Diets used in research to evaluate nutrient requirements of poultry are formulated on a percentage basis. This usually results in requirements expressed as a percentage of the diet, which is the percentage of that nutrient needed for the animals used in that experiment. The US National Research Council's 1994 report suggests percentages of nutrients of the diet for all poultry, based on evaluation of published data. Estimating daily amino acid need and feed intake of laying hens shows the way forward for similar calculations for broiler chickens and turkeys.

For laying hens, the NRC authors suggested amino acids based on a percentage of the diet when the hens were consuming 80, 100 or 120 grams per day of feed. These requirements were ’based on dietary ME (metabolisable energy) concentrations of approximately 2900 kcal/kg and an assumed rate of egg production of 90%.’ They also suggested amino acids needed per day at 100 grams of feed intake per hen per day.

It is generally accepted that for all animals to reach optimal production, they need specific amounts of each nutrient each day. In 1978, Dr Harms and his colleagues developed a programme at the University of Florida to formulate feed for laying hens based on their daily need and daily feed intake. This programme was modified in 2001, and the following formula was developed to calculate the exact percentage of an amino acid (AA) needed in the feed for each flock:

 [(AA/g EM) : (kcal/g EM)] x Energy : mg FI x 100 = % amino acid needed in diet

Subsequently, the formula was simplified as follows for easier calculation:

 [(AA/g EM) : (kcal/g EM)] x Energy : g FI : 10 = % amino acid needed in diet

Where:

AA/g EM = mg amino acid/g egg mass

kcal/g EM = kcal ME/g egg mass

Energy = daily energy intake (kcal)

FI = daily feed intake

Adapting the layer estimation technique for broilers

Since the development of the layer diet formula, it has been modified to estimate the percentage of amino acid needed in the feed of broilers. The term ’mg of amino acid per gram of egg mass’ is replaced with ’mg of an amino acid per gram of gain’. Published data is used to demonstrate the use of this formula for broilers. The amount of dietary amino acid needed for a gram of gain increases as the broiler ages. The threonine (Thr) requirement for broilers at three ages thus can be evaluated.

Broilers aged 4 to 15 days

In 2003, Dr M.T. Kidd and colleagues at Mississippi State University conducted an experiment to evaluate the possible influence of coccidiosis on Thr requirement of the broiler (Table 1).

Table 1

Performance of male broilers when fed diets with three levels of threonine (Thr) from 4 to 15 days of age

The diet containing 0.70% Thr supported near maximum weight gain. This gain was obtained with a feed containing 3.2 kcal ME per gram. This resulted in an intake of 120.6 kcal of ME per day, or 5.00 kcal ME per gram of gain. Also, the intake of Thr was 273.3 mg per day, or 11.30 mg per gram of gain. This resulted in a threonine:energy ratio of 2.260. The formula confirms the percentage of Thr in the diet: (11.30 : 5.00) x 120.6 : 38.9 : 10 = 0.7007 % Thr required.

Broilers aged 3 to 6 weeks

At the University of Illinois in 1996, Dr D.M. Webel and his colleagues conducted an experiment to determine the requirement of digestible Thr for the broiler (Table 2).

Table 2

Performance of male broilers when fed diets with various levels of threonine (Thr) from 3 to 6 weeks of age

 Analysis of the basal diet indicated that it contained 0.50% total Thr. Therefore, the content of other diets was calculated by adding supplemental levels at 0.06%.

Maximum gain was obtained when the diet contained 0.74% Thr. This gain was obtained with 123.7 g of feed containing 3.2 kcal of ME per gram. This resulted in 396 kcal ME per day, or 5.49 kcal ME per gram of gain. There also was a daily intake of 915 mg of Thr, resulting in a threonine:energy ratio of 2.313. The formula confirms the percentage of Thr in the diet: (12.70 : 5.49) x 396 : 123.7 : 10 = 0.7406% Thr required.

Broilers aged 6 to 8 weeks

In other research carried out by Dr M.T. Kidd's group in 2003, maximum gain was produced with a daily intake of 159.8 g of a diet containing 0.66% Thr (Table 3).

Table 3

Performance of female broilers when fed diets with various levels of threonine (Thr) from 42 to 56 days of age

 This resulted in an intake of 1055 mg Thr and 511 kcal of ME. One gram of gain was produced with 16.22 mg of Thr and 7.87 kcal of ME. This resulted in a threonine:energy ratio of 2.061. The following formula confirms the percentage of Thr in the diet: (16.22 : 7.87) x 511 : 159.8 : 10 = 0.6590 % Thr required.

Adaptation of the formula to a range of conditions in broilers

The level of 0.659% Thr is the percentage needed in this particular flock of broilers. However, various factors can cause the Thr level to change with different conditions. This is demonstrated for four different conditions for the 6 to 8-week data, with four values used in the formula. The value, or values, that cause a change in the percentage requirement appear in bold type within the formulas.

-Assume a 5% increase in dietary energy: This will cause a 5% decrease in feed intake (159.8 x 0.95 = 151.8 kcal). (16.22 : 7.87) x 511 : 151.8 : 10 = 0.6942 % Thr required

-Assume a 5% decrease in dietary energy: This will cause a 5% increase in feed intake (159.8 x 1.05 = 167.8 kcal). The diet will contain 3.04 kcal/ g (16.22 : 7.87) x 511 : 167.8 : 10 = 0.6280 % Thr required

-Assume a 5% decrease in energy requirement (485.5 kcal): This reduced energy need will cause a 5% reduction in energy intake (159.8 x 0.95 = 151.8). (16.22 : 7.46) x 485.5 : 151.8 : 10 = 0.6959 % Thr required

-Assume a 5% decrease in energy requirement (485.5 kcal) causes a 5% reduction in energy requirement and the energy content of the feed is increased by 5%: Feed intake will be reduced 5% for decreased energy requirement and 5% for energy increase in the diet, resulting in a 10% decrease in feed intake (159.8 x 0.90 = 143.82g). (16.22 : 7.46) x 485.5 : 143.82 : 10 = 0.7344 % Thr required.

Adapting the layer estimation technique for turkeys

The formula for establishing the amino acid needs of the broiler can also be used for the turkey poult. Published data were used to support the application of the formula for the turkey. These are recent data and with soybean meal diets were used in all three experiments by Professor P.W. Waldroup and his colleagues at the University of Arkansas, published in 1998 and 2002.

Turkeys aged 3 to 6 weeks of age

Maximum gain was produced when the diet contained 0.93% Thr (Table 4).

Table 4

The performance of young male turkeys when fed diets with various levels of threonine (Thr) from 3 to 6 weeks of age

 This resulted in an intake of 1242 mg of Thr and 387 kcal of ME. A gram of gain was produced with 14.13 mg Thr and 4.41 kcal of ME. This resulted in a threonine:energy ratio of 3.205. The formula confirms the percentage of Thr in the diet of this flock of poults: (14.3 : 4.41) x 387 : 133.6 : 10 = 0.93% Thr required.

Turkeys aged 6 to 9 weeks of age

Maximum gain was produced in poults from 6 to 9 weeks of age with a daily intake of 2817 mg and 939 kcal ME of Thr (Table 5).

Table 5

The performance of young male turkeys when fed diets with various levels of threonine (Thr) from 6 to 9 weeks of age