Nutritional constraints to lamb growth at pasture

Abstract

Intake and nutrient supply were measured in early weaned lambs grazing pure species swards of Rere lucerne (Medicago sativa, L), Huia white clover (Trifolium repens, C), Ruanui perennial ryegrass (Lolium perenne, R) and Matua prairie grass (Bromus catharticus, P). A study of the factors regulating intake was undertaken with these young lambs and this area was studied in greater detail with adult sheep, which permitted repeated measurement of rumen digesta content.

A preliminary experiment, conducted indoors, studied the effect of lamb age and cannulation on the development of rumen function in lambs weaned at 6 weeks of age and offered clover hay ad libitum. Cannulation did not affect intake (g.kgW⁻¹), in vivo digestibility or marker retention time in the rumen. However, liveweight gain was 22% lower in cannulated lambs than in intact lambs, but this may have been due, at least partly, to differences in weaning weight, as liveweight gain per kg digestible organic matter intake was similar for both groups. Intake (g.kgW⁻¹) increased rapidly from weaning until approximately 10 weeks of age and remained constant, at 36-38gDM.kgW⁻¹, thereafter. With increasing age there appeared to be a reduction in the proportion of apparently digestible organic matter intake (DOMI) and digestible neutral detergent fibre (NDF) intake apparently digested in the rumen, but this affect was largely explained by changes in the intake of cannulated lambs between the 4 periods of nutrient supply measurement.

In a later experiment lambs weaned at 6 weeks of age grazed single species swards. Growth rate, intake and nutrient supply were measured during the subsequent 6 week period. Liveweight gain was higher in lambs grazing legumes (321 and 308 g.d⁻¹ for C and L respectively) than for lambs grazing grasses (230 and 227 g.d⁻¹ for P and R respectively). Nutrient supply was measured at 8 and 12 weeks of age. There was no difference in intake (g.kgW⁻¹) or site of nutrient digestion between these periods. The higher growth rate of lambs grazing legumes was associated with a 36% higher DOMI (g.kgW⁻¹) and a 33% higher NAN flow at the duodenum (g.kgW⁻¹). The proportion of DOMI apparently digested in the rumen was similar for all pasture species (average, 0.56), although less digestible NDF was digested in the rumen of lambs grazing legumes (0.76) than in the rumen, of lambs grazing grasses (0.88). Substantial losses of dietary nitrogen occurred across the rumen, particularly in lambs grazing legumes. The proportion of ingested nitrogen lost across the rumen was related to the nitrogen content of the diet.

Analysis of the present data and data from the literature suggested that growth rate was better related to the absorption of amino acid nitrogen (aaN) than to ME intake, although intake, which affected both amino acid and ME absorption, exerted the greatest influence. Estimated efficiency of utilisation of ME and aaN, above maintenance, i.e. for growth, appeared low (0.39 and 0.43, respectively) but were within the range commonly observed in animals consuming herbage diets.

The retention time of digesta in the rumen (RT) is an important aspect of intake regulation. Two techniques were identified as having potential for the estimation of RT in grazing animals, and these were evaluated in early weaned lambs and, more comprehensively, in adult sheep. Firstly, RT was estimated from the rate of disappearance of digesta from the rumen of fasted sheep. Secondly, RT was estimated from the average daily rumen fill and intake of grazing sheep. The latter technique appeared to give more reliable results and was recommended for general application. Legumes exhibited shorter RT of OM than grasses (average, 6.7 v 8.8h in adult sheep, 3.5 v 8.5h in early weaned lambs). Early weaned lambs displayed a shorter RT of legumes than adult sheep, but this was not apparent for grasses.

The pattern of rumen fill, in relation to grazing time, was examined in adult sheep. A consistent pattern was observed, with maximum rumen fill occurring around sunset and minimum fill occurring during the forenoon. The ability of sheep to alter the pattern of fill was studied by restricting the rumen capacity of sheep grazing P and L in spring.

This was effected by inserting water filled balloons into the rumen. The presence of balloons was associated with a reduction in OMI (23 and 9% for P and L respectively), although there was no apparent change in the pattern of rumen fill. The volume of digesta in the rumen was reduced in the presence of balloons, particularly in sheep grazing P (average 16 and 9% for P and L respectively), but the total volume of rumen contents (digesta + balloons) increased (7 and 17% for P and L respectively). Therefore, it appeared that intake could be markedly increased if sheep maintained rumen fill at the maximum observed value. This seriously challenged the concept of rumen capacity regulating intake and suggested the involvement of some other factor(s). A conceptual model was developed to study intake regulation. This was based on a combination of physical and metabolic mechanisms interacting in the overall regulation of intake. It provided a qualitative explanation of the response of sheep to restriction of rumen capacity. The model concept has wide potential and warrants further investigation.

In conclusion, it appears that the higher growth rate observed in lambs grazing legumes, compared to grasses, was mainly due to higher intake, although amino acid absorption appeared to be of greater importance than ME intake. The intake of high quality grazed herbage is complex and appears to be regulated by a combination of physical and metabolic mechanisms.