Measuring diet selection in dairy cows: effect of training on choice of dietary protein level

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Abstract
In a 7-week experiment, the ability of lactating Holstein-Friesian cows to select a consistent diet from two similar foods differing in calculated metabolizable protein to energy (MP/ME) yield was investigated. The effect on diet selection of training through previous access to foods separately, was measured. Food intake was recorded with 28 computer-linked feeders. All foods were mixtures of grass silage and concentrates. In week 1, all feeders contained a standard food. In weeks 2 to 7 a low protein food (LP) and a high protein food (HP) were offered in 14 feeders each. Group CHOICE had access to both foods as a choice from week 2. Group TR1 was trained by access to one food during days 8 to 10 and to the other during days 11 to 13. Group TR2 received the same training as group TR1 during days 8 to 13 which was repeated once during days 14 to 19. After training, TR groups had access to both foods as a choice. Groups LOPRO and HIPRO had only access to LP or HP, respectively in weeks 2 to 5 and to both foods as a choice in weeks 6 and 7. In weeks 2 to 5 LOPRO COWS consumed less dry matter and produced less milk than CHOICE or HIPRO cows. After a week of adaptation, untrained CHOICE COWS selected 662 (s.e. 27) g HP per kg of intake, a choice that differed significantly (P < 0·01) from random. In weeks 4 to 7 TR cows established similar diet choice: 696 (s.e. 21) g HP per kg intake and the proportion selected was unaffected by length of training. The between-day variation in diet choice within cows was not affected by treatment. It is concluded that, under the circumstances tested, training was not required for cows to distinguish between two mixed foods with different calculated MP/ME ratios and to select proportions significantly different from random.