Growth curves are gaining attention, but one aspect of the concept can get lost: You have to know more than just lean growth to formulate an accurate feed program.

At Purina Mills, they refer to the “law of unity.” Simply put, everything the pig eats goes into lean growth, fat growth, ash (bone) growth or gut fill. The body weight of these components must equal the real body weight of the pig.

Purina contends on-farm measuring may present some accuracy problems due to instabilities and variations between groups. Body weight may not equal lean, fat, ash and water weight calculations. Purina prefers to use closeout data to figure the lean, fat and ash accretion rates. Those rates of gain let you calculate a set of diets to meet a pig’s protein and energy needs at various weights.

You could change diets every day to fit the average pig’s needs, but that’s not realistic. Purina uses from two to eight phases, but Gawain Willis, director of swine research, notes four to five may better meet most of your pigs’ needs.

As pigs grow, lean accretion curves turn down faster than fat accretion curves, so diets must take that into account. Also consider the environment, health status and genotype: The first two may limit the pig’s growth potential despite its genetic potential.

Other nutritionists agree that you need to consider fat and lean curves to calculate diets. Mike Tokach of Kansas State University says nutritionists there use two methods to develop growth curves. One is based on the Fat-Free-Lean Index and closeout data. It gets a producer in the ballpark on pig needs, Tokach says.

Using the lean curve offers an estimate of the pig’s lysine needs in grams per day. Fat and protein curves provide energy needs and, based on feed intake, let Tokach calculate the percent lysine the diet needs at a given phase to meet the energy and lysine requirements.

Kansas State nutritionists also consider on-farm performance. Every three weeks they perform real-time ultrasound scans on 32 gilts and 32 barrows from 50 pounds to market weight, if possible up to 270 pounds.

They collect backfat depth and loineye area information. That data, together with pig weights and ages, goes to Allan Schinckel, Purdue University swine specialist, who runs it through a computer model to calculate nutrient needs for various weight ranges.

Though they take different routes, Purina and the two universities contend that an effective nutrition program must include accurate fat accretion and lean accretion curves.

Tokach notes formulating diets isn’t a one-time event. Operations change over time, eventually making even the best recommendations obsolete. He suggests reviewing growth curves after any major change in genetics, health status, facilities or management. Otherwise, review protein and fat deposition curves annually or at least every other year.

He also suggests making sure your initial estimates are accurate. If one phase has a lysine shortage, it can throw the group’s performance off and skew the curve. When you correct that error, the rest of the curve may need adjustment.