It's accurate to say that the Iowa State University work on the need to match diets to immune status in the growing pig caused - and is still causing - enormous interest worldwide.

This is certainly true in Europe where we like to think we have the world's leanest stock. We have taken to heart the wide differences in lysine requirements of a pig enjoying only a modest need to build a protective wall against disease, compared to one less fortunate, heavily challenged by pathogens (Table 1).

Table 2 shows why we are concerned. I've boxed the figures we think apply to us. In other words, we have very lean genotypes in pretty old and decrepit housing, a spin-off from years of low profits. This has dissuaded our producers from renewing or refurbishing their buildings. The potential difference is enormous. But how do we know?

And farmers, always pragmatic realists, are asking, "How do I know and more importantly how does the nutritionist know where the farmer is on the immune activation ladder?"

Good question. An absolutely vital question, in fact because getting it wrong could erode, in U.K. terms, 8 British pounds/pig (about $13.30 U.S.) or 90% of our profits.

Three Options I see three possible solutions. 1. Serology. This one uses the veterinarian to collect blood samples and establish a herd disease profile. But even the cutting edge of serology cannot identify certain diseases. This is also expensive and could be time consuming.

2. Challenge or test feeding. This concept takes 50 typical growers, feeds them on what the nutritionist calls a "non-limiting diet" and monitors growth, feed conversion ratio and lean gain (by using a deep muscle scanner). In this way, along with carcass data at slaughter, the nutritionist has a good idea of the grow/finish herd's lean accretion curve. Therefore, a farm-specific diet can be built twice a year to satisfy the curve.

One snag - the scanner is very expensive, so it is the province of a feed manufacturer who can organize it and whose clients have a computerized, wet feeding facility.

Wet feeding is recommended because with this equipment any diet can be made on farm from only two (or three) basic diets. This cuts down a custom mix inventory drastically. In fact, one feed manufacturer I know reduced his normal pig diet list by 50% in two years, despite increasing his custom mix clientele by 60% and his pig business by 300%!

3. Measuring growth rate. There seems to be a possible link between growth rate and immune activation (Table 1). Measuring growth rate accurately is something the producer can do if hesets his mind to it. And it doesn't cost much.

Another potential snag - there are, of course, other things besides immune stimulation which can affect daily gain. Temperature, stress, feeder management, overcrowding, water and wet/dry feeding are just a few. We need research to confirm this is indeed a viable option.

Matching Diets Most commercial nutritionists view the subject as a "nightmare" to quote a leading European. When pigs encounter pathogenic challenge, cytokines (a type of protein chemical messenger) are released. They re-program the animal's metabolism to divert nutrients away from growth, especially lean growth, in order to ensure the immune process is prioritized. Cytokines alter nutrient intake and utilization which - first headache for the nutritionist - need to be compensated to lessen the damage to productivity.

At the same time - second headache - metabolic changes are occurring which both increase and decrease disease nutrient requirements. Fever places demands on energy and while the consequences of fever - reduced activity and more sleep - lessens it, a reduction in growth rate lowers it still further. On top of this, appetite goes down when immune response is high, even if the animal feels healthy enough.

I quote Paul Toplis, a leading European commercial pig nutritionist responsible for making sense of it all: "Appetite changes can be unpredictable. For example, if a healthy, growing pig with an appetite of 1.5 kg. (3.3 lb.) a day requires 15 g. (0.5 oz.) of lysine, then the diet specification for lysine should be set at 1%, l0 g./kg. (0.35 oz./lb.). Now, if this pig encounters an immune challenge, its lysine requirement might fall to 12 g. (0.42 oz.), 13 g. (0.45 oz.) or perhaps 14 g. (0.49 oz.) per day and the feed intake might fall to 1 kg. (2.2 lb.), 1.2 kg. (2.64 lb.) or 1.4 kg. (3.08 lb.) per day, giving the nutritionist nine possible diet specifications to work with." See Figure 1.

But does it matter? Yes, it could well matter. Taking the variables Toplis quotes and the reduction in performance quoted in the Iowa State results, even for the less extreme differences under U.K. prices at September 1999, this could reduce saleable meat sold per U.S. ton of feed fed to slaughter by 26 lb., add 12 days to achieve slaughter weight and might cause you to pay an unnecessary 6% more for your feed.

I don't know what that is in U.S. profit terms but over here using moderate returns, it is a 38% reduction in gross profit - all for getting it modestly wrong.

Where Do We Go From Here? First, understand the considerable drag on performance which a strong immune defense wall requires.

Second, redouble your efforts to provide conditions which do not require the pig to boost these defenses needlessly. A whole book could be written on what to do.

If European calculations are anything to go by, good genetics and poor animal health could be costing us 90% of profits. It must be happening today, on your farm because it sure is on ours. That added profit pays for a lot of improvements.

Third, make sure all future work on nutrient requirements contains some assessment of immune activity. Indeed, an international standard is needed. Meanwhile, researchers, please confirm and refine that linkage between growth and immune status so that we can start to exploit the fruits of your worthy discoveries.