(a) Describe the methods for evaluation of protein quality in ruminant animals. (20 marks) (b) Enlist the hormones secreted from…

(a) Methods of evaluating protein quality in ruminants

Ruminant protein quality is not judged by the dietary amino-acid pattern alone, because rumen microbes degrade feed protein and synthesise microbial protein. Evaluation therefore estimates what reaches the small intestine and is absorbed, as metabolisable protein (MP: digestible microbial true protein + digestible rumen-undegraded protein (RUP) + endogenous protein).

In vivo and in situ methods. The nitrogen balance and digestibility trial measures apparent and true digestibility, nitrogen retention and, with indicators, nitrogen flow from duodenal or ileal cannulae. The in situ nylon-bag technique incubates feed in the rumen for different times to give the soluble (a), degradable (b) fractions and rate (c), so that effective degradability and RUP follow. Post-ruminal digestibility of RUP is measured with the mobile nylon bag technique. Microbial protein supply is estimated by markers, such as purine derivatives excreted in urine, diaminopimelic acid and ¹⁵N.

In vitro methods. Nitrogen solubility (Burroughs), protease enzyme methods (Streptomyces griseus protease), ammonia release, in vitro gas production and the Daisy incubator estimate degradability, and pepsin–pancreatin or the three-step procedure estimate intestinal digestibility of RUP.

Systems. The NRC, ICAR, INRA (PDI system, PDIN and PDIE, for protein digestible in the intestine) and CNCPS systems model RDP, RUP, microbial yield and amino-acid supply, in which lysine and methionine are usually first limiting. Classical indices such as biological value, net protein utilisation and protein efficiency ratio suit monogastrics and are of limited use in ruminants.

(b) Anterior pituitary hormones and their physiological roles

Hormones (adenohypophysis): growth hormone (GH, somatotropin), prolactin (PRL), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH) and luteinising hormone (LH). (MSH comes from the pars intermedia in animals.)

Roles:

• GH promotes growth of bone and muscle through IGF-1, protein synthesis and lipolysis, and raises milk yield in lactating cows.
• PRL supports mammary development, milk synthesis and maintenance of lactation, and maternal behaviour.
• TSH stimulates thyroid growth and thyroxine and triiodothyronine secretion, regulating metabolic rate.
• ACTH stimulates the adrenal cortex to release cortisol for stress, glucose and protein metabolism.
• FSH drives follicle growth in the ovary and, in the male, spermatogenesis through Sertoli cells.
• LH causes ovulation, corpus luteum formation and progesterone secretion, and stimulates testosterone production by Leydig cells.

Hypothalamic releasing hormones control their release, and target-gland hormones provide negative feedback.

(c) Nutritional factors affecting milk composition in dairy cows

Fat is the most variable constituent. Low-fibre, high-concentrate diets, finely chopped forage and too little effective fibre lower rumen pH and acetate production and cause milk fat depression (trans-10 fatty acids also reduce fat synthesis); adequate forage and buffers such as sodium bicarbonate restore it. Polyunsaturated oils, ionophores (monensin) and marine oils lower fat, while rumen-protected fat raises milk yield and alters fatty-acid profile.

Protein changes less: higher energy and protein intake raises milk protein by about 0.1–0.2 percentage points, through more microbial protein and amino acids (lysine and methionine); underfeeding reduces protein and solids-not-fat.

Lactose is largely constant. Minerals and vitamins affect composition, iodine and selenium and vitamin E also affect quality, and inadequate water or heat-induced low feed intake reduces yield and solids. Feeding frequency and total mixed rations stabilise rumen conditions.