Livestock Feeding

Akira Matsushita

Department of Economics, University of Tokyo

From Miranda and Fackler, Applied Computational Economics and Finance, 2002, Section 8.4.8 and 9.7.8

8.4.8 Model Formulation and Solution by hand

• A livestock producer feeds their stock for $T$ periods and sells it at the beginning of period $T+1$
• Each period $t$, the producer determine the amount of feed: $x_t \geq 0$. The fixed unit cost of grain is $\kappa \geq 0$
• $s_t$, the weight of livestock at time $t$, follows the deterministic process: $s_{t+1} = g(s_t, x_t), s_0 = \bar{s}$. Assume $s_t \geq 0$ for all $t$
• The selling price of the livestock at time $T+1$ is given by $s_{T+1} \times p$

Then the profit maximization problem for producer is formulated as:

\begin{align*} \underset{ \{x_t \geq 0 \}_{t=1}^T}{\max} \delta^T p s_{T+1} - \sum_{t=1}^{T} \delta^{t-1} \kappa x_t \hspace{1em} \text{s.t.} \hspace{1em} \begin{cases} s_{t+1} = g(s_t, x_t) \\ s_0 = \bar{s} \end{cases} \end{align*}

where $\delta \geq 0$ is the discount factor

The corresponding Bellman equation is:

\begin{align*} \begin{cases} V_t(s_t) = \underset{x_t \geq 0}{\max} \{ -\kappa x_t + \delta V_{t+1}(g(s_t, x_t)) \} \hspace{1em} t=1, \ldots, T \\ V_{T+1}(s_{T+1}) = ps_{T+1} \end{cases} \end{align*}

Euler Conditions

Let's derive the Euler Equilibrium Conditions from the Bellman equation

Assume $g: \mathbb{R}_+ \times \mathbb{R}_+ \rightarrow \mathbb{R}_+$ is differentiable on the entire domain (so $g$ is partially differentiable by both $s$ and $x$)

Define $g_x = \frac{\partial g}{\partial x}$ and $g_s = \frac{\partial g}{\partial s}$

Assume $g_x(s, 0)$ is large enough so that the nonnegative constraint of $x$ will not be binded at an optimum (ensures an interior point solution)

Let $X_t^*$ be the optimal solution correspondence, i.e.,

\begin{align*} X_t^*(s) = \left\{ x_t^* \in [0, \infty) \mid V_t(s) = -\kappa x_t^* + \delta V_{t+1}(g(s_t, x_t^*)) \right\} \end{align*}

and $x_t^*(s) \in X_t^*(s)$ be a selection of it. Assume $x_t^*(s)$ is differentiable for all $t$

Note that by using $x_t^*(s)$ the optimal value function at time 1 is derived as

\begin{align*} V_1(\bar{s}) &= -\kappa x_1^*(\bar{s}) + \delta V_2(g(\bar{s}, x_1^*(\bar{s}))) \\ &= -\kappa \left[ x_1^*(\bar{s}) + \delta x_2^*(g(\bar{s}, x_1^*(\bar{s}))) \right] + \delta^2 V_3(g(g(\bar{s}, x_1^*(\bar{s})), x_2^*(g(\bar{s}, x_1^*(\bar{s}))))) \\ &= \ldots \end{align*}

Here we show that $V_t$ is differentiable by $s_t$ for all $t=1, 2, \ldots, T+1$

• At $t = T+1$, $V_{T+1}(s_{T+1}) = ps_{T+1}$ so this is differentiable by $s_{T+1}$

• As an induction hypothesis, assume $V_{t+1}$ is differentiable by $s_t$ ($1 \leq t \leq T$). Since

  \begin{align*} V_t(s_t) = -\kappa x_t^*(s_t) + \delta V_{t+1}(g(s_t, x_t^*(s_t))) \end{align*}

  and $x_t^*(s_t)$ and $V_{t+1}(s_t)$ are differentiable by $s_t$ and $g(s_t, x)$ is differentiable by both $s_t$ and $x$, $V_t(s_t)$ is also differentiable. The derivative is

  \begin{align*} \frac{\partial}{\partial s_t} V_t(s_t) = -\kappa \frac{\partial}{\partial s_t} x_t^*(s) + \delta \frac{\partial}{\partial s_t}V_{t+1}(g(s_t, x_t^*(s_t))) \left\{ g_s(s_t, x_t^*(s_t)) + g_x(s_t, x_t^*(s_t)) \frac{\partial}{\partial s_t} x_t^*(s_t) \right\} \end{align*}

Hence by induction, $V_t$ is differentiable by $s_t$ for all $t$

Next consider the optimality condition of the maximization problem in the Bellman equation: $\underset{x_t \geq 0}{\max} \{ -\kappa x_t + \delta V_{t+1}(g(s_t, x_t)) \}$

Define $\lambda_t(s_t) \equiv V_{t}'(s_t) = \frac{\partial V_t}{\partial s_t}$

Using the chain rule, the FOCs are

\begin{align*} \delta \lambda_{t+1}(g(s_t, x_t^*))g_x(s_t, x_t^*) = \kappa \hspace{1em} t=1, \ldots, T \\ \end{align*}

Then the drivatives of the value functions by $s_t$: $\lambda_t(s_t) = \frac{\partial}{\partial s_t} V_t(s_t)$ become

\begin{align*} \lambda_t(s) &= \underbrace{ \bigl\{ -\kappa + \delta \lambda_{t+1}(g(s_t, x_t^*(s_t))) g_x(s_t, x_t^*(s_t)) \bigr\}}_{=0 \text{ by FOC}} \frac{\partial}{\partial s_t} x_t^*(s_t) + \delta \lambda_{t+1}(g(s_t, x_t^*(s_t))) g_s(s_t, x_t^*(s_t)) \\ &= \delta \lambda_{t+1}(g(s_t, x_t^*(s_t))) g_s(s_t, x_t^*(s_t)) \end{align*}

for $t=1, \ldots, T$ and

\begin{align*} \lambda_{T+1}(s_{T+1}) = p \end{align*}

In summary, the optimal path follows the following equations:

\begin{align*} \begin{cases} \delta \lambda_{t+1}(g(s_t, x_t))g_x(s_t, x_t) = \kappa \hspace{1em} t=1, \ldots, T \\ \lambda_t(s_t) = \delta \lambda_{t+1}(g(s_t, x_t)) g_s(s_t, x_t) \hspace{1em} t=1, \ldots, T \\ \lambda_{T+1}(s_{T+1}) = p \end{cases} \end{align*}

Solve the equations by hand

How to solve the above equations and get the optimal polity $\{x_t\}_{t=1}^{T}$?

• At $t=T$, since $\lambda_{T+1}(s) = p$ regardless of the value of $s$, the 1st equation becomes $$ g_x(s_T, x_T) = \frac{\kappa}{\delta p} $$

  So we can get the optimal policy at $t=T$ by solving this equation for $x_T$. Then by the 2nd equation, we get $\lambda_{T}(s_T)$

• At $1 \leq t < T$, we can get $x_t$ and $\lambda_t(s_t)$ in the similar way

• Finally we set $s_1 = \bar{s}$ and then we get the concrete values of $\lambda_1(s_1^*), x_1^*, \lambda_2(s_2^*), x_2^*, \ldots$

Concrete Example

Let

• $T = 6$
• $g(x, s) = \alpha s + \sqrt{x}$
• $p = 1$

Then

• $g_x(x, s) = \cfrac{0.5}{\sqrt{x}}$
• $g_s(x, s) = \alpha$

At $t=6$,

• $g_x(s_6, x_6) = \cfrac{0.5}{\sqrt{x_6}} = \cfrac{\kappa}{\delta} \hspace{2em} \therefore x_6(s_6) = \cfrac{\delta^2}{4 \kappa^2}$
• $\lambda_6(s_6) = \delta \alpha$

At $1 \leq t \leq 5$,

• $g_x(s_t, x_t) = \cfrac{0.5}{\sqrt{x_t}} = \cfrac{\kappa}{\delta^{7-t} \alpha^{6-t}} \hspace{2em} \therefore x_t(s_t) = \cfrac{(\delta^{7-t} \alpha^{6-t})^2}{4 \kappa^2}$
• $\lambda_t(s_t) = \delta^{7-t} \alpha^{7-t}$

So the optimal feeding policy is

\begin{align*} x_t^*(s_t) = \cfrac{(\delta^{7-t} \alpha^{6-t})^2}{4 \kappa^2} \hspace{1em} t=1, \ldots, 6 \end{align*}

Note that in this special case the optimal policy $x_1, \ldots, x_6$ does not depend on the initial weight $\bar{s}$

9.7.8 Solution by computation

In addition to the settings of the example above, assume

• $\alpha = 0.9$
• $\kappa = 0.4$
• $\delta = 0.9$

In order to calculate the value function $V_t(s_t) = \underset{x_t \geq 0}{\max} \{ -\kappa x_t + \delta V_{t+1}(g(s_t, x_t)) \}$ given $s_t$ computationally, we approximate $V_t(s_t)$ as

\begin{align*} V_t(s_t) \simeq \tilde{V_t}(s_t) = \sum_{i=1}^n c_{it} \phi_i(s_t) \end{align*}

where $\{ \phi_i \}_{i=1}^n$ are the predetermined basis functions and $\{ c_{it} \}_{i=1}^n$ are coefficients of them

Also we choose $n$ sample points $\xi_1 < \xi_2 < \ldots < \xi_n$ in the state space

The procedure of calculating optimal policy is as follows:

• At $t=T$

  • Using optimizer, solve $V_T(\xi_j) = \underset{x_T \geq 0}{\max} \{ -\kappa x_T + \delta p g(\xi_j, x_T) \}$ for each $\xi_1, \ldots, \xi_n$ and get optimal $V_T(\xi_j)$ and $x_T(\xi_j)$

  • Solve $n$ simultaneous linear equations

    \begin{align*} \sum_{i=1}^n c_{iT} \phi_i(\xi_j) = V_T(\xi_j) \hspace{1em} j=1, \ldots, n \end{align*}

    to determine the $n$ coefficients $\{ c_{iT} \}_{i=1}^n$ and get $\tilde{V_T}(s_T)$

• At $1 \leq t < T$, given $\tilde{V_{t+1}}(\xi_j)$,

  • Using optimizer, solve $V_t(\xi_j) = \underset{x_t \geq 0}{\max} \{ -\kappa x_t + \delta \tilde{V_{t+1}}(g(\xi_j, x_t)) \}$ for each $\xi_1, \ldots, \xi_n$ and get optimal $V_t(\xi_j)$ and $x_t(\xi_j)$

  • Solve $n$ simultaneous linear equations

    \begin{align*} \sum_{i=1}^n c_{it} \phi_i(\xi_j) = V_t(\xi_j) \hspace{1em} j=1, \ldots, n \end{align*}

    to determine the $n$ coefficients $\{ c_{it} \}_{i=1}^n$ and get $\tilde{V_t}(s_t)$

• Finally set $s_1 = \bar{s}$ and then we can get all optimal policies

Let $\bar{s} = 0.6, 1.0, 1.4$

plot feeding policy and livestock weight ($\bar{s}=0.4$)

compare to the theoretical policy