diff --git a/Herd.java b/Herd.java
index 6647d5a..4f96b8f 100755
--- a/Herd.java
+++ b/Herd.java
@@ -94,6 +94,9 @@
 		//check both parents present
 		if (parents[0] == null) return "You haven't selected a bull!";
 		if (parents[1] == null) return "You haven't selected a cow!";
+		//check previous offspring
+		if (offspring[0] != null || offspring[1] != null)
+			return "You still have calves from the last round of breeding!";
 		//check age difference (max 3 generations)
 		if (Math.abs(parents[0].getGeneration() - parents[1].getGeneration()) > 3)
 			return "The age difference between the parents is too great!";

diff --git a/Herd.java b/Herd.java
index 6647d5a..4f96b8f 100755
--- a/Herd.java
+++ b/Herd.java
@@ -94,6 +94,9 @@
 		//check both parents present
 		if (parents[0] == null) return "You haven't selected a bull!";
 		if (parents[1] == null) return "You haven't selected a cow!";
+		//check previous offspring
+		if (offspring[0] != null || offspring[1] != null)
+			return "You still have calves from the last round of breeding!";
 		//check age difference (max 3 generations)
 		if (Math.abs(parents[0].getGeneration() - parents[1].getGeneration()) > 3)
 			return "The age difference between the parents is too great!";
diff --git a/README.md b/README.md
index ddf2f06..9eb5517 100755
--- a/README.md
+++ b/README.md
@@ -1,8 +1,23 @@
 # MooBreeder
 
-*(c) 2018 Daniel Vedder, Amano Christian School*
-
-*Licensed under the terms of the MIT license.*
-
 MooBreeder is a simple cow breeding game, designed to illustrate the
 fundamentals of selective breeding to KS3 pupils in biology class.
+
+## Installation
+
+Java JRE 7 or higher has to be installed. MooBreeder itself does not
+have to be installed. Simply double-click the executable JAR file to
+start the game.
+
+If you are interested in the source code, you will find the current
+version at <https://git.synoikos.de/daniel/moobreeder>.
+
+## Playing
+
+Once you have started the game, click on the `Help` menu to read the
+instructions.
+
+---
+
+*&copy; 2018 Daniel Vedder, Amano Christian School*  
+*Licensed under the terms of the MIT license.*