Voluntary feed intake and growth performance of boars and barrows in dependence on lysine-to-energy ratio

Boars (entire male pigs) consume less feed than barrows (castrated male pigs) under practical ad libitum feeding conditions, but knowledge on possible reasons is only limited. Since the lysine and energy contents of the diet are known to be important determinants of voluntary feed intake (VFI), it was of interest if sex specific responses to selected dietary lysine-to-energy ratios could explain the differences in feed intake between boars and barrows. A total of ninety-five pigs (48 boars and 47 barrows) were used for this investigation. Four diets were fed with two different precaecally digestible lysine (Lys)-to-metabolise energy (ME) ratios, 0.93 and 0.86 (g/MJ) for grower diets and 0.71 and 0.66 (g/MJ) for finisher diets. The pigs were kept individually and divided into four feeding groups for each sex. A 2 x 2 x 2 factorial design with the factors sex (boars vs. barrows), lysine level and energy level was employed. The sex effect was dominant and influenced all variables ( P <0.001). Superior growth potential with a concomitant lower feed intake was confirmed for boars compared to barrows. Energy and lysine levels of the diets exerted only minor effects on the measured variables, such as the intake ( P <0.05) and conversion rate ( P <0.01) of lysine and energy. It was concluded that boars grew faster than barrows even though they consumed less feed. Boars and barrows responded to different lysine and ME levels of the diets in a similar manner suggesting the marked and dominating effect of sex.