Comparative nutrition and energy metabolism of young red deer (Cervus elaphus) and red x elk hybrid deer

Abstract

Elk (Cervus elaphus spp) are widely used as a terminal sire in the New Zealand deer industry because elk red deer crosses are heavier at 12 months of age than pure-bred red deer (Cervus elaphus) and therefore better fit market demands. However, it is unclear whether nutritional requirements differ between genotypes. A series of experiments compared young (4 - 12 months) red deer and red deer-elk cross (hybrids) in various aspects of their nutrition.

Single genotype groups (10-15) of red deer and hybrid weaner stags were offered one of four pasture allowances (2 to 12 kg DM/head/day) on a rotationally grazed mixed ryegrass - white clover pasture system for 9 weeks in winter (June-July), spring (October-December) and summer (February - March). Stags were weighed and given a new allocation of pasture weekly. Pre-grazing pasture mass ranged from 800 kg DM/ha for low pasture allowances to 4500kg DM/ha for higher allowances. Winter live-weight gain was low (40-80 g/day), relatively unaffected by pasture allowance and similar for both genotypes. In spring however, hybrids gained live-weight on average 100 g/day more than red deer across all pasture allowances and the response to additional pasture allowance was large (110 g/day at 2kg DM/head/day to 300 g/day at 9.5 kg DM/head/day). At the highest pasture allowance, hybrids grew faster (350 g/day) than red deer (250 g/day), although red deer were able to achieve this live-weight gain when offered less pasture (4 vs 12 kg DM/head/day, respectively). Summer live-weight gain was lower for both genotypes and responded less to increases in pasture allowance than during spring.

A second experiment compared the live-weight gain of both genotypes at ad lib feeding in an indoor environment where intake could be accurately measured. A group of red deer (n =15) and a group of hybrid (n =15) weaner stags were housed indoors during winter (3 June - 27 August) and spring (16 October - 16 December) and fed a pelleted grain based ration ad lib. Mean daily intake for each group (kg DM/head/day) was calculated as the difference between feed offered and feed refused. Hybrids had a significantly higher (P< 0.05) absolute DM intake compared with red deer in both seasons, although when expressed on a metabolic body weight basis, there was no difference between genotypes irrespective of season. Live-weight gain during winter did not differ significantly between genotypes regardless of whether it was expressed on an absolute or metabolic weight basis. Spring live-weight gain, expressed both on an absolute and metabolic live-weight basis, was significantly higher for hybrids compared with red deer (P<0.05). Red deer and hybrids increased their feed intake from winter to spring by 20% and 24% respectively on a metabolic body weight basis. Although the difference between genotypes in their seasonal increase in intake was relatively small there was a large difference in their pattern of live-weight gain. Red deer exhibited a 34% and hybrids a 76% seasonal increase in live-weight gain expressed on a metabolic live-weight basis from winter to spring. These results indicate the greater rate of live-weight gain displayed by hybrids compared with red deer was not associated with a greater ad lib intake (expressed on a metabolic body weight basis) and the seasonal increase in live-weight gain is greater for hybrids than for red deer.

A further experiment estimated the energy requirement for maintenance of both genotypes. Five deer of each genotype were housed in separate pens (3.5m²) during winter (3 June - 27 August) and spring (16 October - 16 December) and randomly assigned to one of 5 feeding levels (0.5, 0.6, 0.7, 0.8, or 0.9 times estimated ad lib intake of l.5 and l.7 kg DM/head/day during the winter and 3.0 and 3.3 kg DM/head/day during the spring for red deer and hybrids, respectively. Maintenance requirement was determined by regression analysis of live-weight gain on ME intake. Although there was no seasonal effect on the live weight gain response to intake there was a significant genotype effect. To maintain live weight during either season, hybrids required a higher ME intake (0.52 MJ ME/W0.75/day compared with red deer 0.41 MJ ME/W0.75/day). The rate of increase in live weight gain to increasing intake declined as intake increased and more so for red deer than hybrids.

The final experiment in the series involved individually housed deer and aimed to more precisely determine differences in maintenance requirement and examine the difference in composition of gain between genotypes. In addition, in vivo apparent DM digestibility was measured in both genotypes.

Red deer (n=7) and hybrid weaner stags (n=7) were housed in individual pens for a period of 8 weeks in both winter (July - August) and spring (November - December) and offered one of 7 feeding levels which ranged from maintenance to ad lib. During each 8 week experimental period, live weight gain, apparent digestibility and feed intake were measured. Immediately prior to, and at the conclusion of each 8 week period body composition was estimated using computer-assisted topography (CT scan).

In winter, there was no significant difference in the live weight gain response to intake although red deer tended to have a higher (44 vs 55 MJ/kg) requirement for gain than hybrids. In spring, red deer had a lower requirement for maintenance (0.35 vs 0.47 MJ ME/W0.75/day) but a greater requirement for live weight gain (64 vs 35 MJ/kg) than hybrids. In spring, mean ad lib intake was about 30% higher than in winter and was greater for hybrids than for red deer. Energy retention in whole body (kJ/W0.75/day) did not differ between genotypes in either winter or spring but both the energy requirement for zero energy balance (0.59 vs 0.48 MJ ME/W0.75/day) and the efficiency of utilisation (0.37 vs 0.24) was greater in spring than in winter. The disparity between live weight gain and whole body weight gain may have been due to differences in gut fill.

There was no significant difference between genotypes in relative growth coefficients for lean, bone or adipose tissue in whole body. However hybrids tended to have a higher winter and lower spring growth coefficient for fat compared with red deer. Growth coefficients for adipose, lean and bone, respectively were 0.983, 1.063 and 1.026 for winter and 1.02, 0.708 and 1.727 for spring. At the same whole body weight, deer in October had less adipose tissue than in August. It is unclear whether this represents a strategy for rapid spring growth or is an artefact of experimental protocol.

Apparent dry matter digestibility (DMD) did not differ between genotypes but was higher by between 7 and 15 percentage units in winter compared with spring. Unexpectedly, digestibility was positively correlated with intake. Digestibility increased by 2.6 percentage units for every 10g DM/W0.75/day increase in either season in one group and 4.1 and 2.1 percentage units for deer in winter and spring respectively in another group. Errors in faecal collection were discounted as causes of the unexpected result.