Answer: 8
Step-by-step explanation:
The degrees of freedom for the t-distribution used while testing the difference between two population means can be calculated using the formula:

, where
represents the size of the first sample and
indicates the size of the second sample.
In this case,
the veterinarian selects five horses that have previously been diagnosed with enteroliths and assesses their monthly intake of alfalfa flakes against that of five horses without enteroliths.
This means we consider
and 
To derive a two-sample confidence interval manually for the average flakes difference between those horses with and without enteroliths, the degrees of freedom required would thus be

Consequently, the final answer is 8.