Photoperiodic entrainment of seasonal changes in the appetite, feeding behaviour, growth rate and pelage of pony colts

Relationships among photoperiod and changes in voluntary food intake, feedi ng behaviour growth and pelage were determined in seven, 2-year-old pony co lts (182.4 (s.e. 5.4) kg). Individually housed colts were provided with ad libitum access to a complete pelleted diet (gross energy = 16.7 MJ/kg dry m atter). Voluntary food intake (VFI, kg/day) was calculated daily and body w eights were recorded weekly throughout the 36-week study. Feeding behaviour was evaluated at approximately 4-week intervals by continous observation ( 24 h), and the hair weight density (HWD, mg/cm(2)) of shoulder pelage was d etermined fortnightly. Day length was artificially manipulated to mimic the prevailing mid-summer photoperiod (16 h light : 8 h dark, 16L:8D). After 1 week of the study (and the preceding fortnight), day length was abruptly d ecreased and thereafter animals were exposed to alternating 14-week periods of short (SD, 8L:16D) and long days (LD, 16L:8D). The mean daily VFI of in dividual ponies was calculated weekly and normalized for digestible energy (DE) content and metabolic body weight (DEI, MJ/kg M-0.75). The average dai ly gain (ADG, kg/day) in body weight of each individual was calculated week ly. The apparent digestibility of dietary energy (digestibility) was determ ined over 72 h (no.=6) on two occasions (days 92 to 95 and 190 to 193) duri ng the study. Digestibility was similar in both periods (0.48, s.e. 0.01). DEI, ADG and HWD changed in a cyclic manner throughout the study. the perio d of the appetite cycle (24.4 (s.e. 1.3) weeks) did not differ from that of the 28-week photoperiodic regime. DEI decreased from a maximum of 1.4 (s.e . 0.03) MJ/kg M-0.75 per day (day 21), to a nadir of 0.75 (s.e. 0.02) MJ/kg M-0.75 per day (day 154, (P<0.001) and had increased (P<0.001) to attain a second zenith (0.93 (s.e. 0.01) MJ/kg M-0.75 per day) before the end of th e study. Ponies ate discrete meals of similar duration, but meal frequency was associated with changes in VFI (r = 0.77) as was proportion of time spe nt feeding (r = 0.79). Changes in ADG reflected those of DEI. Body weight w as stable for 4 weeks at the nadir of the appetite cycle. Maximal HWD was c oincident with the nadir of the appetite and growth cycles. Regression of i ndividual values for DEI on ADG described a linear relationship (R-2 = 0.80 ) which could be used to predict the energy requirements of growing ponies maintained under similar conditions: DEItotal (MJ/kg M-0.75 per day) = 0.65 4ADG (kg/day) + 0.789 The duration of the photoperiod appetite, growth and pelage cycles were sim ilar, suggesting a causal relationship. Physiological responses to photoper iodic change were not immediate and exhibited a delay of 5 to 8 weeks.