energy in spring grass is good but it can vary; if there is a cold spell energy can fall. Even when protein levels are high, low energy levels cause weight loss and risk poor fertility. Negative energy status, especially in early lactation cows, delays the onset of normal ovarian activity which limits the number of oestrus cycles before breeding and may account for reduced fertility.

Cows with a low energy status also have lower progesterone concentrations during the second and third luteal phases which may impair implantation of the embryo in the uterine wall and its survival in the early stages of pregnancy.

Maize silage is a good buffer to spring grass when grass is in short supply and extra feed is needed to maintain yields. Its high energy but low protein content complements the high protein levels in spring grass.

The effect of protein on fertility is not known but some studies show high levels of crude protein, especially high rumen degradable protein, can reduce fertility. Lower conception rates have been reported in cows with high levels of blood urea, suggesting that diffusion of excess ammonia out of the rumen is the difficulty. This can arise when the main component of the diet is spring grass containing high levels of rumen degradable protein. Raised concentrations of blood urea may reduce fertility by raising urea levels in the uterus. This impairs conception and embryo survival.

It is also important to supply adequate mineral intakes and these are best achieved with a water-based supplementation. In-feed supplementation has the difficulty of low or nil intakes at turnout as many cows wont eat concentrate for a while after introduction to grazing.

Free access minerals have the difficulty that not all cows take equal amounts, some taking more, some less than required.

Magnesium is the main mineral to watch. The high gut transit time with spring grass prevents good uptake of all minerals and especially magnesium. Buffer feeding helps alleviate this by slowing the intestine and increasing absorption. Gradual introduction to spring grass also reduces the difficulty.

Cows drink less when they are eating wet, spring grass. If a water-based mineral is being used, it may be necessary to increase the inclusion rate.

Special attention should be paid to freshly calved cows and late lactation cows – these are usually getting low levels of concentrate. Avoid pastures heavily dosed with fertiliser as they are rich in K, which decreases magnesium absorption from the gut.

A useful method of checking nutritional status is by a metabolic profile taken about two weeks after the cows have settled onto the grazing.

The aim of metabolic profiling is to get the earliest possible indication of the cows opinion of their diet. There are other methods of assessing the rations effectiveness but they are slower and less specific than blood testing. &#42

All right for them – or is it? The herd may need more than plenty of tasty spring grass in the diet if fertility is not to suffer after turn-out.