Effect of dietary protein to energy ratio on growth performance of pre-and post-weaned lambs

Abstract

Milk replacers adjusted for crude protein to metabolizable energy (CP:ME) ratio to match the lambs’ changing requirement during first few weeks post-birth have resulted in higher pre-weaning lamb growth rate compared to commercial milk replacers with a static CP:ME ratio. This study aimed to determine if milk replacer with an adjusted CP:ME ratio to meet the lambs changing CP:ME requirement over time improved lambs growth performance during both the pre-and post-weaning periods. Twenty-seven Romney twin-born ram lambs were assigned to one of two pre-weaning milk feeding treatments: i) a commercial milk replacer (CMR, n = 14; CP:ME 12.28 g/MJ); or ii) a regularly adjusted blend of commercial milk replacer and milk protein concentrate (MB, n = 13; CP:ME starting at 16.6 g/MJ and adjusted down to approximately 13 g/MJ). All lambs were fed at 2.1 times their maintenance energy requirement. The CP:ME ratio of the MB treatment was adjusted twice-weekly to match the changing CP:ME requirements. All the lambs had ad libitum access to pellets. Lambs were weaned at 22 kg live weight (LW), transferred to a ryegrass and white clover-based pasture and reared approximately to 46 kg LW. The LW during pre- and post-weaning was recorded twice weekly and fortnightly, respectively. In the pre-weaning period, total daily CP intake and daily CP intake from milk were greatest (P < 0.05) in MB compared to CMR lambs. Daily ME intake of lambs in the pre-weaning period did not differ (P> 0.05) between treatments. Lambs fed MB had greater total and milk CP:ME intake (P < 0.0001), compared to CMR lambs. Average daily LW gain (ADG) during the pre-weaning phase did not differ (P> 0.05) between treatments. The ME intake per kilogram LW gain during the pre-weaning did not differ (P > 0.05) between treatments. The CP intake per kilogram LW gain during the pre-weaning was higher (P < 0.05) for MB lambs than CMR lambs. In the post-weaning period lamb LW gain and ADG did not differ (P> 0.05) between treatments. There was no correlation (P > 0.05) in overall ADG of lambs between pre-and post-weaning periods. The ADG during day one to ten at the start of experiment had a positive and significant effect on the overall post-weaning ADG (r = 0.570, P = 0.007) and ADG in first two weeks (r = 0.470, P = 0.032) of the post-weaning phase. Similar pre-and post-weaning growth rates were observed for lambs in both treatment groups, despite the pre-weaning adjustment of CP:ME to meet lambs requirements. The CMR feeding was cost effective as the pre-weaning feed cost per kilogram live weight gain was lower than that of milk blend feeding. As CMR lambs likely obtaining additional CP from pellet intake during pre-weaning, further research is warranted on the effect of CP:ME ratio of pellets, pellet intake and digestibility on growth performance of lambs reared artificially.